Biochemistry

Lecture 3

Łukasz Łopusiewicz PhD DSc Eng.

Associate Professor
School of Medical & Health Sciences
University of Economics and Human Sciences
in Warsaw
1. Primary structure : The linear sequence of amino acids
forming the backbone of proteins (polypeptides).
2. Secondary structure : The spatial arrangement of protein by
twisting of the polypeptide chain.
3. Tertiary structure : The three dimensional structure of a
functional protein.
4. Quaternary structure : Some of the proteins are composed of
two or more polypeptide chains referred to as subunits. The
spatial arrangement of these subunits is known as quaternary
structure.
• Enzymes are important group of biomolecules synthesized by the living cells.
• Enzymes are biological catalysts that speed up the rate of the biochemical reaction.
• Enzymes only accelerate the rate of chemical reaction, but do not initiate them, chemical reactions can take
place without enzymes, but then reactions will be extremely slow. Enzymes are neither consumed nor
permanently altered as a consequence of their participation in a reaction.
• Most enzymes are three dimensional globular proteins (tertiary and quaternary complex3D structure).
• Actions of most enzymes are under strict regulation in a variety of ways.
• Enzymes that catalyze the conversion of one or more compounds (substrates) into one or more different
compounds (products) enhance the rates of the corresponding non-catalyzed reaction by factors of at least 10.
• Some special RNA types also act as enzymes and are called ribozymes
 Enzymes speed up the reaction by lowering the activation
energy of the reaction.
 Their presence does not effect the nature and properties of
end product.
 They are highly specific in their action that is each enzyme
can catalyze one kind of substrate.
 Small amount of enzymes can accelerate chemical reactions.
 Enzymes are sensitive to change in pH, temperature and
substrate concentration.
 Turnover number is defined as the number of substrate
molecules transformed per minute by one enzyme molecule.

Catalase turnover number = 6 x106/min

• Each enzyme has its own tertiary structure and specific
conformation.
• Functional unit is holoenzyme, made up of
1-Apoenzyme (protein part),
2-coenzyme (non protein part)
3-cofactor

Holoenzyme Apoenzyme coenzyme

• Active enzyme protein part non-protein part

cofactor

non-protein part
 Enzymes lower activation energy by forming an
enzyme/substrate complex

Substrate + Enzyme

Enzyme/substrate complex

Enzyme/product complex

Product + Enzyme
• Catalyst
compound that increase the velocity of
chemical reaction without itself
undergoing any change.

• Substrate
compounds on which enzymes act.

• Product
Compound produced by chemical
reaction.

• Active site
are on enzymes where substrate binds.
• Enzymes with only one polypeptide
chain are called monomeric
enzymes

• Those with more than one

polypeptide chain are called
oligomeric enzymes

• Multi-enzyme complex when many

different enzyme catalyzing reaction
sites are located at different site of
the same macro molecule.
Active site
• The active site of an enzyme is the region that binds substrates, co-factors and
prosthetic groups and contains residue that helps to hold the substrate.

• Active sites generally occupy less than 5% of the total surface area of enzyme.

• Active site has a specific shape due to tertiary structure of protein.

• A change in the shape of protein affects the shape of active site and function of the
enzyme.
o Active site can be further divided into:

Active Site

Binding Site Catalytic Site

It chooses the substrate It performs the catalytic

and binds it to active site. action of enzyme.
 Cofactors
Cofactor is the non protein molecule which carries out chemical
reactions that can not be performed by standard 20 amino acids.

Cofactors are of two types:

- Organic cofactors
- Inorganic cofactors
 INORGANIC COFACTORS
o These are the inorganic molecules required for the proper
activity of enzymes.
Examples:
 Enzyme carbonic anhydrase requires Zn2+ for activity.
 Hexokinase has co-factor Mg2+

ORGANIC COFACTORS
o These are the organic molecules required for the proper
activity of enzymes.
Example:
 Glycogen phosphorylase requires the small organic
molecule pyridoxal phosphate.
 TYPES OF ORGANIC COFACTORS
Prosthetic Group Coenzyme

A prosthetic group is a tightly bound organic cofactor e.g.

Flavins, heme groups and biotin.
o A coenzyme is loosely bound organic co-factor. factor e.g.
heme E.g. NAD+
• An enzyme with its cofactor removed it is called a apoenzyme

• The complete complex of a protein with all necessary small

organic molecules, metal ions and other components is
termed as holoenzyme of holoprotein.
 SUBSTRATE

 The reactant in biochemical reaction is termed as substrate.

 When a substrate binds to an enzyme it forms an enzyme-

substrate complex.

Substrate Joins Enzyme

 SITES OF ENZYMES SYNTHESIS

o Enzymes are synthesized by ribosomes which are attached to

the rough endoplasmic reticulum.

o Information for the synthesis of enzyme is carried byDNA.

o Amino acids are bonded together to form specific enzyme

according to the DNA code
INTRACELLULAR AND EXTRACELLULAR ENZYMES
o Intracellular enzymes are synthesized and retained in the cell
for the use of cell itself.
o They are found in the cytoplasm, nucleus, mitochondria and
chloroplast.
Example :
 Oxydoreductase catalyses biological oxidation.
 Enzymes involved in reduction in the mitochondria.

o Extracellular enzymes are synthesized in the cell but

secreted from the cell to work externally.
Example :
 Digestive enzyme produced by the pancreas, are not used by
the cells in the pancreas but are transported to the duodenum.
 ACTIVATION

 Activation is defined as the conversion of an inactive form of an

enzyme to active form which processes the metabolic activity.

TYPES OF ACTIVATION
 Activation by co-factors.
 Conversion of an enzyme precursor.
ACTIVATION BY
COFACTORS
• Many enzymes are activated by cofactors.

• Examples:

• DNA polymerase is a holoenzyme that

catalyzes the polymerization of
deoxyribonucleotide into a DNA strand. It
uses Mg ion for catalytic activity.
• Horse liver dehydrogenase uses Zn ion for
activation
CONVERSION OF AN ENZYME
PRECURSOR

• Specific proteolysis is a common

method of activating enzymes and
other proteins in biological system.

• Example:

• The generation of trypsin from

trypsinogen leads to the activation of
other zymogens.
 REMINDER: SIMILIARITY
INSULIN – PEPTIDE HORMONE
• Insulin has two polypeptide chains. The A chain (Glycine chain) has 21 amino acids and B (Phenylalanine) chain has 30 amino acids.
• They are held together by two interchain disulfide bonds. A chain 7th cysteine and B chain 7th cysteine are connected. Similarly A chain 20th
cysteine and B chain 19th cysteine are connected.
• There is another intrachain disulfide bond between 6th and 11th cysteine residues of A chain.
• Beta cells of pancreas synthesize insulin as a prohormone. Proinsulin is a single polypeptide chain with 86 amino acids. Biologically active
insulin (2 chains) is formed by removal of the central portion of the pro-insulin before release.
MECHANISMS OF ENZYMES ACTION
 Chemical reactions
• Chemical reactions need an initial
input of energy THE ACTIVATION
ENERGY
• During this part of the reaction the
molecules are said to be in a
transition state.
• At this stage energy of activation is
at its peak.
• Enzymes are power full catalysts
and they accelerate reactions
millions of time by reducing the
energy of activation.
• Requirement for catalysis is
formation of enzyme-substrate
complex.
Enzymes
Lower a
Reaction’s
Activation
Energy
• In the early 20th century it was observed
that presence of substrates renders
enzymes more resistant to the
denaturing effects of elevated
temperature
• Emil Fischer in 1894 proposed that
enzymes and substrates interact to form
ENZYME-SUBSTRATE Complex (ES)
whose thermal stability is far better than
the enzyme itself.
• Fischer reasoned that high specificity with which enzyme recognizes and
makes complex with substrate is just like lock and key.
• This enzymatic lock is referred to as ACTIVE SITE
• Lock and key hypothesis assumes the active sites of enzymes are rigid in their
shape
• There is no change in the active site before and after enzymatic action
• In most enzymes the active site takes the form of a cleft or pocket on the
enzyme’s surface
Lock and Key Model
Two substrates

Enzyme

Active site of the enzyme

Lock and Key Model
The substrates fit like a key in a lock

Enzyme

The active site is like a lock

 Lock and Key Model
The activation energy for these substrates to bind
together has been lowered by the enzyme.

Chemical reaction!!!

Enzyme
Active site is due to
tertiary structure of
protein.
Made up of amino
acids (known as
catalytic residue).
As clefts or pockets
occupying a small
region.
 Flexible to promote the specific substrate binding.
• Possessing a substrate binding site and a catalytic site(for
catalysis of specific reaction).
• Co enzymes are present as a part of catalytic site.
• The substrate binds at the active site by weak non-covalent
bonds.
• The active site is much more than simply a binding site
for substrates
• It provides a 3 dimensional environment that both
shields substrates from solvents and facilitates catalysis
• Binds co-factors or prosthetic groups needed for
catalysis
• Within active site substrates are aligned in close proximity and optimal orientation to the
co-factors and prosthetic groups responsible for catalysis and their chemical
transformation into PRODUCTS.

• Product have a different shape and chemical composition then the substrate

• Once formed, they are released from the active site

• Leaving it free to become attached to another substrate

 INDUCED FIT MODEL
 More recent studies have revealed that the process is much
more likely to involve an induced fit model (proposed by Daniel
Koshland in 1958). He proposed that when substrate
approaches and binds to the enzyme they induce a
conformational change which is analogous to placing a hand
(SUBSTRATE) into a glove (ENZYME).
 According to this exposure of an enzyme to substrate cause a
change in enzyme, which causes the active site to change its
shape to allow enzyme and substrate to bind. The enzyme brings
about reciprocal changes in the substrates, by decreasing the
energy of activation to facilitate the transformation of
SUBSTRATES into PRODUCTS
 MECHANISM OF ENZYME ACTION
 The catalytic efficiency of enzymes is explained by two
perspectives:

Thermodynamic Processes at the

changes active site
 THERMODYNAMIC CHANGES
 All chemical reactions have energy barriers between reactants
and products.

 The difference in transitional state and substrate is called

activational barrier.
 ENZYMES LOWER THE ACTIVATION ENERGY OF A REACTION
Energy levels of molecules

Activation energy
of uncatalysed
Initial energy state Activation energy reactions
of substrates of enzyme catalysed
reaction

Final energy state of

products

Progress of reaction (time)

RATES OF REACTION AND THEIR DEPENDENCE ON ACTIVATION ENERGY

Activation Energy (Ea):

“The least amount of energy needed for a chemical reactiontotake place.”
 Enzyme (as a catalyst) acts on substrate in such a way that they lower the
activation energy by changing the route of the reaction.
 The reduction of activation energy (Ea) increases the amount of reactant
molecules that achieve a sufficient level of energy, so that they reach the
activation energy and form the product.
Example:
 Carbonic anhydrase catalyses the hydration of 10⁶ CO₂ molecules per second
which is 10⁷x faster than spontaneous hydration.
 THERMODYNAMIC CHANGES

 Only a few substances cross the activation barrier

and change into products.
 That is why rate of uncatalyzed reactions is much
slow.
 Enzymes provide an alternate pathway for
conversion of substrate into products.
 Enzymes accelerate reaction rates by forming
transitional state having low activational energy.
 Hence, the reaction rate is increased many
folds in the presence of enzymes.
 The total energy of the system remains the
sameand equilibrium state is not disturbed.
 ENZYME UNIT

• The enzyme unit (U) is a unit for the amount of a

particular enzyme.
• One U is defined as the amount of the enzyme that
produces a certain amount of enzymatic activity, that is,
the amount that catalyses the conversion of 1 micro
mole of substrate per minute.
• The conditions also have to be specified: one usually
takes a temperature of 25°C and the pH value and
substrate concentration that yield the maximal substrate
conversion rate.
PROCESSES AT THE ACTIVE SITE

Covalent
catalysis

Acid base
Catalysis catalysis
by strain
Catalysis
by
proximity
 MECHANISM OF CATALYSIS

• 1. CATALYSIS BY PROXIMITY:
For molecules to react they must come within bond
forming distance of one another. The higher their
concentration the more frequently they will encounter
one another, and the greater will be the rate of their
reaction.
 When enzyme binds:
 A region of high substrate concentration is produced at active site.
 This will orient substrate molecules especially in a position
ideal for them.
2. ACID-BASE CATALYSIS:
• The ionizable functional groups of
aminoacyl side chains and of prosthetic
groups present on active site contribute to
catalysis by acting as acids or bases. Acid-
base catalysis can either be SPECIFIC or
GENERAL.

• In specific acid or specific base catalysis,

the rate of reaction will be sensitive to
changes in the concentration of protons but
independent of concentration of other
acids(proton donors) or bases(proton
acceptors) present in solution or active site.
• 3. CATALYSIS BY STRAIN:
• Enzymes that catalyze –lytic (lyases) reactions
that involve breaking a covalent bond, typically
bind their substrates in a conformation slightly
unfavorable for the bond that will undergo
cleavage.
• The resulting strain distorts the targeted bond,
weakening it and making it more vulnerable to
cleavage.
• The enzyme-substrate binding causes
reorientation of the structure of site due to in a
strain condition.
• Transitional state is required and here bond is
unstable and eventually broken.
• In this way bond between substrate is broken
and converted into products.
4 COVALENT CATALYSIS:

• The process of covalent catalysis

involves the formation of a covalent
bond between the enzyme and one or
more substrates.
• The modified enzyme then becomes a
reactant. Enzyme is released
unaltered after completion of reaction
• Covalent catalysis introduces a new
reaction pathway whose activation
energy is low and therefore is faster
than the reaction pathway in
homogenous solution.
 KINETICS OF ENZYMES CATALYSIS

“It is a branch of biochemistry in which the rate of enzyme

catalyzed reactions is studied.

 Kinetic analysis reveals the number and order of the individual

steps by which enzymes transform substrate into products

 Studying an enzyme's kinetics in this way can reveal the

catalytic mechanism of that enzyme, its role in metabolism,
how its activity is controlled, and how a drug or an agonist
might inhibit the enzyme
PHARMACEUTICAL IMPORTANCE
• Enzymes are virtually involved in all physiological processes which makes them the
targets of choice for drugs that cure or ameliorate human disease.

• Applied enzyme kinetics represents the principal tool by which scientist identify and
characterize therapeutic agents that selectively inhibit the rates of specific enzymes
catalyzed processes.

• Enzymes kinetics thus play a critical role in drug discovery as well as elaborating the
mode of action of drugs.
 Factors affecting enzyme activity

The contact between the enzyme and substrate is

the most essential factor for enzyme activity.

1. Enzyme concentration
2. Substrate concentration
3. Temperature
4. Hydrogen ion concentration (pH)
5. Product concentration
6. Presence of activators
7. Time
8. Light and radiation
 Denaturing the protein
ACTIVE SITE CHANGES SHAPE
SO SUBSTRATE NO LONGER FITS

Even if temperature lowered –

enzyme can’t regain its correct shape
 INHIBITION

• INHIBITOR - Any chemical entity (or compound)

which either critically retards or specially blocks the
phenomenon of enzyme catalysis.
• Inhibitors are the chemicals that reduce the rate of
enzymatic reactions.
• They block the enzyme but they do not usually destroy
it.
 Importance of enzyme inhibition
• For understanding the regulation of enzyme
activity within the living cells.
• To elucidate the kinetic mechanism of an
enzyme catalyzing in a multi substrate reaction.
• Useful in elucidating the cellular metabolic
pathways by causing accumulation of
intermediates.
• Identification of the catalytic groups at the
active site.
• Provide information about substrate
specificity of the enzyme.
 Types of inhibitors
1) Reversible inhibitor:
• Inhibitor binds to enzyme reversibly through non
covalent interactions.
• An equilibrium is established between the free
inhibitor and EI complex and is defined by an
equilibrium constant (Ki).

E
+ I E I

• The activity of Enzyme is fully restored on removing

the inhibitor (for example by dialysis).
 TYPES OF REVERSIBLE INHIBITION

o There are four types:

 Competitive inhibition.

 Uncompetitive inhibition.

 Mixed inhibition.

 Non-competitive inhibition.
 a) Competitive inhibitors
• A competitive inhibitor often has structural
features similar to those of the substrate whose
reactions they inhibit.
• This means that a competitive inhibitor and
enzyme’s substrate are in direct competition for
the same binding active site on the enzyme.

Substrate
Enzyme
ES- complex
Products

Competitive inhibitor

Inactive enzyme
Example:

The antibiotic sulfanilamide is similar in structure to para-

aminobenzoic acid (PABA), an intermediate in the
biosynthetic pathway for folic acid. Sulfanilamide can
competitively inhibit the enzyme that has PABA as it's
normal substrate by competitively occupying the active
site of the enzyme.
 b) Non competitive inhibitor
• These are not influenced by the concentration of
the substrate. It inhibits by binding irreversibly to
the enzyme but not at the active site.
• They also bind with the same affinity to the free
enzyme and form the Enzyme-Substrate complex.
• It change the shape of enzyme and active site.
Example:
• Silver ions (heavy metal) react with -SH groups in
the side groups of cysteine residues in the protein
chain:

• If the cysteine residue is somewhere on the protein

chain which affects the way it folds into its tertiary
structure, then altering this group could have an
effect on the shape of the active site, and so stop
the enzyme from working.
 c) Uncompetitive inhibitor
• Uncompetitive inhibitors do not bind to the free enzyme. They bind only
to the enzyme-substrate complex to yield an inactive E. S. I complex.
• In this type of inhibition, inhibitor does not compete with the substrate
for the active site of enzyme instead it binds to another site known as
allosteric site.
• Uncompetitive inhibitors frequently observed in multi substrate
reaction.
• Inhibition can’t be reversed by increasing the [S] since I doesn't
compete with S for the same binding site.

Enzyme
Enzyme

S
I
Enzyme
EXAMPLES OF UNCOMPETITIVE INHIBITION
• Drugs to treat cases of poisoning by methanol or ethylene glycol act as
uncompetitive inhibitors.

• Tetramethylene sulfoxide and 3- butylthiolene 1-oxide are uncompetitive inhibitors

of liver alcohaldehydrogenase.
MIXED • In this type of inhibition both E.I and E.S.I
complexes are formed.

INHIBITION • Both complexes are catalytically inactive.

 2) Irreversible inhibitor:
• Inhibitor binds at or near the active site of the
enzyme irreversibly, usually by covalent bonds, so
it can’t dissociate from the enzyme.
• Irreversible inhibitors combine with the functional
groups of the amino acids in the active site,
irreversibly.
• Irreversible inhibitors occupy or destroy the active
sites of the enzyme permanently and decrease the
reaction rate.
• Enzyme activity is not regained on dialysis.

E I E I
 EXAMPLES OF IRREVERSIBE INHIBITION

 Aspirin which targets and covalently modifies a key enzyme

involved in inflammation is an irreversible inhibitor.

 Action of nerve gas poisons on acetylcholinesterase – an enzyme

that has an important role in the transmission of nerve impulses
 a) Active site directed inhibitor
• Active site directed inhibitor is also called as
affinity label. It is a chemically reactive compound
that is designed to resemble the substrate of an
enzyme so that it binds at the active site and forms
a stable covalent bond with a susceptible group of
the nearby residue in the enzyme protein.

• Affinity labels are very useful for identifying

catalytically important residues.
 b) Suicide inhibitor
• A suicide inhibitor is a relatively inert molecule that
is transformed by an enzyme at its active site into a
reactive compound that irreversibly inactivates the
enzyme
• They are substrate analogs designed so that via
normal catalytic action of the enzyme, a very
reactive group is generated.
• The latter forms a covalent bond with a nearby
functional group within the active site of the
enzyme causing irreversible inhibition.
 TYPES OF INHIBITION

Inhibition

Reversible Irreversible

Competitive Uncompetitive Mixed Non-

competitive
 ENZYME SPECIFICITY
 Enzymes are highly specific in nature, interacting with one or
few substrates and catalyzing only one type of chemical
reaction.
 Substrate specificity is due to complete fitting of active site and
substrate .
Example:
 Oxydoreductase do not catalyze hydrolase reactions and
hydrolase do not catalyze reaction involving oxidation and
reduction.
 Enzymes show different
degrees of specificity:

 Bond specificity.
 Group specificity.
 Absolute specificity.
 Optical or stereo-specificity.
 Dual specificity.
 BOND SPECIFICITY
 In this type, enzyme acts on substrates that are similar in structure and
contain the same type of bond.

Example :
 Amylase which acts on α-1-4 glycosidic, bond in starch dextrin and
glycogen, shows bond specificity.
 GROUP SPECIFICITY
 In this type of specificity, the enzyme is specific not only to the
type of bond but also to the structure surrounding it.
Example:
 Pepsin is an endopeptidase enzyme, that hydrolyzes central
peptide bonds in which the amino group belongs to aromatic
amino acids e. g phenyl alanine, tyrosine and tryptophan.
 SUBSTRATE SPECIFICITY
 In this type of specificity, the enzymes acts only on one
substrate
Example :
 Uricase, acts only on uric acid, shows substrate
specificity.

 Maltase, which acts only on maltose, shows substrate

specificity.
 OPTICAL / STEREO-SPECIFICITY

 In this type of specificity , the enzyme is not specific to

substrate but also to its optical configuration
Example:
 D-amino oxidase acts only on D-amino acids.

 L-amino acid oxidase acts only on L-aminoacids.

 DUAL SPECIFICITY
 There are two types of dual specificity.
 The enzyme may act on one substrate by two different
reaction types.
Example:

 Isocitrate dehydrogenase enzyme acts on isocitrate (one

substrate) by oxidation followed by decarboxylation(two
different reaction types) .
 DUAL SPECIFICITY
 The enzyme may act on two substrates by one reactiontype

Example:
• Xanthine oxidase enzyme acts on xanthine and
hypoxanthine(two substrates) by oxidation (one reaction type)
 Classification on the basis of site of action

a) Endoenzymes- Which act only inside the cell are known as endoenzymes or
intracellular enzymes.
- It involves the synthesis of cell components, food reserves & bioenergetic i.e
liberation of energy from food.
- Ex. Isomerases, Phosphorylases.
b) Exoenzymes- The enzymes which are secreted outside the cell are
known as exoenzymes or extracellular enzymes.
- These are normally digestive in their function.
- They hydrolyse very complex molecules into simple compounds i.e.
proteases, lipases acting on proteins, lipids respectively.
83
c) Constitutive Enzymes: These enzymes are produced in absence of
substrate are known as constitutive enzymes.
- Which produced in constant rate, in constant amounts of metabolic state
of organism.
- These are part of basic & permanent enzymic action of cell.
- Ex. Enzymes of glycolytic series.

d) Induced Enzymes: These are present in trace amounts but their conc. Gets
immediately increased in presence of substrate on which they act.
- Microorganisms produce them in response to the presence of substrate in
the environment only known as induced enzymes.
- Ethanol, barbiturates are powerful in inducing hepatic microsomal enzymes.
 NOMENCLATURE OF ENZYMES
o An enzyme is named according to the name of the substrate it
catalyses.
o Some enzymes were named before a systematic way of
naming enzyme was formed.
Example: pepsin, trypsin and rennin
o By adding suffix -ase at the end of the name of the
substrate, enzymes are named.
o Enzyme for catalyzing the hydrolysis is termed as hydrolase.
Example :

maltase
maltose + water glucose + glucose
 EXAMPLES
substrate enzymes products
lactose lactase glucose + galactose
maltose maltase Glucose
cellulose cellulase Glucose
lipid lipase Glycerol + fatty acid
starch amylase Maltose
protein protease Peptides +
polypeptide
 CLASSIFICATION OF ENZYMES
 A systematic classification of enzymes has been developed by
International Enzyme Commission.

 This classification is based on the type of reactions catalyzed by

enzymes.

 There are six major classes.

 Each class is further divided into sub classes, sub sub-classes

and so on, to describe the huge number of different enzyme-
catalyzed reactions.
ENZYME CLASS REACTION TYPE EXAMPLES
Oxidoreductases Reduction- Lactate
oxidation dehydrogena
(redox) se
Transferases Transfer of chemical Hexokinase
group
Hydrolases Hydrolysis; bond Lysozyme
cleavage with
transfer of
functional group of
water
Lyases Non-hydrolytic Fumarase
bond cleavage
Isomerases Intramolecular Triose
group transfer phosphate
(isomerization) isomerase
Catalyze geometric
or structural
changes within a
molecule
Ligases Synthesis of new RNA polymerase
covalent bond
between
substrates, using
ATPhydrolysis
 • The class, subclass and sub-subclass provide
additional information about the reaction classified.
EC-NUMBER:
Each enzyme is given a 4-digit number:
hexokinase is given 2.7.1.1
2- means class of enzyme transferases
7-means subclass phospho transferase
1-means sub-sub class: phospho transferase with –OH group
acceptor
1-means serial no of acceptors-here phosphate group acceptor.

90
 Isoenzymes

o The enzymes which have multiple molecular forms in the

same organism, catalysing the same biochemical reaction
are called isoenzymes.
Lactate + NAD+ Pyruvate + NADH+ H+
o These reaction is catalysed by enzyme lactate
dehydrogenase which is present in five different molecular
forms in the tissues.
o All have different amino acid composition and sequence.
 Allosteric Enzymes
o These are regulatory enzyme.
o The catalytic activity is regulated by itself.
o The regulation is mediated via specific metabolites.
o These metabolites are called allosteric modulators.
o Allosteric enzymes have allosteric site, in addition to the active site to which
allosteric modulator binds.
o 2 types of allosteric modulators,
o Positive allosteric modulators if they increase the activity of an enzyme.
o Negative allosteric modulators which reduces the activity of an enzyme.