Unit-3

Proteins
By Girum Getachew
RVU
 AMINO ACIDS
• Introduction
• 300 amino acids present in various animals, plants,
and microbial systems,
• Only 20 amino acids are coded by DNA to appear in
proteins.
• Cells produce proteins with different properties and
activities by joining the same 20 amino acids in
many different combinations and sequences.
• The properties of proteins are determined by the
physical and chemical properties of their monomer
units, the amino acids.
 Definition:

• Amino acids are the basic structural units

of proteins consisting of
– An amino group, (-NH2)
– A carboxyl (-COOH) group
– A hydrogen (H) atom and
– A (variable) distinctive (R) group.
• All of the substituents in amino acid are
attached (bonded) to a central α carbon
atom.
• This carbon atom is called α because it is
bonded to the carboxyl (acidic) group.
The general formula for the naturally occurring
amino acids would be:

H • A basic amino group

|α • (-NH2)
R – C – COOH • An acidic carboxyl group
| • (-COOH)
• A hydrogen atom
NH2
• (-H)
• A distinctive side chain
• (-R)
• In neutral solution (PH = 7):
– Both the α- amino and α carboxyl
group are ionized
– Resulting the charged form of an
amino acids
– Called zwitterion (dipolar).
• In dipolar (zwitterion) form:
– The amino group is protonated
COO-
• (-NH3 +) and
– The carboxyl group is |α
dissociated (deprotonated) NH3+ – C – H
• (-COO-)
|
– Leading to a net charge zero.
R
 Stereochemistry (Optical activity)

• Stereochemistry mainly emphasizes the

configuration of amino acids at the α
carbon atom:
– Having either
• D-isomers or
• L- isomers.
 COOH COOH
| |
H- C – NH2 H2N – C – H
| |
R R
• D (+) amino acid • L (-) amino acid
 Classification of Amino Acids

• L-Amino acids are the building blocks of

proteins.
• They are frequently grouped according to the
chemical nature of their side chains.
• Common groupings of amino acids are
– Aliphatic, Hydroxyl/sulfur,
– Cyclic, Aromatic,
– Basic, acidic and acid amides.
 I. Structural Classification

• This classification is based on the side

chain radicals (R-groups)
• Amino acid is designated:
– by three letter abbreviation
• eg. Aspartate as Asp and
– by one letter symbol
• Eg.Aspartate as D.
Abbreviations and symbols for commonly
occurring amino acids
Structural classification of Amino acids
 Location of nonpolar amino acids
in soluble and membrane proteins.
• Out of the 20 amino acids,
– Proline is not an α amino acid
– rather an α - imino acid.
• Except for glycine,
– All amino acids contain at least one
asymmetric carbon atom (the α -
carbon atom).
 II. Electrochemical classification

• Amino acids could also be classified

based on their acid – base properties
• Acid amino acids
• (Negatively charged at pH = 6.0)
• Example:
• Aspartic acid - CH2 – COO-
• Glutamic acid - CH2 – CH2 – COO
• Neutral amino acid
• Example:
• Serine - CH2- OH
• Threonine - CH2- OH
|
CH3
• Asparagine - CH2- CO-NH2
• Glutamine - CH2- CH2 - CO-NH2
 III. Biological or Physiological Classification

• This classification is based on the

functional property of amino acids for
the organism.
• 1. Essential Amino Acids
• 2. Non- Essential Amino Acids
• 3. Semi-essential amino acids
 1. Essential Amino Acids

• Amino acids which are not synthesized in the

body and must be provided in the diet to
meet an animal’s metabolic needs are called
essential amino acids.
• About ten of the amino acids are grouped
under this category indicating that mammals
require about half of the amino acids in their
diet for growth and maintenance of normal
nitrogen balance.
 2. Non- Essential Amino Acids

• These amino acids are need not be

provided through diet,
• Because:
– they can be biosynthesized in
adequate amounts within the
organism.
 3. Semi-essential amino acids

• Two amino acids are grouped under semi-

essential amino acids:
– since they can be synthesized within the
organism but their synthesis is not in
sufficient amounts.
• In that they should also be provided in the diet.
• Semi essential amino acids include Arginine
and Histidine.
 Classification Based on the Fate of Each
Amino acid in Mammals.
• Amino acids can be classified here as
– Glucogenic (potentially be converted
to glucose),
– Ketogenic (potentially be converted to
ketone bodies) and
– Both glucogenic and ketogenic.
 I. Glucogenic Amino Acids

• Those amino acids in which their carbon

skeleton gets degraded to pyrurate, α
ketoglutarate, succinyl CoA, fumrate and
oxaloacetate and then converted to Glucose
and Glycogen, are called as Glucogenic amino
acids.
• These include:-
• Alanine, cysteine, glycine,
• Arginine, glutamine, Isoleucine, tyrosine.
 II. Ketogenic Amino Acids

• Those amino acids in which their carbon skeleton

is degraded to Acetoacetyl CoA, or acetyl CoA.
• Then converted to acetone and β-hydroxy
butyrate which are the main ketone bodies are
called ketogenic amino acids.
• These includes:-
• Phenylalanine, tyrosine, tryptophan,
• Isoleucine, leucine, and lysine.
 III. Ketogenic and glucogenic Amino Acids

• The division between ketogenic and glucogenic

amino acids is not sharp for amino acids
• (Tryptophan, phenylalanine, tyrosine and
Isoleucine are both ketogenic and glucogenic).
• Some of the amino acids that can be converted
in to pyruvate, particularly (Alanine, Cysteine
and serine, can also potentially form
acetoacetate via acetyl CoA
• Especially in severe starvation and untreated
diabetes mellitus.
 Acid Base Properties of Amino Acids

• When a crystalline amino acid, such as Alanine

is dissolved in water, it can act as either an acid
(proton donor) or a base (proton acceptor)
• According to theory of acid and bases,
– An acid is a proton donor and
– A base is a proton acceptor.
• Substances having such dual nature are said to
be Amphoteric and are often called
Ampholytes.
 B. Buffers

• A buffer is a solution that resists change in pH

following the addition of an acid or base.
• A buffer can be created by mixing a weak acid
(HA) with its conjugate base (A–).
• If an acid such as HCl is then added to such a
solution, A– can neutralize it, in the process
being converted to HA.
• If a base is added, HA can neutralize it, in the
process being converted to A–.
 Peptides

• The peptide bond and its characteristics

• Proteins are macromolecules with a backbone
formed by polymerization of amino acids in a
polyamide structure.
• These amide bonds in protein, known as
peptide bonds formed by linkage of
– α - carboxyl group of one amino acid with
– α- amino groups of the next amino acid
– by amide bonds.
• A peptide chain consisting of:
– two amino acid residues is called a dipeptide,
– three amino acids tripeptide (e. g
Glutathione) etc.
• By convention, peptide structures are written:
– With amino terminal residues on the left and
– With the carboxyl terminal residue at the
right.
• E.g. A tripeptide formed from Cysteine,
Glycine and Alanine.
 Peptides of Physiological Significance

• Glutathione
• Glutathione is a tripeptide formed from amino
acids glutamate, cysteine and Glycine, linked
together in that order.
• The glutamate is linked to cysteine through
the γ- carboxyl group and α - amino group of
cysteine.
• Glutathione is virtually present in all cells often at
high levels and can be thought as a kind of redox
buffer, which probably helps to maintain.
– 1) Sulfhydryl groups of proteins in the reduced
state
– 2) Keeps the iron of heme in the ferrious
(Fe2+) state
– 3) Serves as a reducing agent for glutaredoxin.
Summery
Amino acids
 Proteins
• The word protein is derived from Greek word,
proteious meaning primary.
• So, proteins are the major components of any
living organism.
• Proteins are natural substances with high
molecular weights ranging from 5,000 to many
millions.
• Besides Carbon, Hydrogen and Oxygen, they
also contain Nitrogen, and
• Sometimes, Sulfur and Phosphorous.
• Proteins are most important constituent of
cell membranes and cytoplasm.
• Muscle and blood plasma also contain certain
specific proteins.
• Protein containing foods are essential for
living organism, because protein is the most
important biological molecules in building up
and maintenances of the structure of body,
• Giving as much energy as carbohydrates in the
course of metabolism in the body.
• Many of the body proteins perform
innumerable chemical reactions constantly
taking place inside the body.
• Proteins are the molecular instruments in which
genetic information is expressed;
• Hormones, antibodies, transporters, muscle,
• the lens protein, antibiotics, mushroom
poisons, and
• a myriad of other substances having distinct
biological activities are derived.
• Definition
• Proteins are macromolecules with a backbone
formed by polymerization of amino acids in a
polyamide structure.
• Classification
• Proteins may be classified on the basis of their
composition, solubility, shape, biological
function and on their three dimensional
structure.
 I. Composition:

• A. Simple protein:
– Yields only amino acids and no other
major organic or inorganic hydrolysis
products
– i.e. most of the elemental
compositions.
• B. Conjugated Proteins
• Yields amino acids and other organic and
inorganic components. E.g.
• Nucleoprotein (a protein containing Nuclei acids)
• Lipoprotein (a protein containing lipids)
• Phosphoprotein (a protein containing
phosphorous)
• Metalloprotein (a protein containing metal ions
of Fe2+)
• Glycoprotein (a protein containing
carbohydrates)
 II. Solubility

• a) Albumins:
• These proteins such as egg albumin and serum
albumin are readily soluble in water and
coagulated by heat.
• b) Globulins:
• These proteins are present in serum, muscle
and other tissues and are soluble in dilute salt
solution but sparingly in water.
• c) Histones:
• Histones are present in glandular tissues
(thymus, pancreas etc.)
• Soluble in water;
• They combine with nucleic acids in cells and
on hydrolysis yield basic amino acids
 III. Overall Shape

• A. Fibrous proteins
• In these protein, the molecule are
constituted by several coiled cross-linked
polypeptide chains,
• They are insoluble in water and highly
resistant to enzyme digestion.
• A few sub groups are listed below.
• 1. Collagens: the major protein of the
connective tissue, insoluble in water, acids or
alkalis.
• But they are convertible to water-soluble
gelatin, easily digestible by enzymes.
• 2. Elastins: present in tendons, arteries and
other elastic tissues, not convertible to gelatin.
• 3. Keratins: protein of hair, nails etc.
• B. Globular proteins:
• These are globular or ovoid in shape,
• Soluble in water and constitute the
enzymes, oxygen carrying proteins,
hormones etc.
• Subclasses include:-
– Albumin,
– Globulins and
– Histones.
 IV. On their Biological Functions:
• Enzymes: kinases, transaminases etc.
• Storage proteins- myoglobin, ferretin
• Regulatory proteins- peptide hormones, DNA
binding proteins
• Structural protein- collagen, proteoglycan
• Protective proteins -blood clotting factors,
Immunoglobins,
• Transport protein- Hemoglobin, plasma lipoproteins
• Contractile or motile Proteins- Actin, tubulin
 V. On their level of organization

• Primary, secondary, tertiary and quaternary.

• The amino acid composition of a peptide
chain has a profound effect on its physical and
chemical properties of proteins.
• Protein rich in polar amino acids are more
water soluble.
• Proteins rich in aliphatic or aromatic amino
groups are relatively insoluble in water and
more soluble in cell membranes (can easily
cross the cell membrane).
 a) Primary Structure of Proteins

• The primary structure of a protein is defined by

the linear sequences of amino acid residues.
• Protein contain between 50 and 2000 amino
acid residues.
• The mean molecular mass of an amino acid
residue is about 110 Dalton units (Da).
• The primary structure cannot represent the 3D-
nature of a protein molecule since the
extended chain of amino acids is co-planar as
the covalent bind of peptide is right.
 b) Secondary Structure

• The secondary structure of a protein refers to

the local structure of a polypeptide chain,
which is determined by Hydrogen bond.
• The Interactions are between the carbonyl
oxygen group of one peptide bond and the
amide hydrogen of another near by peptide
bond.
• There are two types of secondary structure,
the ∝ - helix and the β- pleated sheet.
 Protein Secondary Structure….

• Secondary structure is the regular

arrangement of amino acid residues in a
segment of a polypeptide chain, in which each
residue is spatially related to its neighbors in
the same way.
• The most common secondary structures are
– the ∝ - helix,
– the β- conformation, and
– the β- turns.
 The α - helix

• The α - helix is a rod like structure with

peptide chains tightly coiled and
• The side chains of amino acid residues
extending outward from the axis of spiral.
• The α- helix form more readily than many
other possible conformations
 β- pleated sheet
• The Conformation Organizes Polypeptide Chains
into Sheets
• This is a more extended conformation of polypeptide
chains, and its structure has been confirmed by x-ray
analysis.
• In the conformation, the backbone of the
polypeptide chain is extended into a zigzag rather
than helical structure .
• The zigzag polypeptide chains can be arranged side
by side to form a structure resembling a series of
pleats. In this arrangement, called a sheet,
Some common structural motifs combining
α-helices and/or β-sheets.
 c) Tertiary Structure

• The three
dimensional, folded
and biologically active
conformation of a
protein is referred to
as tertiary structure.
• The structure reflects
the overall shape of
the molecule.
The three dimensional structure of Myoglobin
• The three - dimensional tertiary structure
of a protein is stabilized by interactions
between
– Side-chain functional group,
– Covalent bonds, disulfide bonds,
hydrogen bonds,
– Salt bridges, and hydrophobic
interactions.
 d) Quaternary Structure

• Quaternary structure refers to a complex or an

assembly of two or more separate peptide
chains that are held together by non- covalent
or, in some case, covalent interactions.
• If the subunits are identical, it is a
homogeneous quaternary structure; but if
there are dissimilarities, it is heterogeneous.
• For instance insulin consists of α and β chain
which are different.
• Hemoglobin has 4 chains, two of them are α
and two are β.
• These, the polymers may be:
– Dimers,
– Trimers,
– Tetramers and so on.
Hemoglobin as an example of quaternary structure of a
protein
 Denaturation of Proteins
• Proteins have finite lifetimes.
• They are also subject to environmental
damages like:
– Oxidation
– Proteolysis,
– Denaturation and
– Other irreversible modifications.
• Denaturation involves the destruction of the
higher level structural organization:
– Secondary Structure
– Tertiary Structure
– Quaternary Structure
• With the retention of the primary structure by
denaturing agents.
• A denatured protein loses its native physico-
chemical and biological properties since the
bonds that stabilize the protein are broken down.
• Thus the polypeptide chain unfolds itself
and remain in solution in the unfolded
state.
• The denatured protein may retain its
biological activity by refolding
(renaturing) when the denaturing agent
is removed.
 Factors that Affect Denaturation

• Denaturing agents
• 1. Physical factors-Causes the protein to lose
its biological activity.
• Temperature,
• Pressure,
• Mechanical shear force,
• Ultrasonic vibration and
• Ionizing radiation
• 2. Chemical factors- Cause the denaturation.
– Acids and alkalis,
– Organic solvents (actone, ethanol),
– Detergents (cleaning agents),
– Certain amides urea, guandidine
hydrochloride, alkaloids, and
– Heavy metal salts (Hg, Cu, Ba, Zn, Cd…)
 Properties of a Denatured Protein

• A. An increase in number of reactive

and functional group in the composition
of the native protein molecule ( side
chain group of amino acids, COOH, NH2,
SH, OH … etc)
• B. Reduced solubility and pronounced
propensity for precipitation
• C. Configurational alteration of the
protein molecule.
• D. Loss of biological activity evoked by
the disarrangement of the native
structural molecular organization.
• E. Access of proteolytic enzymes in
comparasion with the native protein
Thank You!
 Globular Proteins
• GLOBULAR HEMEPROTEINS
• Hemeproteins are a group of specialized
proteins that contain heme as a tightly
bound prosthetic group.
• Heme is the O2 – binding molecule
common to Mb and Hb
• Protophorphyrin IX is the backbone of
heme
• When iron is complexed with
protophorphyrin IX it is called Heme.
• Heme is the prosthetic group in
• Hemoglobin,
• Myoglobin and
• Cytochrome b, c, and c1.
• The two most abundant heme-proteins
in humans are:
• Hemoglobin and
• Myoglobin
• The heme group serves to reversibly bind
oxygen.
• Myoglobin (Mb)
• Found primarily in skeletal and striated
muscle
• Serves as a store of O2 in the cytoplasm and
• Deliver it on demand to the mitochondria.
• Hemoglobin (Hb)
• Restricted to the erythrocytes (RBCs)
• Responsible for the movement of O2
between lungs and other tissues .
 A. Structure of heme

• Heme is a complex of protoporphyrin IX

and ferrous iron (Fe2+).
• The iron is held in the center of the heme
molecule by bonds to the four nitrogens
of the porphyrin ring.
• The heme Fe2+ can form two additional
bonds, one on each side of the planar
porphyrin ring.
• In myoglobin and hemoglobin, one of these
positions is coordinated to the side chain of a
histidine residue of the globin molecule,
• Whereas the other position is available to
bind oxygen.
 Myoglobin(Mb)
• Structure and function of myoglobin
• Myoglobin is a hemeprotein
• Present in heart and skeletal muscle,
• Functions both as a reservoir for
oxygen, and as an oxygen carrier
• That increases the rate of transport of
oxygen within the muscle cell.
• Myoglobin consists of a single polypeptide chain
• 8-stretches of α-helixcal regions, labeled A to H
• Myoglobin, is an oxygen storage protein.
• Oxygen transported to tissues must be released
for utilization.
• In tissues, such as muscle, with high oxygen
demands, myoglobin provides large oxygen
reserves.
A. Model of myoglobin showing helices A to H.
B. Schematic diagram of the oxygen-binding
site of myoglobin.
 Hemoglobin(Hb)
• Structure and function of hemoglobin
• Hemoglobin is found exclusively in red blood
cells (RBCs),
• Main function is to transport oxygen (O2) from
the lungs to the capillaries of the tissues.
• Hemoglobin is a tetrameric protein.
• Each polypeptide subunit closely resembles
myoglobin.
 Hemoglobin A
• Adult Hb (HbA)
• The major hemoglobin in adults
• Contains two types of globin:
• Two α - chains (141 residues each) and
• Two β - chains (146 residue each).
• Held together by noncovalent interactions
• Each subunit has stretches of α-helical structure,
and a heme-binding pocket
• The two subunits are identical at 27 positions.
• Adult Hb:
– HbA, has the structure α2 β2.
– HbA2 (α2δ 2) is minor adult hemoglobin.
• Fetal Hb:
– HbF (α2γ2) predominates during most
of gestation
• Just after conception fetuses synthesize:
– zeta(ζ) chain (quite like α -chain)
– ε- chains just like β - chain
• Later zeta replaced by α – and ε- by γ.
• The HbF variant barely detectable
• The globin genes.
– The α-like genes (α,ζ) are encoded on
chromosome 16
– The β -like genes (β,γ,δ,ε) are encoded
on chromosome 11.
• The ζ and ε genes encode embryonic
globins.
Normal adult human hemoglobins.
• The tetrameric hemoglobin molecule is structurally and
functionally more complex than myoglobin.
• For example, hemoglobin:
– Can transport H+ and CO2 from the tissues to the
lungs, and
– Can carry four molecules of O2 from the lungs to the
cells of the body.
– Oxygen-binding properties of hemoglobin are
regulated by interaction with allosteric effectors.
• Carbon monoxide (CO) binds tightly (but reversibly) to
the hemoglobin iron, forming carbon monoxy
hemoglobin (Hb CO).
A. Structure of hemoglobin showing the
polypeptide backbone.
B. Simplified drawing showing the helices.
 1. Quaternary structure of hemoglobin:

• The hemoglobin tetramer composed of

two identical dimers,
• (αβ)1 and
• (αβ)2,
• In which the numbers refer to dimers
one and two.
• The two polypeptide chains within each dimer are
held tightly together, primarily by hydrophobic
interactions
• a. T form:
• The deoxy form of hemoglobin is called the “T,” or
taut (tense) form.
• In the T form, the two αβ dimers interact through a
network of ionic bonds and hydrogen bonds that
constrain the movement of the polypeptide chains.
• The T form is the low oxygen- affinity form of
hemoglobin.
• b. R form:
• The binding of oxygen to hemoglobin causes
the rupture of some of the ionic bonds and
hydrogen bonds between the αβ dimers.
• This leads to a structure called the “R,” or
relaxed form, in which the polypeptide chains
have more freedom of movement.
• The R form is the high oxygen- affinity form of
hemoglobin.
 Oxygen dissociation curve:

• A plot of Y measured at different partial

pressures of oxygen (pO2) is called the oxygen
dissociation curve.
• The curves for myoglobin and hemoglobin
show important differences.
• This graph illustrates that myoglobin has a
higher oxygen affinity at all pO2 values than
does hemoglobin.
• The partial pressure of oxygen needed to
achieve half-saturation of the binding
sites (P50) is approximately 1 mm Hg for
myoglobin and 26 mm Hg for
hemoglobin.
• The higher the oxygen affinity (that is,
the more tightly oxygen binds), the lower
the P50.
• [Note: pO2 may also be represented as
PO2.]
 Hemoglobin-oxygen
dissociation curve.

• The hemoglobin tetramer can bind up to four

molecules of oxygen in the iron-containing sites of the
heme molecules.
• As oxygen is bound, 2,3-BPG and CO2 are expelled.
• Salt bridges are broken, and each of the globin
molecules changes its conformation to facilitate
oxygen binding.
• Oxygen release to the tissues is the reverse process,
salt bridges being formed and 2,3-BPG and CO2
bound.
• Deoxyhemoglobin does not bind oxygen
efficiently until the cell returns to
conditions of higher pH, the most
important modulator of O2 affinity (Bohr
effect).
• When acid is produced in the tissues, the
dissociation curve shifts to the right,
facilitating oxygen release and CO2
binding.
• Alkalosis has the opposite effect,
reducing oxygen delivery.
 a. Myoglobin (Mb):

• The oxygen dissociation curve for myoglobin

has a hyperbolic shape.
• This reflects the fact that myoglobin
reversibly binds a single molecule of oxygen.
• Thus, oxygenated (MbO2) and deoxygenated
(Mb) myoglobin exist in a simple equilibrium:
• Mb + O2 MbO2
• The equilibrium is shifted to the right or to the
left as oxygen is added to or removed from the
system.
• [Note: Myoglobin is designed to bind oxygen
released by hemoglobin at the low pO2 found
in muscle.
• Myoglobin, in turn, releases oxygen within the
muscle cell in response to oxygen demand.]
 b. Hemoglobin (Hb):
• The oxygen dissociation curve for hemoglobin is
sigmoidal in shape
• Indicating that the subunits cooperate in binding
oxygen.
• Cooperative binding of oxygen by the four
subunits of hemoglobin means that the binding
of an oxygen molecule at one heme group
increases the oxygen affinity of the remaining
heme groups in the same hemoglobin molecule.
• This effect is referred to as heme-heme
interaction.
• Although it is more difficult for the first
oxygen molecule to bind to hemoglobin,
the subsequent binding of oxygen occurs
with high affinity, as shown by the steep
upward curve in the region near 20–30
mm Hg.
 Steps in globin chain synthesis

• Expression of a globin gene begins in the

nucleus of red cell precursors, where the DNA
sequence encoding the gene is transcribed.
• The RNA produced by transcription is actually
a precursor of the messenger RNA (mRNA)
that is used as a template for the synthesis of
a globin chain.
• Before it can serve this function, two
noncoding stretches of RNA (introns) must be
removed from the mRNA precursor sequence,
and
• the remaining three fragments (exons) joined
in a linear manner.
• The resulting mature mRNA enters the
cytosol, where its genetic information is
translated, producing a globin chain.
 HEMOGLOBINOPATHIES
• Hemoglobinopathies defined as a family
of genetic disorders caused by
production of a structurally:
– Abnormal hemoglobin molecule,
– Synthesis of insufficient quantities of
normal hemoglobin, or,
– Rarely, both.
• Hemoglobinopathies includes:
• Sickle cell anemia (Hb S),
• Hemoglobin C disease (Hb C),
• Hemoglobin SC disease (Hb S + Hb C)
• The thalassemia syndromes
• The first three conditions result from
production of hemoglobin with an
altered amino acid sequence (qualitative
hemoglobinopathy), whereas
• The thalassemias are caused by
decreased production of normal
hemoglobin (quantitative hemo -
globinopathy).
 A. Sickle cell anemia (Hb S disease)

• Sickle cell anemia, the most common of the

red cell sickling diseases,
• A genetic disorder of the blood caused by a
single nucleotide alteration (a point mutation)
in the gene for β-globin.
• The mutation is Glu6 by Val a surface
localized charged amino acids is replaced by a
hydrophobic residue
• Sickle cell anemia is a homozygous, recessive
disorder.
• It occurs in individuals who have inherited two
mutant genes (one from each parent) that
code for synthesis of the β chains of the globin
molecules.
• [Note: The mutant β-globin chain is
designated βS, and the resulting hemoglobin,
α2βS2, is referred to as Hb S.
• Such substitution of Valine (non - polar) for
Glutamate (polar) have the following consequence
• 1. Place A non - polar residue on the outside of HbS
which markedly reduce solubility of deoxy HbS.
• But has little effect on oxy - HbS (causes Hb to
clump when deoxygenated)
• 2. Creates sticky patches on the outside surface of
each β - chains (not present HbA)
• 3. The sticky patches interact with complementary
sites of another HbS (oxy) and forms large
aggregates that distort the whole RBC structure.
 Sickle Cell Trait

• The heterozygote individuals (sickle cell trait)

(HbA/HbS) is associated with increased
resistance to malaria.
• Specifically growth of the infectious agent,
Plasmodium falciparum in the erythrocyte.
• The HbA/ HbS heterozygote exhibits
advantage over the HbA/HbA normal or the
HbS/HbS homozygte.
 normal erythrocytes
(a) with the variably shaped erythrocytes seen in sickle-cell
anemia
(b), which range from normal to spiny or sickle-shaped.
 Hemoglobin C disease
• Like Hb S, Hb C is a hemoglobin variant that has a
single amino acid substitution in the sixth position
of the β-globin chain
• In this case, however, a lysine is substituted for the
glutamate (as compared with a valine substitution
in Hb S).
• Patients homozygous for hemoglobin C generally
have a relatively mild, chronic hemolytic anemia.
• These patients do not suffer from infarctive crises,
and no specific therapy is required.
 Methemoglobinemias

• Oxidation of the heme component of hemoglobin to

the ferric (Fe3+) state forms methemoglobin, which
cannot bind oxygen.
• This oxidation may be caused by the action of certain
drugs, such as nitrates, or endogenous products, such
as reactive oxyge intermediates
• Symptoms are related to the degree of tissue hypoxia,
and include anxiety, headache, and dyspnea.
• In rare cases, coma and death can occur. Treatment is
with methylene blue, which is oxidized as Fe+3 is
reduced.
 Thalassemias
• The thalassemias are hereditary hemolytic
diseases in which an imbalance occurs in the
synthesis of globin chains.
• As a group, they are the most common single
gene disorders in humans.
• Normally, synthesis of the α- and β-globin
chains is coordinated, so that each α-globin
chain has a β-globin chain partner.
• This leads to the formation of α2β2 (Hb A).
• In the thalassemias, the synthesis of
either the α- or the β-globin chain is
defective.
• A thalassemia can be caused by a variety
of mutations, including:
– Entire gene deletions, or
– Substitutions or
– Deletions Of one to many nucleotides
in the DNA.
• [Note: Each thalassemia can be classified
as either a disorder in which:
– No globin chains are produced (αo- or
βo-thalassemia), or
– One in which some chains are
synthesized, but at a reduced level
(α+- or β+-thalassemia).]
 1. β-Thalassemias:

• In these disorders, synthesis of β-globin

chains is decreased or absent, typically as a
result of point mutations that affect the
production of functional mRNA;
• however, α-globin chain synthesis is normal.
• α-Globin chains cannot form stable tetramers
and, therefore, precipitate, causing the
premature death of cells initially destined to
become mature RBCs
• There are only two copies of the β-globin
gene in each cell (one on each
chromosome 11).
• β-thalassemia trait (β-thalassemia
minor)
– If they have only one defective β-
globin gene, or
• β-thalassemia major (Cooley anemia)
– If both genes are defective
A. β-Globin gene mutations in the β-thalassemias.
B. Hemoglobin tetramers formed in β-thalassemias.
 2. α-Thalassemias:

• These are defects in which the synthesis of α-

globin chains is decreased or absent, typically
as a result of deletional mutations.
• Because each individual’s genome contains
four copies of the α-globin gene (two on each
chromosome 16),
• there are several levels of α-globin chain
deficiencies
• If one of the four genes is defective,
– the individual is termed a silent carrier
of α-thalassemia,
– because no physical manifestations of
the disease occur.
• If two α-globin genes are defective,
– the individual is designated as having
α -thalassemia trait.
• If three α-globin genes are defective,
– the individual has HbH (β4) disease—
– a mildly to moderately severe hemolytic
anemia.
• If all four α-globin genes are defective,
– Hb Bart (γ4) disease with hydrops fetalis
and fetal death results,
– because α-globin chains are required for
the synthesis of Hb F.
Thank You!
 Fibrous Proteins
• Fibrous proteins of the extracellular matrix
that serve structural functions in the body.
• Examples of common fibrous proteins are:
– Collagen and Elastin
• For example, collagen and elastin are found as
components of:
– Skin, Connective tissue,
– Blood vessel walls, and
– Sclera and cornea of the eye.
• Each fibrous protein:
– Exhibits special mechanical properties,
– Resulting from its unique structure,
– Which are obtained by combining
specific amino acids into regular,
– Secondary structural elements.
 COLLAGEN

• Collagen is the most

abundant protein in the
human body.
• A typical collagen molecule
is a long, rigid structure
• Three polypeptides(“α
chains”) are wound around
one another in a rope-like
triple helix
 Types of collagen
• More than 25 collagen types
• 1. Fibril-forming collagens:
• Types I, II, and III
– are the fibrillar collagens, and
– have the rope-like structure described
above for a typical collagen molecule.
• 2. Network-forming collagens:
• Types IV and VII form a three-
dimensional mesh, rather than distinct
fibrils
• 3. Fibril-associated collagens:
• Types IX and XII bind to the surface of
collagen fibrils, linking these fibrils to one
another and to other components in the
extracellular matrix
• Collagen is rich in proline and glycine, both of
which are important in the formation of the
triple-stranded helix.
• The polypeptide precursors of the collagen
molecule are formed in fibroblasts (or in the
related osteoblasts of bone and chondro
blasts of cartilage), and are secreted into the
extracellular matrix.
 Collagen diseases: Collagenopathies
• Defects in any one of the many steps in
collagen fiber synthesis can result in:
– A genetic disease
– Involving an inability of collagen to form
fibers properly and,
– Thus, not provide tissues with the needed
tensile strength normally provided by
collagen.
• Examples of diseases that are the result
of defective collagen synthesis:
• 1. Ehlers-Danlos syndrome (EDS):
– A heterogeneous group of generalized
connective tissue disorders
– That result from inheritable defects in
the metabolism of fibrillar collagen
molecules.
• 2. Osteogenesis imperfecta (OI):
– Known as brittle bone syndrome,
– A heterogeneous group of inherited
disorders
– Distinguished by bones that easily
bend and fracture
 ELASTIN

• In contrast to collagen, which forms fibers that

are tough and have high tensile strength.
• Elastin is a connective tissue protein with
rubber-like properties.
• Elastic fibers composed of elastin and
glycoprotein microfibrils are found in:
– The lungs,
– The walls of large arteries, and
– The elastic ligaments.
• Elastin can be stretched to several times their
normal length, but recoil to their original
shape when the stretching force is relaxed.
• Elastin is an insoluble protein polymer
synthesized from a precursor, tropoelastin,
• Which is a linear polypeptide composed of
about 700 amino acids that are primarily small
and nonpolar (for example, glycine, alanine,
and valine).
 Disorders of elastin degradation
• α1-Antitrypsin deficiency
• In the alveoli, elastase released by activated and
degenerating neutrophils is normally inhibited
by α1-antitrypsin.
• Genetic defects in α1-antitrypsin can lead to
emphysema and cirrhosis.
• Smoking increases risk.
• The deficiency of elastase inhibitor can be
reversed by weekly intravenous administration
of α1-AT.
• COPD and its subtypes (emphysema,
chronic bronchitis, and chronic
obstructive asthma)
• Emphysema –
– also pulmonary emphysema
– A condition in which the air sacs of
the lungs are damaged and enlarged,
causing breathlessness.
Thank You!
 Enzymes
• Virtually all reactions in the body are
mediated by enzymes,
• Enzymes are protein catalysts that
increase the rate of reactions without
being changed in the overall process.
• Enzymes thus direct all metabolic events.
 ENZYMES
• General Properties
– Enzymes are protein catalysts for
chemical reaction in biological systems.
– They increase the rate of chemical
reactions taking place within living
cells with out changing themselves.
 Nature of Enzymes

• Most enzymes are protein in nature.

• Depending on the presence and absence
of a nonprotein component with the
enzyme enzymes can exist as:
• 1. Simple-enzyme or
• 2. Holo-enzyme
• 1. Simple enzyme:
• It is made up of only protein molecules
not bound to any nonproteins.
• Example:
• Pancreatic Ribonuclease.
• 2. Holo enzyme
• Made up of protein groups and non-
protein component.
• The protein component of this holo
enzymes is called apoenzyme
• The non-protein component of the
holo enzyme is called a cofactor.
• If this cofactor is an organic compound it
is called a coenzyme and
• If it is an inorganic groups it is called
activator.
• (Fe 2+, Mn 2+, or Zn 2+ ions).
• If the cofactor is bound so tightly to the
apoenzyme and is difficult to remove
without damaging the enzyme it is
sometimes called a prosthetic group
 COENZYMES
• Coenzymes are derivatives of vitamins
without which the enzyme cannot exhibit any
reaction.
• One molecule of coenzyme is able to convert a
large number of substrate molecules with the
help of enzyme.
• Coenzyme accepts a particular group,
removed from the substrate or donates a
particular group to the substrate
• Coenzymes are called co-substrate
because the changes that take place in
substrates are complimentary to the
changes in coenzymes.
• The coenzyme may participate in forming
an intermediate enzyme-substrate
complex
• Example: NAD, FAD, Coenzyme A
 Metal ions in enzymes

• Many enzymes require metal ions like ca2+,

K+, Mg2+, Fe2+, Cu2+, Zn2+, Mn2+ and
Co2+for their activity.
• Metal-activated enzymes-form only loose and
easily dissociable complexes with the metal
and can easily release the metal without
denaturation.
• Metalloenzymes hold the metal tightly on the
molecule and do not release it even during
extensive purification.
• Metal ions promote enzyme action by:
– a. Maintaining or producing the active
structural conformation of the enzyme (e.g.
glutamine synthase)
– b. Promoting the formation of the enzyme-
substrate complex (e.g: Enolase and
carboxypeptidase A.)
– c. Acting as electron donors or acceptors
(e.g: Fe-S proteins and cytochromes)
– d. Causing distortions in the substrate or the
enzyme Example: phosphotransferases).
 NOMENCLATURE
• The International Union of Biochemistry
and Molecular Biology developed:
– A system of nomenclature
– On which enzymes are divided in to six
major classes,
– Each with numerous sub groups.
• As an example,

• The formal systematic name of the enzyme

catalyzing the reaction is:
• ATP: glucose phosphotransferase
• Which indicates that it catalyzes the transfer
of a phosphoryl group from ATP to glucose.
• Its Enzyme Commission number (E.C. number) is
2.7.1.1.
• (2) -The first number denotes the class name
(transferase);
• (7)-The second number, the subclass
(phosphotransferase);
• (1)-The third number, a phosphotransferase with
a hydroxyl group as acceptor; and
• (1)- The fourth number, D-glucose as the
phosphoryl group acceptor.
• Each enzyme is assigned two names.
• 1. Common name:
– Short, recommended name, convenient for
everyday use.
• 2. Systematic name
– More complete systematic name,
– Used when an enzyme must be identified
without ambiguity.
 A. Recommended name

• Most commonly used enzyme names

have the suffix “-ase”
• Attached to the substrate of the reaction:
– E.g glucosidase and urease
• Attached to a description of the action
performed:
– E.g hydrogenase and adenylyl cyclase
• Some enzymes retain their original trivial
names,
• Which give no hint of the associated
enzymic reaction,
– E.g trypsin and pepsin
 B. Systematic name

• In the systematic naming system, enzymes are divided

into six major classes each with numerous subgroups.
• For a given enzyme, the suffix -ase is attached to a
fairly complete description of the chemical reaction
catalyzed, including the names of all the substrates;
E.g, lactate: NAD+ oxidoreductase.
• Note: Each enzyme is also assigned a classification
number.
• The systematic names are unambiguous and
informative, but are frequently too cumbersome to be
of general use.
 Classification of Enzymes
• Enzymes are classified based on the reactions
they catalyze.
• Each enzyme is characterized by a code
number comprising four digits separated by
points.
• The four digits characterize class, sub-class,
sub-sub-class, and serial number of a
particular enzyme.
The six major classes of enzymes with
examples
THF = tetrahydrofolate.
 PROPERTIES OF ENZYMES
• Enzymes are protein catalysts that increase
the velocity of a chemical reaction, and are
not consumed during the reaction.
• A. Active sites
• B. Catalytic efficiency
• C. Specificity
• D. Holoenzymes
• E. Regulation
• F. Location within the cell
 A. Active sites

• Enzyme molecules contain a special pocket or

cleft called the active site.
• The active site contains amino acid side chains
that participate in substrate binding and
catalysis.
• The substrate binds the enzyme, forming an
enzyme–substrate (ES) complex.
• Binding is thought to cause a
conformational change in the enzyme
(induced fit) that allows catalysis.
• ES is converted to an enzyme–product
(EP) complex that subsequently
dissociates to enzyme and product.
• Enzyme
• Substrate
• Enzyme–Substrate (ES)
• Enzyme–Product (EP)
• Enzyme
• Product
 B. Catalytic efficiency

• Enzyme-catalyzed reactions are highly

efficient, proceeding from 103–108 times
faster than uncatalyzed reactions.
• The number of molecules of substrate
converted to product per enzyme
molecule per second is called the
turnover number, or Kcat and typically is
102–104s-1.
 C. Specificity

• Enzymes are highly specific, interacting

with one or a few substrates and
catalyzing only one type of chemical
reaction.
• [Note: The set of enzymes made in a cell
determines which metabolic pathways
occur in that cell.]
• Enzymes are specific for their substrate.
• Specificity of enzymes are divided into:
• a. Absolute specificity:-this means one
enzyme catalyzes or acts on only one
substrate.
• For example: Urease catalyzes hydrolysis of
urea but not thiourea.
• b. Stereo specificity- some enzymes are
specific to only one isomer even if the
compound is one type of molecule:
• For example:
• Glucose oxidase catalyzes the oxidation of β-
D-glucose but not α-Dglucose,and
• Arginase catalyzes the hydrolysis of L-arginine
but not D-arginine.
• Maltase catalyzes the hydrolysis of α- but not
β –glycosides.
• Bond Specificity
• Enzymes that are specific for a bond or linkage
such as ester, peptide or glycosidic belong to
this group
• Examples:
• 1. Esterases- acts on ester bonds
• 2. Peptidases-acts on peptide bonds
• 3. Glycosidases- acts on glycosidic bonds.
 D. Holoenzymes

• Some enzymes require molecules other than

proteins for enzymic activity.
• The term holoenzyme refers to the active
enzyme with its nonprotein component,
• Whereas the enzyme without its nonprotein
moiety is termed an apoenzyme and is
inactive.
• If the nonprotein moiety is a metal ion such as
Zn2+ or Fe2+, it is called a cofactor.
• If it is a small organic molecule, it is termed a
coenzyme.
• Coenzymes that only transiently associate with
the enzyme are called cosubstrates.
• Cosubstrates dissociate from the enzyme in an
altered state (NAD+ is an example).
• If the coenzyme is permanently associated with
the enzyme and returned to its original form, it
is called a prosthetic group (FAD is an example).
• Coenzymes frequently are derived from
vitamins.
• For example, NAD+ contains niacin and FAD
contains riboflavin
• Zymogens (- inactive form of enzyme)
• Some enzymes are produced in nature in an
inactive form which can be activated when
they are required. Such type of enzymes are
called Zymogens (Proenzymes).
• Many of the digestive enzymes and enzymes
concerned with blood coagulation are in this
group
• Examples:
• Pepsinogen - This zymogen is from gastric
juice.
• When required Pepsinogen converts to Pepsin
• Trypsinogen - This zymogen is found in the
pancreatic juice, and when it is required gets
converted to trypsin.
• Pepsinogen + H+  Pepsin
• Trypsinogen Enteropeptidase  Trypsin
 E. Regulation

• Enzyme activity can be regulated,

• That is, increased or decreased,
• So that the rate of product formation
responds to cellular need.
 F. Isoenzymes (Isozymes)

• These are enzymes having:

– Similar catalytic activity, act on the same
substrate and produces
– The same product but originated at
different site and exhibiting different
physical and chemical characteristics
– Such as electrophoretic mobilities, amino
acid composition and immunological
behavior.
• Example:
– LDH (Lactate dehydrogenase)
– Exists in five different forms
– Each having four polypeptide chains.
– H= Heart and M=Muscle.
• Example.
– CPK (Creatine phospho kinase)
– Exists in three different forms
– Each having two polypeptide chains.
– Characteristic sub units are
• B=Brain and
• M= Muscle.
 G. Location within the cell

• Many enzymes are localized in specific

organelles within the cell.
• Such compartmentalization serves to isolate
the reaction substrate or product from other
competing reactions.
• This provides a favorable environment for the
reaction, and organizes the thousands of
enzymes present in the cell into purposeful
pathways.
MECHANISM OF ACTION OF ENZYMES

• Lock and Key Model

• Emil Fischer’s 1890.
– implies that the active site of the
enzyme is complementary in shape to
that of its substrate,
– i.e. the shape of the enzyme molecule
and the substrate molecule should fit
each other like a lock and Key
Lock and Key Theory of Fischer
 Induced Fit Model
• Koshland in 1963 in the form of ‘induced fit
mechanism’.
• The essential feature of this theory is the flexibility of
the enzyme active site.
• In Fisher model, the active site is presumed to be a
rigid preshaped structure to fit the substrate,
• while in the induced fit model the substrate induces
the conformational change in the enzyme,
• So that the substrate and active site come close to
each other in such a way that the substrate fits the
active site in a more convenient manner.
Induced Fit Model of Koshland
 Mechanism of Enzyme Action (1913)

• Michaels and Menten have proposed a

hypothesis for enzyme action, which is most
acceptable.
• According to their hypothesis, the enzyme
molecule (E) first combines with a substrate
molecule (S) to form an enzyme substrate (ES)
complex which further dissociates to form
product (P) and enzyme (E) back.
 MICHAELIS-MENTEN EQUATION

• Where:
• S is the substrate
• E is the enzyme
• ES is the enzyme–substrate complex
• P is the product
• k1, k-1, and k2 are rate constants
 Factors Affecting Enzyme Activity

• Physical and chemical factors are

affecting the enzyme activity.
• These include
• 1. Temperature
• 2. pH
• 3. Substrate/enzyme concentration etc.
• Temperature:
• The temperature at which an enzyme
shows maximum activity is known as
• the optimum temperature for the
enzyme.
• For most body enzymes the optimum
temperature is around
• 37 0c, which is body temperature.
• Effect of pH
• The concentration of H+ affects reaction
velocity in several ways.
• The pH at which maximum enzyme activity is
achieved is different for different enzymes
• For example, pepsin, a digestive enzyme in the
stomach, has maximum action at pH 2,
• Optimum Ph of blood 7.35-3.45
Effect of pH on enzyme-catalyzed reactions.
• Substrate concentration
• An increase in the substrate concentration
increases the enzyme activity till a maximum
is reached.
• Further increase in substrate concentration
does not increase rate of reaction.
• The active Sites become saturated.
• At this state the enzyme is obtained it
maximum rate (V max).
Relationship between [S] and Km
 Enzyme Inhibition
• Any substance that can diminish the
velocity of an enzyme-catalyzed reaction
is called an inhibitor and
• The process is known as inhibition.
• There are two major types of enzyme
inhibition:
• Irreversible and
• Reversible.
• Three important classes of inhibitors
• Competitive inhibitors
– Resemble the substrate and compete
for binding active site of the enzyme.
• Irreversible inhibition
– The type of inhibition that can not be
reversed by increasing substrate
concentration or removing the
remaining free inhibitor
• Non-competitive inhibition
– The inhibitor binds at different site
rather than the substrate-binding site.
– When the inhibitor binds at this site
there will be a change in conformation
of the enzyme molecules, which leads
to the reversible inactivation of the
catalytic site.
 Regulation of enzyme activity
• 1. Irreversible covalent Activation / Zymogen
activation
• Some enzymes are secreted in an inactive form called
Proenzymes or zymogens. E.g.
• Pepsinogen  pepsin,
• Trypsinogen trypsin,
• Plasminogenplasmin.
• After hydrolysis when it is activated, it cannot be
reconverted into proenzyme form.
• 2. Reversible Covalent Modification
• By addition of or removal of phosphate or
adenylate, certain enzymes are reversibly
activated and inactivated as per the
requirement.
• Protein kinase of muscle phosphorylate
phosphorylase kinase, glycogen synthetase by
making use of ATP.
 3. Allosteric Modulation

• A class of enzymes that bind

small, physiologically
important molecules and
modulate activity in ways other
than those described above.
• These are known as allosteric
enzymes;
• The small regulatory molecules
to which they bind are known
as effectors.
• 4. Feedback inhibition
• In allosteric regulation in which end products
inhibit the activity of the enzyme is called
“feedback inhibition”.
ENZYMES IN CLINICAL DIAGNOSIS
• Plasma enzymes can be classified into two
major groups
• 1. Those, relatively, small group of enzymes
secreted into the plasma by certain organs
• (i.e. Enzymes those have function in plasma)
• For example: the liver secretes zymogens of
the enzymes involved in blood coagulation.
• 2. Those large enzyme species released
from cells during normal cell turnover.
• Measurement of enzymes concentration
of mostly the in plasma gives valuable
information about disease involving
tissues of their origin.
 1. Lipase:

• It is an enzyme catalyzing the hydrolysis of fats.

• It is secreted by pancreas and Liver.
• The plasma lipase level may be low in:
• Liver disease, Vitamin A deficiency,
• Some malignancies, and Diabetes mellitus.
• It may be elevated in acute pancreatitis and
pancreatic carcinoma.
 2. α- Amylase

• α- amylase is the enzyme concerned with the

break down of dietary starch and glycogen to
maltose.
• It is present in pancreatic juice and saliva as
well as in liver fallopian tubes and muscles.
• The enzyme is excreted in the Urine.
• The main use of amylase estimations is in the
diagnosis of acute pancreatitis.
• The plasma amylase level may be
– Low in liver disease and
– Increased in high
• Intestinal obstruction,
• Mumps,
• Acute pancreatitis and diabetes.
• 3. Trypsin
• Trypsin is secreted by pancreas.
• Elevated levels of trypsin in plasma occur
during acute pancreatic disease.
• 4. Alkaline phosphates (ALP)
• The alkaline phosphates are a group of
enzymes, which hydrolyze phosphate
esters at an alkaline pH.
• They are found in bone, liver, kidney, intestinal
wall, lactating mammary gland and placenta.
• In bone the enzyme is found in osteoblasts and
important for normal bone function.
• The level of these enzymes may be increased in
rickets and osteomalacia, hyperparathyroidism,
paget's disease of bone, obstructive jaundice,
and metastatic carcinoma.
• Serum alkaline phosphatase levels may be
increase in congestive heart failure result of
injury to the liver.
 5. Acid Phosphatase (ACP)

• Acid phosphatases catalyzing the

hydrolysis of various phosphate esters at
acidic pH is found in the prostate, liver,
red cells, platelets and bone.
• It may be elevated in metastatic
prostatic carcinoma.
 6. Transaminases

• Two transaminases are of clinical

interest.
• 1. Aspartate Transaminase, AST
• ( Glutamate oxaloacetate transaminase,
GOT )
• Catalyzes the transfer of the amino group
of aspartic acid to α- ketoglutarate
forming glutamate and oxaloacetate.
• AST or GOT is widely distributed, with
high concentration in:
– The heart, Liver, Skeletal muscle,
– Kidney and Erythrocytes
• Damage to any of these tissues may
cause raised levels.
• 2. Alanine transaminase, ALT
• (Glutamate pyruvate transaminase, GPT )
• Transfer the amino group of alanine to α-
ketoglutarate, forming glutamate and
pyruvate.
• It is present in high concentration in liver
and to a lesser extent in skeletal muscle,
kidney and heart.
• Serum levels of glutamate-
pyruvatetransaminase (SGPT)(ALT) is
– Useful in the diagnosis of liver
parenchymal damage
– In liver damage, both enzymes are
increased, but SGPT increases more.
• Serum Glutamateoxaloacetate-
transaminase (SGOT)(AST) is
– Useful in the diagnosis myocardial
damage respectively.
– In myocardial infarction SGOT is
increased with little or no increase in
SGPT.
 7. Lactate Dehydrogenase (LDH)

• It catalyzes the reversible interconversion

of lactate and pyruvate.
• It is widely distributed with high
concentrations in the heart, skeletal
muscle, liver, kidney, brain and
erythrocytes.
• The enzyme is increased in plasma in
myocardial infarction, acute leukemias,
generalized carcinomatosis and in acute
hepatitis.
• Estimation of it isoenzymes is more
useful in clinical diagnosis to differentiate
hepatic disease and myocardial
infarction.
 8. Creatine kinase (CK) or
ceratin phosphokinase (CPK)

• CK (CPK) is found in heart muscle brain and

skeletal muscle.
• Measurement of serum creatine
phosphokinase activity is of value in the
diagnosis of disorders affecting skeletal and
cardiac muscle.
• The level of CPK in plasma highly increased in
myocardial infarction.
 Eg. Enzymes Whose Catalytic Activity Is Altered by
Covalent Phosphorylation-Dephosphorylation

• Enzyme Low activity High act

• Acetyl-CoA carboxylase EP E
• Glycogen synthase EP E
• Pyruvate dehydrogenase EP E
• HMG-CoA reductase EP E
• Glycogen phosphorylase E EP
• Citrate lyase E EP
• Phosphorylase b kinase E EP
• HMG-CoA reductase kinase E EP

N:B E, dephosphoenzyme; EP, phosphoenzyme.

Thank You!