GENERAL SCIENCE

11: Biochemistry - Macromolecules

Engr. Irene R. Billones-Flores, RCh, RChE, LPT

Carbohydrates
Carbohydrates are
• a major source of energy from our diet.
• made from the elements carbon, hydrogen, and oxygen.
• also called saccharides, which means “sugars.”
Types of Carbohydrates
The types of carbohydrates are
• monosaccharides, the simplest carbohydrates;
• disaccharides, which
consist of two
monosaccharides; and
• polysaccharides,
which contain many
monosaccharides.
Monosaccharides
• contain several hydroxyl groups attached to a chain of three to
eight carbon atoms.
• that contain an aldehyde group are classified as aldoses.
• that contain a ketone group are
classified as ketoses.
• have hydroxyl groups on all carbons
except the carbonyl carbon.
Types of Monosaccharides
Monosaccharides are also classified by the number of carbon
atoms present.
triose (three C atoms) tetrose (four C atoms)
pentose (five C atoms) hexose (six C atoms)

An aldopentose is a five-carbon saccharide with an aldehyde

group.
A ketohexose is a six-carbon saccharide with a ketone group.
Types of Monosaccharides
Fisher Projections
A Fischer projection, used to represent carbohydrates,
• places the aldehyde group (most oxidized) at the top.
• shows the — H and — OH groups on the horizontal intersecting
line.
• places the achiral
— CH2OH group at
the bottom.
D and L Notations
In a Fischer projection, the — OH group on the
• chiral carbon farthest from
the carbonyl group
determines an L or D isomer.
• left is assigned the letter L.
• right is assigned the letter D.
Important Monosaccharides
• D-Glucose, D-galactose, and D-fructose are the most important
monosaccharides. They are all hexoses with the molecular
formula C6H12O6 and are isomers of each other.
D-Glucose
D-Glucose
• is the most common hexose.
• is found in fruits, vegetables, corn syrup, and honey.
• is also known as dextrose and blood sugar in the body.
• is a building block of the disaccharides sucrose and lactose and
of polysaccharides such as cellulose and glycogen.
D-Galactose
D-galactose, an aldohexose with the formula C6H12O6,
• is obtained from the disaccharide lactose, found in milk.
• is important in the cellular membranes of the brain and
nervous system.
In a condition called galactosemia,
• the enzyme needed to convert D-galactose to
D-glucose is missing.
• galactose accumulates in the blood and tissue, which
can lead to cataracts, mental retardation, failure to thrive,
and liver disease.
D-Fructose
D-Fructose
• is a ketohexose with the formula C6H12O6.
• is the sweetest of the carbohydrates, twice as sweet as sucrose
(table sugar).
• is obtained as one of the hydrolysis products of sucrose.

High-fructose corn syrup (HFCS) is a sweetener that is produced

by using an enzyme to break down sucrose to glucose and
fructose.
Practice Question
Identify the following structure of a monosaccharide as D- or L-
ribulose.
Hyperglycemia and Hypoglycemia
In the body,
• glucose has a normal blood level of 70–90 mg/dL.
• a glucose tolerance test measures blood glucose for several
hours after ingesting glucose.
Hyperglycemia and Hypoglycemia
Diabetes mellitus can cause hyperglycemia, which
• occurs when the pancreas is unable to produce sufficient
quantities of insulin.
• allows glucose levels in the body fluids to rise as high as 350
mg/dL of plasma.
Symptoms of diabetes include
• thirst and excessive urination.
• increased appetite and weight loss.
In older adults, diabetes is sometimes a consequence of
excessive weight gain.
Hyperglycemia and Hypoglycemia
When a person is hypoglycemic,
• the blood glucose level rises and then decreases rapidly to
levels as low as 40 mg/dL.
• low blood sugar may occur as a result of an overproduction of
insulin by the pancreas.
• symptoms may appear, such as dizziness, general weakness,
and muscle tremors.
• a diet may be prescribed that consists of several small meals
high in protein and low in carbohydrates.
Hayworth Structures
• The most stable forms of pentose and hexose
sugars are five- or six-atom rings.
• Haworth structures are produced from the
reaction of a carbonyl group and a hydroxyl
group in the same molecule.
Drawing Hayworth Structures
Turn the Fischer projection clockwise by 90°. The — H and
— OH groups on the
• right of the vertical carbon
chain are below the
horizontal carbon chain.
• left of the open chain are
above the horizontal carbon
chain.
Drawing Hayworth Structures
Fold the horizontal carbon chain into a hexagon and bond the O
on carbon 5 to carbon 1. Carbons 2 and 3 form the base of the
hexagon, as the remaining carbons move upward.
Drawing Hayworth Structures
Draw the — OH group on carbon 1 below
the ring to give the α anomer or above
the ring to give the β anomer.
Mutarotation α- and β-D-Glucose
When placed in solution,
• cyclic structures open and close.
• α-D-glucose converts to β-D-glucose and vice versa.
• at any time, only a small amount of open chain forms.
Oxidation of Monosaccharides
Sugar acids
• are produced from the oxidation of the aldehyde as Cu2+ is
reduced to Cu+.
• are named by replacing the ose ending of the monosaccharide
with onic acid.

A carbohydrate (such as the open-chain form of D-glucose) that

reduces another substance is called a reducing sugar.
Reduction of Monosaccharides
The reduction of the carbonyl group in monosaccharides
• converts an aldehyde group to alcohol producing sugar
alcohols, which are also called alditols.
• converts D-glucose to the sugar
alcohol D-sorbitol.
Reducing Sugars
The sugar alcohols
• are named by replacing the ose
ending of the monosaccharide
with itol.
• include D-sorbitol, D-xylitol from
D-xylose, and D-mannitol from D-
mannose.
• are used as sweeteners in
many sugar-free products such
as diet drinks and sugarless
gum.
Disaccharides
A disaccharide
• consists of two monosaccharides linked together.
• is formed when two monosaccharides combine in a dehydration
reaction.
Monosaccharides Disaccharide
glucose + glucose maltose + H2O
glucose + galactose lactose + H2O
glucose + fructose sucrose + H2O

The most common disaccharides are maltose, lactose, and sucrose.

Maltose
Maltose is
• a disaccharide also known as malt sugar.
• composed of two D-glucose molecules.
• obtained from the hydrolysis of starch.
• used in cereals, candies, and brewing.
• found in both the α and β forms.
Lactose
Lactose
• is a disaccharide found in milk and
milk products.
• makes up 6–8% of human milk and
about 4–5% of cow’s milk.
Sucrose
Sucrose, or table sugar,
• consists of α-D-glucose and β-D-fructose.
• has an α,β-(1 2)-glycosidic bond between carbon 1 of glucose
and carbon 2 of fructose.
• form an open chain or be oxidized.
• react with Benedict’s reagent and is not a reducing sugar.

The sugar we use to sweeten our cereal, coffee, or tea is

sucrose. Most of the sucrose for table sugar comes from sugar
cane (20% by mass) or sugar beets (15% by mass).
Polysaccharides
Polysaccharides
• are formed when many monosaccharides
are joined together.
• include amylose, amylopectin, cellulose,
and glycogen, which are polymers of D-
glucose and differ by branching and types
of glycosidic bonds.
Starch
Starch is
• a storage form of glucose in plants, found as insoluble granules
in rice, wheat, potatoes, beans, and cereals.
• composed of two kinds of polysaccharides: amylose and
amylopectin.
Amylose
Amylose, which makes up about 20% of starch, consists of 250
to 4000 α-D-glucose molecules connected by α-(1 → 4)-glycosidic
bonds in a continuous chain. Polymer chains of amylose are
coiled in a helical fashion.
Amylopectin
Amylopectin
• makes up as much as 80% of starch.
• is a branched-chain polysaccharide.
• contains glucose molecules connected by α-(1 → 4)- and α-(1
→ 6)-glycosidic bonds.

Starches hydrolyze easily in water and acid to give smaller

saccharides, called dextrins, which then hydrolyze to maltose
and finally glucose.
Starch
In our bodies, these complex carbohydrates
• are digested by the enzymes amylase in saliva and maltase in
the intestines.
• provide about 50% of our nutritional calories from the glucose
obtained in digestion.
Animal Starch: Glycogen
Glycogen is
• a polymer of glucose that is stored in the liver and muscle of
animals.
• hydrolyzed in our cells at a rate that maintains the blood level of
glucose and provides energy between meals.
• similar to amylopectin but more highly branched.

The glucose units in glycogen are joined by α-(1 → 4)-glycosidic

bonds, with branches attached by α-(1 → 6)-glycosidic bonds
that occur every 10–15 glucose units.
Cellulose
Cellulose, the major structural unit of wood and plants,
• is a polysaccharide of glucose units in unbranched chains with
β-(1 → 4)-glycosidic bonds.
• cannot form hydrogen bonds with water, making it insoluble in
water.
• gives a rigid structure to the cell walls in wood and fiber.
• is more resistant to hydrolysis than are the starches.
• cannot be digested by humans because humans cannot break
down β-(1 → 4)-glycosidic bonds.
Cellulose
• The polysaccharide
cellulose is
composed of
glucose units
connected by β(1
→ 4)-glycosidic
bonds.
Lipids
• biomolecules that contain fatty acids
or a steroid nucleus.
• soluble in organic solvents but not in
water.
• named for the Greek word lipos,
which means “fat.”
• an important feature in cell
membranes, fat-soluble vitamins,
and steroid hormones.
Types of Lipids
Lipids are characterized by their structures:

Lipids such as
• waxes,
• triacylglycerols,
• glycerophospholipids, and
• sphingolipids
are esters that can be hydrolyzed to give fatty acids and other
molecules.
Lipids Containing Glycerol
• Triacylglycerols and glycerophospholipids contain the alcohol
glycerol.
Lipids Containing Sphingosine
• Sphingolipids contain the amino alcohol sphingosine.
Steroids
• Steroids, which have a completely different structure, do not
contain fatty acids and cannot be hydrolyzed. They are
characterized by the steroid nucleus of four fused carbon rings.
General Structure of Lipids
Fatty Acids
Fatty acids, the simplest type of lipids, are
• long, unbranched carbon chains with a carboxylic acid group at
the end.
• typically 12–18 carbon atoms long.
• insoluble in water because of the long carbon chain.
• saturated when they do not contain C = C double bonds in the
carbon chain.
• unsaturated when they contain C = C double bonds in the
carbon chain.
Saturated Fatty Acids
• Fatty acids can be saturated, with only C — C single bonds in
the carbon chain.
Monounsaturated Fatty Acids
• Fatty acids can be monounsaturated, with only one double
C = C bond in the carbon chain.
Polyunsaturated Fatty Acids
Fatty acids can be polyunsaturated, with at least two double
C = C bonds in the carbon chain.
Cis and Trans Unsaturated Fatty Acids
• Unsaturated fatty acids can be drawn as cis and trans isomers.
• Oleic acid is a monounsaturated fatty acid found in olives, with
one double bond at carbon 9.
• The trans isomer of oleic acid is called elaidic acid and has a
straight chain without
a kink.
• Almost all naturally
occurring unsaturated
fatty acids have one
or more cis double
bonds.
Essential Fatty Acids
Humans
• are capable of synthesizing some fatty acids from
carbohydrates or other fatty acids.
• cannot synthesize sufficient amounts of polyunsaturated fatty
acids such as linoleic acid, linolenic acid, and arachidonic acid.

Because these polyunsaturated fatty acids must be obtained

from the diet, they are known as essential fatty acids.
Properties of Saturated Fatty Acids
Saturated fatty acids
• contain only single C — C bonds and fit close together in a
regular pattern, with strong dispersion forces between carbon
chains.
• require a significant amount of energy and high temperatures to
separate and melt.
Properties of Saturated Fatty Acids
• In unsaturated fatty acids, cis
double bonds cause the carbon
chain to bend or kink, giving the
molecules an irregular shape
and thus allowing fewer
interactions between
molecules.
• The reduced interactions in
fatty acids with cis bonds
reduce the melting point of the
molecules.
Waxes
Waxes are found in plants and animals. They
• are found on the surface of fruits and on the leaves and stems
of plants, where they help prevent loss of water and damage
from pests.
• are found on the skin, fur, and feathers of animals and provide a
waterproof coating.
Common Waxes
Triacylglycerols
In the body, fatty acids are stored as triacylglycerols
(triglycerides), which are
• esters of glycerol, a trihydroxy alcohol, and fatty acids.
• formed when three hydroxyl groups of glycerol react with the
carboxyl groups of three fatty acids.
• named by changing glycerol to glyceryl and naming the fatty
acids as carboxylates.
Triacylglycerols
• The general formula of a triacylglycerol consists of three
hydroxyl groups of glycerol forming ester bonds with the
carboxyl groups of three fatty acids.
• Glycerol and three molecules of stearic acid form a
triacylglycerol. In
the name, glycerol
is named glyceryl
and the fatty acids
are named as
carboxylates.
Triacylglycerols
Triacylglycerols are the major form of energy storage for animals.
• Animals that hibernate eat large quantities of plants, seeds, and
nuts that are high in calories.
• As the external temperature drops, the animal goes into
hibernation and the body temperature drops to nearly freezing,
reducing cellular activity, respiration, and heart rate.
Fats and Oils
A fat
• is usually solid at room temperature.
• usually comes from animal sources such as meat, whole milk,
butter, and cheese.
An oil
• is usually liquid at room temperature.
• is usually obtained from a plant source such as corn and olive.
Fats and Oils
Oils from
• olive and peanut are monounsaturated because they contain
large amounts of oleic acid.
• corn, cottonseed, safflower seed, and sunflower seed are
polyunsaturated because they contain large amounts of fatty
acids with two or more double bonds.
• palm and coconut are solids at room temperature because they
consist mostly of saturated fatty acids.
Saturated and Unsaturated Melting Points
Saturated fatty acids
• have higher melting points than do unsaturated fatty acids
because they pack together more tightly.
• are usually found in animal fats, coconut oil, and palm oil.
Saponification and Soap
Saponification
• is the reaction of a fat with a strong base such as NaOH in the
presence of heat.
• splits triacylglycerols into glycerol and the sodium salts of fatty
acids.
• is the process of forming “soaps” (salts of fatty acids).
• gives solid soaps that can be molded into different shapes when
mixed with NaOH.
• gives softer, liquid soaps when mixed with KOH.
Saponification and Soap
Phospholipids
Phospholipids are a family
of lipids similar in structure to
triacylglycerols; they include
glycerophospholipids and
sphingomyelin.
Glycerophospholipids
Glycerophospholipids contain
• two fatty acids that form ester
bonds with the first and second
hydroxyl groups of glycerol.
• a hydroxyl group that forms an
ester with phosphoric acid, which
forms another phosphoester bond
with an amino alcohol.
Sphingomyelin
Sphingomyelin
• contains sphingosine instead of
glycerol.
• contains a fatty acid, phosphate,
and an amino alcohol.
Amino Alcohols
Amino alcohols found in glycerophospholipids
• are choline, serine, and ethanolamine.
• are ionized at physiological pH of 7.4.
Lecithin and Cephalin
Lecithin and cephalin are two types of glycerophospholipids
that are
• abundant in brain
and nerve tissues.
• found in egg yolk,
wheat germ, and
yeast.
Steroids
All steroids contain a steroid nucleus, which consists of
• three cyclohexane rings and one cylopentane ring, fused together.
• rings designated as A, B, C, and D.
• numbered carbon atoms beginning in ring A.
• two methyl groups at positions 18 and 19.
Cholesterol
Cholesterol
• is the most important and abundant
steroid in the body.
• has a hydroxyl group (— OH)
on carbon 3.
• has a double bond between carbons 5
and 6.
• has methyl groups on carbons
10 and 13.
• has an alkyl chain at carbon 17.
Cholesterol in the Body
Cholesterol
• is obtained from meats, milk, and eggs.
• is synthesized in the liver.
• is needed for cell membranes, brain and nerve
tissue, steroid hormones, and vitamin D.
• clogs arteries when high levels form plaque.

Cross-sections of arteries show how plaque clogs

the arteries.
Cholesterol in Foods
Cholesterol
• is considered elevated if plasma cholesterol exceeds
200 mg/dL.
• is synthesized in the liver and obtained from foods.
A diet that is low in foods containing cholesterol and saturated fats
appears to be helpful in reducing the serum cholesterol level.

The American Institute for Cancer Research recommends that we limit

our intake of foods high in cholesterol, such as eggs, nuts, french
fries, fatty or organ meats, cheeses, butter, and coconut oil.
Lipoproteins
Lipoproteins
• surround nonpolar lipids
with polar lipids and
protein for transport
to cells.
• are soluble in
water because the surface
consists of polar lipids.
Cell Membrane
Cell membranes
• are semipermeable so that nutrients can enter the cell and
waste products can leave.
• separate cellular contents from the external environment.
• consist of a lipid bilayer made of two rows of phospholipids.
• have an inner portion made of the nonpolar tails of
phospholipids, with the polar heads at the outer and inner
surfaces.
Lipid Bilayer
The lipid bilayer
• contains phospholipids with unsaturated fatty acids with kinks in
the carbon chains.
• contains proteins, carbohydrates, and cholesterol molecules.
• is not a rigid, fixed structure but one that is dynamic and fluid-
like.
• has proteins and carbohydrates on the surface that
communicate with hormones and neurotransmitters.
Proteins
Proteins
• in the body are polymers made from 20 different amino acids.
• differ in characteristics and functions that depend on the order
of amino acids that make up the protein.
• form structural components such as cartilage, muscles, hair,
and nails.
• function as enzymes to regulate biological reactions such as
digestion and cellular metabolism.
• including hemoglobin and myoglobin transport oxygen in the
blood.
Structural Classification of Proteins and Functions
Amino Acids
Amino acids, the molecular building blocks of proteins, have a
central carbon atom called the α carbon, bonded to
• two functional groups: an ammonium group (— NH3+) and a
carboxylate group (— COO−)
• a hydrogen atom and an R
group or side chain in addition
to the carboxylate and
ammonium groups.
Zwitterions
At physiological pH, the
• ionized ammonium and carboxylate groups give an amino acid
a balance of positive and negative charge, which gives an
overall zero charge.
• neutral amino acid, which is
called a zwitterion, occurs
at a specific pH value known
as its isoelectric point (pI).
Classification of Amino Acids
Amino acids are classified using their specific R groups. The
• nonpolar (hydrophobic) amino acids have hydrogen, alkyl, or
aromatic R groups.
• polar amino acids have R groups that interact with water, which
makes them hydrophilic.
• polar neutral amino acids contain an hydroxyl
(— OH), a thiol (— SH), or an amide (— CONH2) R group
• polar acidic amino acids contain a carboxylate
(— COO−) R group.
• polar basic amino acids contain an ammonium
(— NH3+) R group.
Classification of Amino Acids
Nonpolar Amino Acids
• An amino acid is nonpolar when the R group is H, alkyl, or
aromatic.
Polar Amino Acids, Neutral R Groups
• An amino acid is polar when the R group is an alcohol, a thiol,
or an amide.
Amino Acids, Charged R Groups
• An amino acid is charged when the R group is a carboxylate (—
COO−) or an ammonium (— NH3+) group.
Amino Acid Abbreviations
Amino acids have
• three-letter abbreviations derived from their names, given in Table
19.2.
• one-letter abbreviations to allow the faster transfer of data.

Of the 20 amino acids,

• 11 have one-letter abbreviations that are the same as the first letter
in their names and 9 use different letters.
• arginine R and tyrosine Y use the second letters in their names;
phenylalanine is F; and lysine is K, which is near L in the alphabet.
Amino Acid Stereoisomers
Amino acids can exist as D and L stereoisomers called
enantiomers. We draw Fischer projections for α-amino acids by
placing
• the carboxylate group at the top.
• the R group at the bottom.
• the — NH3+ group of the L isomer
on the left.
• the — NH3+ group of the D isomer
on the right.
Essential Amino Acids
Of the 20 amino acids used to build the proteins in the body,
• only 11 can be synthesized in the body;
• the other 9 are essential amino acids that must be obtained
from the proteins in the diet.
Formation of Peptides
The linking of two or more amino acids by peptide bonds forms a
peptide.

Peptides formed from

• two amino acids are called dipeptides.
• three amino acids are called tripeptides.
• four amino acids are called tetrapeptides.
• five amino acids are called pentapeptides
• longer chains of amino acids are called polypeptides.
Formation of Peptides
A peptide bond
• is an amide bond.
• forms between the — COO− group of one amino acid and the —
NH3+ group of the next amino acid.
Naming Peptides
The name of a peptide
• begins with the name of the N-terminal amino acid residue.
• gives all the following amino acid residues up to the
C-terminal amino acid in which the ine or ate endings are
replaced with yl.
• retains the complete name of the C-terminal amino acid.
A tripeptide consisting of alanine, glycine, and serine residues is
named as
alanylglycylserine
Primary Structure of Proteins
A protein is a polypeptide of 50 or more amino acids with
biological activity.
The primary structure of a protein is the particular sequence of
amino acids held together by peptide bonds.
Insulin
Insulin
• was the first protein to have its primary structure
determined.
• has a primary structure of two polypeptide chains
linked by disulfide bonds.
• has a chain A with 21 amino acids and a chain B
with 30 amino acids.
Secondary Structure of Proteins
The secondary structure of a
protein describes the structure
that forms when amino acids form
hydrogen bonds between the
atoms in the backbone and atoms
on the same or another peptide
chain.
• α-helix
• β-pleated sheet
Tertiary and Quaternary Structure of Proteins
• The tertiary structure of a protein is an overall three-
dimensional shape formed by the interactions and repulsions of
amino acid residues in different parts of the chain.
• Biologically active proteins with two or more polypeptide chains
or subunits have a quaternary structure.
Protein Structures
Denaturation of Proteins
Denaturation of a protein
• occurs when a change disrupts the interactions between
residues that stabilize the secondary, tertiary, or quaternary
structure.
• does not affect the amide bonds between amino acids.
Denaturation of Proteins
The loss of secondary and tertiary structures in a protein occurs when
conditions change, such as
• increasing the temperature.
• making the pH very acidic or basic.
• adding certain organic compounds or heavy metal ions.
• adding mechanical agitation.
When the interactions between the residues are disrupted,
• a globular protein unfolds.
• the tertiary structure is disrupted and the protein is no longer
biologically active.
Enzymes
Enzymes are biological catalysts that
• increase the rate of a reaction by changing
the way a reaction
takes place.
• are not changed in the process of the reaction.
• lower the activation energy of the reaction.
Enzymes
Enzymes increase the rate of a chemical reaction by reducing the
energy required to convert reactant molecules to products.

An enzyme in the blood called carbonic anhydrase catalyzes

• the rapid interconversion of carbon dioxide and water to
bicarbonate and H+.
• the reverse reaction, converting bicarbonate and H+ to carbon
dioxide and water.
Enzymes
Nearly all enzymes
• are globular proteins with a unique three-dimensional shape
that recognizes and binds a small group of reacting molecules,
called substrates.
• have a tertiary structure that includes a region called the active
site where one or more small groups of substrates bind to
create a chemical reaction.
• have specific amino acid residues within the active site that
interact with functional groups of the substrate to form hydrogen
bonds, salt bridges, and hydrophobic interactions.
Enzymes
• Some enzymes show absolute specificity by catalyzing only one
reaction for one specific substrate.
• Other enzymes catalyze a reaction of two or more substrates.
• Some enzymes catalyze a reaction for a specific type
of bond.
Enzymes
• The combination of an enzyme and a substrate forms an
enzyme–substrate (ES) complex.
• The ES provides an alternative pathway for the reaction with
lower activation energy.
• Within the active site, amino acid R groups catalyze the
reaction to form an enzyme-product (EP) complex.
Models of Enzyme Action
• A lock-and-key model has a rigid substrate binding to a rigid
enzyme, much like a key fitting into a lock.
• The induced-fit model, a more dynamic model of enzyme
action, states that the active site is flexible enough to adapt to
the shape of the substrate.
• The induced-fit model has the substrate and enzyme working
together to acquire a geometrical arrangement that lowers the
activation energy.
Naming Enzymes
The name of an enzyme
• usually ends in ase.
• identifies the reacting substance; for example, sucrase
catalyzes the reaction of sucrose.
• describes the function of the enzyme; for example, oxidases
catalyze oxidation.
• can be a common name, particularly for the digestive enzymes,
such as pepsin and trypsin.
Inhibitors
Inhibitors
• are molecules that cause a loss of catalytic activity.
• prevent substrates from fitting into the active sites.
• can be classified as either reversible inhibitors or irreversible
inhibitors.
Reversible Inhibition
Reversible inhibitors
• cause a loss of enzyme activity that can be restored.
• can act in different ways but do not form covalent bonds with
the enzyme.

Reversible inhibition can be competitive or noncompetitive.

• Competitive inhibitors compete for the active site.
• Noncompetitive inhibitors act on another site that is not the
active site.
Competitive Inhibitors
A competitive inhibitor
• has a chemical structure and polarity similar to the substrate.
• competes with the
substrate for the
active site.
• has its effect reversed
by increasing
substrate
concentration.
Noncompetitive Inhibitors
A noncompetitive inhibitor
• has a structure that is much different from that of the substrate.
• does not compete for the active site.
• distorts the shape of the
enzyme, which prevents
the binding of the
substrate at the active site.
• cannot have its effect
reversed by adding more
substrate.
Irreversible Inhibition
In irreversible inhibition, enzyme activity is destroyed when
• the inhibitor covalently bonds with R groups of an amino acid
that may be near the active site.
• the inhibitor changes
the shape of the enzyme,
which prevents the
substrate from entering
the active site.
Cofactors
• A simple enzyme is an active enzyme
that consists only of protein.
• Many enzymes are active only when they
combine with cofactors such as metal
ions or small molecules.
• A coenzyme is a cofactor that is a small
organic molecule such as a vitamin.
Vitamins and Coenzymes
Vitamins
• are organic molecules that are essential for
normal health and growth.
• are required in trace amounts.
• need to be obtained from the diet.
• are grouped into water-soluble vitamins and
fat-soluble vitamins.
Water-Soluble Vitamins
Water-soluble vitamins must come from our food each
day; they
• are soluble in aqueous solutions and cannot be stored in the body.
• are cofactors for many enzymes.
• are excreted in urine each day.
• are easily destroyed by heat, oxygen, and ultraviolet light, so care
must be taken in food preparation.
Many of the water-soluble vitamins are precursors of cofactors
required by many enzymes to carry out certain aspects of catalytic
action.
Fat-Soluble Vitamins
Fat-soluble vitamins
• include A, D, E, and K and are not involved as coenzymes in
catalytic reactions.
• are soluble in lipids but not in aqueous solutions.
• are stored in the body and not eliminated in urine.
• are important in vision, bone formation, antioxidants, and blood
clotting.
Nucleic Acids
There are two types of nucleic acids:
• deoxyribonucleic acid (DNA), which may contain several million
nucleotides, and ribonucleic acid (RNA), which may contain
several thousand nucleotides.
• both are unbranched polymers of repeating monomer units
known as nucleotides.
Each nucleotide has three components:
• a base that contains nitrogen.
• a five-carbon sugar.
• a phosphate group.
Bases in DNA
In DNA,
• the purine bases with double rings are
adenine (A) and guanine (G).
• the pyrimidine bases with single rings
are cytosine (C) and thymine (T).
Bases in RNA
In RNA,
• the purine bases with double rings are
adenine (A) and guanine (G).
• the pyrimidine bases with single rings are
cytosine (C) and uracil (U).
Pentose Sugar
The five-carbon sugar
• in RNA is ribose.
• in DNA is deoxyribose, with no O atom on
C2′.
• has carbon atoms numbered with primes
to distinguish them from the atoms in the
bases.
Nucleosides
A nucleoside
• is composed of a nitrogen-containing base and a sugar, either
ribose or deoxyribose.
• has a base linked by
a β-N-glycosidic bond
to C1′ of a sugar
(ribose or deoxyribose).
Nucleotides
• A nucleotide has a phosphate group attached to the C5′ — OH
group of a nucleoside.
• The addition of a phosphate to a nucleoside forms a nucleotide.
Nucleic Acid Base Sequence
Each nucleic acid has its own unique sequence of bases, which
• is known as its primary structure.
• carries the genetic information.
• is read from the sugar with the free 5ʹ phosphate to the sugar
with the free 3ʹ — OH group.
• is often written using the letters of the bases to represent the
correct sequence.
5ʹ A C G T 3ʹ
DNA, Ratio of Bases
Scientists determined that
• adenine is paired (1:1) with thymine.
• guanine is paired (1:1) with cytosine.

This relationship, known as Chargaff’s rules, can be summarized:

Number of purine molecules = Number of pyrimidine molecules
Adenine (A) = Thymine (T)
Guanine (G) = Cytosine (C)
Complementary Base Pairs: A and T
• DNA contains complementary base pairs in which adenine is
always linked by two hydrogen bonds to thymine (AT).
Complementary Base Pairs: G and C
• DNA contains complementary base pairs in which guanine is
always linked by three hydrogen bonds to cytosine (GC).
DNA Double Helix
The DNA structure is a double helix that
• consists of two strands of nucleotides
that form a double helix structure like a
spiral staircase.
• has two strands held together by the
hydrogen bonds between the bases AT
and GC.
• has bases along one strand that
complement the bases along the other.
Recombinant DNA
DNA can be used in
• genetic engineering that permits scientists to cut and recombine DNA fragments
to form recombinant DNA.
• the identification of a person by examining bands on film that represent DNA
fingerprints.
In preparing recombinant DNA,
• a DNA fragment from one organism is combined with DNA from another.
• restriction enzymes are used to cleave a gene from a foreign DNA and open DNA
plasmids in Escherichia coli.
• DNA fragments are mixed with the plasmids in E. coli and the ends are joined by
ligase.
• the new gene in the altered DNA produces protein.
Recombinant DNA
Recombinant DNA is formed by
placing
a gene from another organism in a
plasmid DNA of a bacterium. This
causes the bacterium to produce a
nonbacterial protein.
Polymerase Chain Reaction
A polymerase chain reaction (PCR)
• made it possible to produce multiple copies of a
DNA in a short time.
• separates the sample DNA strands by heating.
• mixes the separated strands with enzymes and
nucleotides to form complementary strands.
• is repeated many times to produce a large sample of
the DNA.
• Each cycle of the polymerase chain reaction doubles
the number of copies of the DNA section.
Genetic Testing
Polymerase chain reaction (PCR)
• allows screening for defective genes.
• can be used to screen for genes associated with breast cancer.

Multiple defects in two known breast cancer genes, called

BRCA1 and BRCA2, correlate to a higher risk of breast cancer.
DNA Fingerprinting
In DNA fingerprinting,
• restriction enzymes cut a DNA sample into smaller fragments
(RFLPs).
• the sample is placed on a gel and separated using
electrophoresis.
• the banding pattern on the gel is called a DNA fingerprint and is
unique to each individual.
The Human Genome
The Human Genome Project
• was completed in 2003 and showed that our DNA is composed
of 3 billion bases and 21 000 genes coding for protein, which
represents only 3% of the total DNA.
• has since identified stretches of DNA that code for other RNA
molecules.
Much of our DNA
• regulates genes and serves as recognition sites for proteins.
• has been assigned a function leading to understanding errors in
DNA replication, transcription, or regulation.
Viruses
Viruses
• are small particles of DNA or
RNA that require a host cell to
replicate and cause infection.
• can replicate only in cells, by
taking over the machinery and
materials necessary for
protein synthesis and growth.
Viral Infections
A virus causes infection when
• an enzyme in the protein coat of the virus makes a hole in the
outside of the host cell;
• the virus enters the cell and the viral nucleic acid mixes with the
materials in the host cell;
• a protease processes proteins to produce a protein coat
encasing the new viral RNA or DNA; and
• the new virus particles are released from the cell, ready to infect
more cells.
Diseases Caused by Viruses
Reverse Transcription
In reverse transcription,
• a retrovirus, which contains viral RNA but no viral DNA, enters a
cell.
• the viral RNA uses reverse transcriptase to produce a viral DNA
strand.
• the viral DNA strand forms a complementary DNA strand.
• the new DNA uses the nucleotides and enzymes in the host cell
to synthesize new virus particles.
HIV Virus and AIDS
The HIV-1 virus
• is a retrovirus that infects and
destroys T4 lymphocyte cells.
• leaves the immune system unable to
destroy harmful organisms.
• is associated with an
increased chance of
developing pneumonia and
skin cancer associated
with AIDS.
Retroviruses
After a retrovirus injects its viral RNA into a cell, it forms a DNA
strand by reverse transcription.

The single-stranded DNA forms a double-stranded DNA called a

provirus, which is incorporated into the host cell DNA.

When the cell replicates, the provirus produces the viral RNA
needed to produce more virus particles.