Chapter 21
Enzymes and
Vitamins
Chapter 21
Table of Contents
21.1
21.2
21.3
21.4
21.5
21.6
21.7
21.8
21.9
21.10
21.11
21.12
21.13
General Characteristics of Enzymes
Enzyme Structure
Nomenclature and Classification of Enzymes
Models of Enzyme Action
Enzyme Specificity
Factors That Affect Enzyme Activity
Enzyme Inhibition
Regulation of Enzyme Activity
Antibiotics That Inhibit Enzyme Activity
Medical Uses of Enzymes
General Characteristics of Vitamins
Water-Soluble Vitamins
Fat-Soluble Vitamins
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Section 21.1
General Characteristics of Enzymes
• Enzymes are catalysts and are not consumed in the reactions
• Enzymes are proteins that act as a catalyst for biochemical
reactions
• The human body has 1000s of enzymes
• Enzymes are the most effective catalysts known
• Most enzymes are globular proteins
• A few enzymes are now known to be ribonucleic acids (RNA)
• Enzymes undergo all the reactions of proteins including
denaturation
• Enzyme activity is dramatically affected by:
– Alterations in pH
– Temperature
– Other protein denaturants
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Section 21.2
Enzyme Structure
Simple and Conjugated Enzymes
• Enzymes are of two types: simple enzymes and
conjugated enzymes
• Simple enzyme: composed only of protein (amino acid
chains)
• Conjugated enzyme: Has a nonprotein part in addition to
a protein part.
–
–
–
–
Apoenzyme: Protein part of a conjugated enzyme.
A cofactor : Nonprotein part of a conjugated enzyme.
A holoenzyme is the biochemically active conjugated enzyme
Apoenzyme + cofactor = holoenzyme (conjugated enzyme)
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Section 21.2
Enzyme Structure
Cofactors
• Cofactors are important for the chemically reactive
enzymes
• Cofactors are small organic molecules or Inorganic ions
– Organic molecule cofactors: also called as co-enzymes or cosubstrates
– Co-enzymes/co-substrates are derived from dietary vitamins
– Inorganic ion cofactors
– Typical metal ion cofactors - Zn2+, Mg2+, Mn2+, and Fe2+
– Nonmetallic ion cofactor - Cl– Inorganic ion cofactors derived from dietary minerals
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Section 21.3
Nomenclature and Classification of Enzymes
• Nomenclature: Most commonly named with reference to
their function
– Type of reaction catalyzed
– Identity of the substrate
• A substrate is the reactant in an enzyme-catalyzed
reaction:
– The substrate is the substance upon which the
enzyme “acts.”
– E. g., In the fermentation process sugar to be
converted to CO2, therefore in this reaction sugar is
the substrate
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Section 21.3
Nomenclature and Classification of Enzymes
Three Important Aspects of the Naming Process
1. Suffix -ase identifies it as an enzyme
– E.g., urease, sucrase, and lipase are all enzyme designations
– Exception: The suffix -in is still found in the names of some
digestive enzymes, E.g., trypsin, chymotrypsin, and pepsin
2. Type of reaction catalyzed by an enzyme is often used
as a prefix
– E.g., Oxidase - catalyzes an oxidation reaction,
– E.g., Hydrolase - catalyzes a hydrolysis reaction
3. Identity of substrate is often used in addition to the type
of reaction
– E.g. Glucose oxidase, pyruvate carboxylase, and succinate
dehydrogenase
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Section 21.3
Nomenclature and Classification of Enzymes
Practice Exercise
• Predict the function of the following enzymes.
a. Maltase
b. Lactate dehydrogenase
c. Fructose oxidase
d. Maleate isomerase
Answers:
a. Hydrolysis of maltose;
b. Removal of hydrogen from lactate ion;
c. Oxidation of fructose;
d. Rearrangement (isomerization) of maleate ion
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Section 21.3
Nomenclature and Classification of Enzymes
Six Major Classes
•
Enzymes are grouped into six major classes based on the types of
reactions they catalyze
Class
Reaction Catalyzed
1. Oxidoreductases
Oxidation-reductions
2. Transferases
Functional group transfer reactions
3. Hydrolases
Hydrolysis reactions
4. Lyases
Reactions involving addition or removal of groups form
double bonds
5. Isomerase
Isomerisation reactions
6. Ligases
Reactions involving bond formation coupled with ATP
hydrolysis
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Section 21.3
Nomenclature and Classification of Enzymes
Oxidoreductase
• An oxidoreductase enzyme catalyzes an oxidation–
reduction reaction:
– Oxidation and reduction reactions are always linked
to one another
– An oxidoreductase requires a coenzyme that is either
oxidized or reduced as the substrate in the reaction.
– E.g., Lactate dehydrogenase is an oxidoreductase
and the reaction catalyzed is shown below
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Section 21.3
Nomenclature and Classification of Enzymes
Transferase
• A transferase is an enzyme that catalyzes the transfer of
a functional group from one molecule to another
• Two major subtypes:
– Transaminases - catalyze transfer of an amino group
to a substrate
– Kinases - catalyze transfer of a phosphate group
from adenosine triphosphate (ATP) to a substrate
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Section 21.3
Nomenclature and Classification of Enzymes
Hydrolase
• A hydrolase is an enzyme that catalyzes a hydrolysis
reaction
• The reaction involves addition of a water molecule to a
bond to cause bond breakage
• Hydrolysis reactions are central to the process of
digestion:
– Carbohydrases hydrolyze glycosidic bonds in oligoand polysaccharides (see reaction below)
– Proteases effect the breaking of peptide linkages in
proteins,
– Lipases effect the breaking of ester linkages in
triacylglycerols
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Section 21.3
Nomenclature and Classification of Enzymes
Lyase
• A lyase is an enzyme that catalyzes the addition of a
group to a double bond or the removal of a group to form
a double bond in a manner that does not involve
hydrolysis or oxidation
– Dehydratase: effects the removal of the components
of water from a double bond
– Hydratase: effects the addition of the components of
water to a double bonds
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Section 21.3
Nomenclature and Classification of Enzymes
Isomerase, and Ligase
• An isomerase is an enzyme that catalyzes the
isomerization (rearrangement of atoms) reactions.
• A ligase is an enzyme that catalyzes the formation of a
bond between two molecules involving ATP hydrolysis:
– ATP hydrolysis is required because such reactions
are energetically unfavorable
– Require the simultaneous input of energy obtained by
a hydrolysis of ATP to ADP
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Section 21.3
Nomenclature and Classification of Enzymes
Practice Exercise
Answers:
a.Transferase
b.Lyase
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Section 21.4
Models of Enzyme Action
Enzyme Active Site
• The active site:
Relatively small part of
an enzyme’s structure
that is actually involved
in catalysis:
–
–
–
–
Place where substrate
binds to enzyme
Formed due to folding
and bending of the
protein.
Usually a “crevice like”
location in the enzyme
Some enzymes have
more than one active
site
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Section 21.4
Models of Enzyme Action
Enzyme Substrate Complex
• Needed for the activity of enzyme
• Intermediate reaction species formed when
substrate binds with the active site
• Orientation and proximity is favorable and
reaction is fast
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Section 21.4
Models of Enzyme Action
Two Models for Substrate Binding to Enzyme
• Lock-and-Key model:
– Enzyme has a pre-determined shape for the active
site
– Only substrate of specific shape can bind with active
site
• Induced Fit Model:
– Substrate contact with enzyme will change the shape
of the active site
– Allows small change in space to accommodate
substrate (e.g., how a hand fits into a glove)
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Section 21.4
Models of Enzyme Action
Forces That Determine Substrate Binding
• H-bonding
• Hydrophobic interactions
• Electrostatic interactions
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Section 21.5
Enzyme Specificity
• Absolute Specificity:
– An enzyme will catalyze a particular reaction for only one
substrate
– This is most restrictive of all specificities (not common)
– E.g., Urease is an enzyme with absolute specificity
• Stereochemical Specificity:
– An enzyme can distinguish between stereoisomers.
– Chirality is inherent in an active site (amino acids are chiral
compounds)
– L-Amino-acid oxidase - catalyzes reactions of L-amino acids but
not of D-amino acids.
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Section 21.5
Enzyme Specificity
• Group Specificity:
– Involves structurally similar compounds that have the same
functional groups.
– E.g., Carboxypeptidase: Cleaves amino acids one at a time from
the carboxyl end of the peptide chain
• Linkage Specificity:
– Involves a particular type of bond irrespective of the structural
features in the vicinity of the bond
– Considered most general of enzyme specificities
– E.g., Phosphatases: Hydrolyze phosphate–ester bonds in all
types of phosphate esters
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Section 21.6
Factors That Affect Enzyme Activity
Temperature
• Higher temperature results in higher
kinetic energy which causes an
increase in number of reactant
collisions, therefore there is higher
activity.
• Optimum temperature: Temperature
at which the rate of enzyme
catalyzed reaction is maximum
• Optimum temperature for human
enzymes is 37ºC (body temperature)
• Increased temperature (high fever)
leads to decreased enzyme activity
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Section 21.6
Factors That Affect Enzyme Activity
pH
• pH changes affect enzyme
activity
• Drastic changes in pH can result
in denaturation of proteins
• Optimum pH: pH at which
enzyme has maximum activity
• Most enzymes have optimal
activity in the pH range of 7.0 7.5
• Exception: Digestive enzymes
• Pepsin: Optimum pH = 2.0
• Trypsin: Optimum pH = 8.0
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Section 21.6
Factors That Affect Enzyme Activity
Substrate Concentration
• Substrate Concentration: At a
constant enzyme concentration,
the enzyme activity increases with
increased substrate concentration.
• Substrate saturation: the
concentration at which it reaches
its maximum rate and all of the
active sites are full
• Turnover Number: Number of
substrate molecules converted to
product per second per enzyme
molecule under conditions of
optimum temperature and pH
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Section 21.6
Factors That Affect Enzyme Activity
Enzyme Concentration
• Enzyme Concentration:
• Enzymes are not consumed
in the reactions they
catalyze
• At a constant substrate
concentration, enzyme
activity increases with
increase in enzyme
concentration
– The greater the enzyme
concentration, the greater
the reaction rate.
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Section 21.6
Factors That Affect Enzyme Activity
Practice Exercise
• Describe the effect that each of the following changes
would have on the rate of a reaction that involves the
substrate sucrose and the intestinal enzyme sucrase.
a.
b.
c.
d.
Decreasing the sucrase concentration
Increasing the sucrose concentration
Lowering the temperature to 10ºC
Raising the pH from 6.0 to 8.0 when the optimum pH is 6.2
Answers:
a. Decrease rate
b. Increase rate
c. Decrease rate
d. Decrease rate
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26
Section 21.7
Enzyme Inhibition
• Enzyme Inhibitor: a substance that slows down or stops
the normal catalytic function of an enzyme by binding to
it.
• Competitive Inhibitors: Compete with the substrate for
the same active site
– Will have similar charge & shape
– Noncompetitive Inhibitors: Do not compete with the
substrate for the same active site
– Binds to the enzyme at a location other than active
site
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Section 21.7
Enzyme Inhibition
Reversible Competitive Inhibition
• A competitive enzyme inhibitor: resembles an enzyme
substrate in shape and charge
• Binds reversibly to an enzyme active site and the
inhibitor remains unchanged (no reaction occurs)
• The enzyme - inhibitor complex formation is via weak
interactions (hydrogen bonds, etc.).
• Competitive inhibition can be reduced by simply
increasing the concentration of the substrate.
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Section 21.7
Enzyme Inhibition
Reversible Noncompetitive Inhibition
• A noncompetitive enzyme inhibitor decreases enzyme
activity by binding to a site on an enzyme other than the
active site.
• Causes a change in the structure of the enzyme and
prevents enzyme activity.
• Increasing the concentration of substrate does not
completely overcome inhibition.
• Examples: Heavy metal ions Pb2+, Ag+, and Hg2+.
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Section 21.7
Enzyme Inhibition
Irreversible Inhibition
• An irreversible enzyme inhibitor inactivates enzymes by
forming a strong covalent bond with the enzyme’s active
site.
– The structure is not similar to enzyme’s normal
substrate
– The inhibitor bonds strongly and increasing substrate
concentration does not reverse the inhibition process
– Enzyme is permanently inactivated.
– E.g., Chemical warfare agents (nerve gases) and
organophosphate insecticides
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Section 21.8
Regulation of Enzyme Activity
• Cellular processes continually produces large amounts
of an enzyme and plentiful amounts of products if the
processes are not regulated.
• General mechanisms involved in regulation:
– Proteolytic enzymes and zymogenscovalent
modification of enzymes
– Feedback control Regulation of enzyme activity by
various substances produced within a cell
– The enzymes regulated are allosteric enzymes
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Section 21.8
Regulation of Enzyme Activity
Properties of Allosteric Enzymes
•
•
•
•
All allosteric enzymes have quarternary structure:
– Composed of two or more protein chains
Have at least two of binding sites:
– Substrate and regulator binding site
Active and regulatory binding sites are distinct from each other:
– Located independent of each other
– Shapes of the sites (electronic geometry) are different
Binding of molecules at the regulatory site causes changes in the
overall three dimensional structure of the enzyme:
– Change in three dimensional structure of the enzyme leads to
change in enzyme activity
– Some regulators increase enzyme activity – activators
– Some regulators decrease enzyme activity - inhibitors
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Section 21.8
Regulation of Enzyme Activity
Feedback Control
• Feedback Control: A process in which activation or
inhibition of the first reaction in a reaction sequence is
controlled by a product of the reaction sequence.
• Regulators of a particular allosteric enzyme may be:
– Products of entirely different pathways of reaction
within the cell
– compounds produced outside the cell (hormones)
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Section 21.8
Regulation of Enzyme Activity
Proteolytic Enzymes and Zymogens
• 2nd mechanism of regulating enzyme activity:
– Production of enzymes in an inactive forms (zymogens)
– Zymogens are “turned on” at the appropriate time and place
– Example: proteolytic enzymes: Most digestive and blood-clotting
enzymes are proteolytic enzymes
– Hydrolyze peptide bonds in proteins
– Proteolytic enzymes are generated in an inactive form and then
converted to their active form
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Section 21.8
Regulation of Enzyme Activity
Covalent Modification of Enzymes
• 3rd Mechanism for regulation of enzyme activity
• Covalent modification: A process in which enzyme activity is altered
by covalently modifying the structure of the enzyme:
– Involves adding or removing a group from an enzyme
• Most common covalent modification: addition and removal of
phosphate group:
– Phosphate group is often derived from an ATP molecule.
– Addition of the phosphate (phosphorylation) catalyzed by a
Kinase enzyme
– Removal of the phosphate group (dephosphorylation) catalyzed
by a phosphatase enzyme.
– Phosphate group is added to (or removed from) the R group of a
serine, tyrosine, or threonine amino acid residue in the enzyme
regulated.
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Section 21.9
Antibiotics That Inhibit Enzyme Activity
• An anitibiotic is a substance that kills bacteria or inhibits
their growth
• Antibiotics usually inhibit specific enzymes essential to
life processes of bacteria
• Two families of antibiotics considered in this discussion
are sulfa drugs and penicillins
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Section 21.9
Antibiotics That Inhibit Enzyme Activity
Sulfa Drugs
• Many derivatives of sulfanilamide collectively called sulfa
drugs exhibit antibiotic activities
• Sulfanilamide is structurally similar to PABA (paminobenzoic acid)
• Many bacteria need PABA to produce coenzyme, folic
acid
• Sulfanilamide is a competitive inhibitor of enzymes
responsible for converting PABA to folic acid in bacteria
• Folic acid deficiency retards bacterial growth and that
eventually kills them
• Sulfa drugs don’t affect humans because we absorb folic
acid from our diet
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Section 21.9
Antibiotics That Inhibit Enzyme Activity
Penicillins
• Accidently discovered by Alexander Fleming in 1928
• Several naturally occurring penicillins and numerous
synthetic derivatives have been produced
• All have structures containing a four-membered Betalactam ring fused with a five-membered thiazolidine ring
• Selectively inhibits transpeptidase by covalent
modification of serine residue
• Transpeptidase catalyzes the formation of peptide cross
links between polysaccharides strands in bacterial cell
walls
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Section 21.9
Antibiotics That Inhibit Enzyme Activity
Cipro
• The antibiotic ciprofloxacin hydrochloride (Cipro for
short)
• Considered the best broad-spectrum antibiotics because
it is effective against skin and bone infections as well as
against infections involving the urinary, gastrointestinal,
and respiratory systems
• It is the drug of choice for treatment of traveler’s diarrhea
• Bacteria are slow to acquire resistance to Cipro.
– Biochemical threats associated with terrorism has
thrust Cipro into the spotlight because it is effective
against anthrax.
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Section 21.10
Medical Uses of Enzymes
• Diagnose certain diseases:
– Enzymes produced in certain organ/tissues if found in
blood may indicate certain damage to that
organ/tissue
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Section 21.11
General Characteristics of Vitamins
•
•
•
•
•
•
•
•
•
•
•
Organic compounds
Must be obtained from dietary sources
Human body can’t synthesize in enough amounts
Essential for proper functioning of the body
Needed in micro and milligram quantities
1 Gram of vitamin B is sufficient for 500,000 people
Enough vitamin can be obtained from balanced diet
Supplemental vitamins may be needed after illness
Many enzymes contain vitamins as part of their structures - conjugated
enzymes
Two Classes
– Water Soluble and Fat Soluable
Synthetic and natural vitamins are same
– 13 Known vitamins
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Section 21.12
Water-Soluble Vitamins
Vitamin C
•
•
•
•
•
Humans, monkeys, apes and guinea pigs need dietary vitamins
Co-substrate in the formation of structural protein collagen
Involved in metabolism of certain amino acids
100 mg/day saturates all body tissues - Excess vitamin is excreted
RDA (mg/day):
– Great Britain: 30
– United States and Canada: 60
– Germany: 75
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Section 21.12
Water-Soluble Vitamins
Vitamin B
•
•
•
•
•
•
•
•
•
•
•
The preferred and alternative names for the B vitamins
Thiamin (vitamin B1)
Riboflavin (vitamin B2)
Niacin (nicotinic acid, nicotinamide, vitamin B3)
Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine)
Folate (folic acid)
Vitamin B12 (cobalamin)
Pantothenic acid (vitamin B5)
Biotin
Exhibit structural diversity
Major function: B Vitamins are components of coenzymes
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Section 21.13
Fat-Soluble Vitamins
Vitamins A, D, E, K
• Involved in plasma membrane processes
• More hydrocarbon like with fewer functional groups
• Vitamin A
– Has role in vision - only 1/1000 of vitamin A is in retina
– 3 Forms of vitamin A are active in the body
– Derived from b-carotine
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Section 21.13
Fat-Soluble Vitamins
Functions of Vitamin A
• Vision: In the eye- vitamin A combines with opsin protein to form the
visual pigment rhodopsin which further converts light energy into
nerve impulses that are sent to the brain.
• Regulating Cell Differentiation - process in which immature cells
change to specialized cells with function.
– Examples: Differentiation of bone marrow cells white blood cells
and red blood cells.
• Maintenance of the healthy of epithelial tissues via epithelial tissue
differentiation.
– Lack of vitamin A causes such surfaces to become drier and
harder than normal.
• Reproduction and Growth: In men, vitamin A participates in sperm
development. In women, normal fetal development during
pregnancy requires vitamin A.
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Section 21.13
Fat-Soluble Vitamins
Vitamin D
• Two forms active in the body: Vitamin D2 and D3
• Sunshine Vitamin: Synthesized by UV light from sun
• It controls correct ratio of Ca and P for bone
mineralization (hardening)
• As a hormone it promotes Ca and P absorption in
intestine
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Section 21.13
Fat-Soluble Vitamins
Vitamin E
• Four forms of Vitamin Es: a-, b-, g- and d-Vitamin E
• Alpha-tocopherol is the most active biological active
form of Vitamin E
• Peanut oils, green and leafy vegetables and whole
grain products are the sources of vitamin E
• Primary function: Antioxidant – protects against
oxidation of other compounds
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Section 21.13
Fat-Soluble Vitamins
Vitamin K
•
•
•
•
Two major forms; K1 and K2
K1 found in dark green, leafy vegetables
K2 is synthesized by bacteria that grow in colon
Dietary need supply: ~1/2 synthesized by bacteria and
1/2 obtained from diet
• Active in the formation of proteins involved in regulating
blood clotting
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