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
21.14
21.15
General Characteristics of Enzymes
Enzyme Structure
Nomenclature and Classification of Enzymes
Models of Enzyme Action
Enzyme Specificity
Factors That Affect Enzyme Activity
Extremozymes
Enzyme Inhibition
Regulation of Enzyme Activity
Prescription Drugs That Inhibit Enzyme Activity
Medical Uses of Enzymes
General Characteristics of Vitamins
Water-Soluble Vitamins: Vitamin C
Water-Soluble Vitamins: The B Vitamins
Fat-Soluble Vitamins
Copyright © Cengage Learning. All rights reserved
2
Section 21.1
General Characteristics of Enzymes
• Enzymes are usually proteins that act as
biological catalysts.
• Each cell in the human body contains
thousands of different enzymes.
• Enzymes cause cellular reactions to occur
millions of times faster than corresponding
uncatalyzed reactions
• An enzyme speeds a reaction by lowering
the activation energy, changing the reaction
pathway that provides a lower energy route
for conversion of substrate to product.
• As catalysts enzymes are not consumed in
the reactions
• A few enzymes are now known to be
ribonucleic acids (RNA)
Copyright © Cengage Learning. All rights reserved
3
Section 21.2
Enzyme Structure
Simple and Conjugated Enzymes
•
•
•
Most enzymes are globular proteins; some are
simple proteins, others are conjugated proteins
Simple enzyme: composed only of protein
(amino acid chains)
It is the 3o structure of the simple
enzymes that makes it biologically active
Conjugated enzyme: has a non-protein part in
addition to a protein part.
1. apoenzyme  protein part; inactive in itself
2. cofactor /coenzyme  nonprotein organic
(coenzyme /co-substrate) or inorganic
(cofactor) moiety; the activator; loosely
bound to protein
• apoenzyme + cofactor = holoenzyme
(biologically active conjugated enzyme)
Copyright © Cengage Learning. All rights reserved
4
Section 21.3
Nomenclature and Classification of Enzymes
• 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 is converted
to alcohol, therefore in this reaction sugar is the
substrate
Copyright © Cengage Learning. All rights reserved
5
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
Copyright © Cengage Learning. All rights reserved
6
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
Copyright © Cengage Learning. All rights reserved
7
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
Copyright © Cengage Learning. All rights reserved
8
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 of a group to a double bond
or removal of groups to form double bonds
5. Isomerase
Isomerization reactions
6. Ligases
Reactions involving bond formation coupled with ATP
hydrolysis
Copyright © Cengage Learning. All rights reserved
9
Section 21.3
Nomenclature and Classification of Enzymes
Copyright © Cengage Learning. All rights reserved
10
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 NAD+ is
the coenzyme in this reaction.
Copyright © Cengage Learning. All rights reserved
11
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:
1. kinases - catalyze
transfer of a
phosphate group
from adenosine
triphosphate (ATP)
to a substrate
Copyright © Cengage Learning. All rights reserved
12
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:
2. transaminases catalyze transfer of
an amino group to a
substrate
Copyright © Cengage Learning. All rights reserved
13
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 oligo- and
polysaccharides
– proteases effect the breaking of peptide linkages in proteins
– lipases effect the breaking of ester linkages in triacylglycerols
Copyright © Cengage Learning. All rights reserved
14
Section 21.3
Nomenclature and Classification of Enzymes
Lyase
• A lyase is an enzyme that catalyzes the addition or the removal of a
group in a manner that does not involve hydrolysis or oxidation
– dehydratase: effects the removal of the components of water to
form a double bond
– hydratase: effects the addition of the components of water to a
double bond
Copyright © Cengage Learning. All rights reserved
15
Section 21.3
Nomenclature and Classification of Enzymes
Lyase
• A lyase is an enzyme that catalyzes the addition or the removal of a
group in a manner that does not involve hydrolysis or oxidation
– decarboxylase: effects the removal of carbon dioxide from a
substrate
– deaminase: effects the removal of ammonia from a substrate
Copyright © Cengage Learning. All rights reserved
16
Section 21.3
Nomenclature and Classification of Enzymes
Isomerase
• An isomerase is an enzyme that catalyzes the isomerization
(rearrangement of atoms) of a substrate in a reaction, converting it
into a molecule isomeric with itself.
racemases – conversion of D- to L- isomer or vice versa
mutases – transfer of a functional group within a molecule
Copyright © Cengage Learning. All rights reserved
17
Section 21.3
Nomenclature and Classification of Enzymes
Ligase
•
A ligase is an enzyme that
catalyzes the formation of a bond
between two molecules involving
ATP hydrolysis to ADP:
– ATP hydrolysis is required
because such reactions are
energetically unfavorable
– synthetases – formation of
new bond between two
substrates with participation
of ATP
– carboxylases – formation of
new bond between substrate
and carbon dioxide with
participation of ATP
Copyright © Cengage Learning. All rights reserved
18
Section 21.3
Nomenclature and Classification of Enzymes
Practice Exercise
To what main enzyme class do the enzymes that catalyze
the following chemical reactions belong?
Copyright © Cengage Learning. All rights reserved
19
Section 21.3
Nomenclature and Classification of Enzymes
Practice Exercise
To what main enzyme class do the enzymes that catalyze
the following chemical reactions belong?
Answers:
a.Transferase
b.Lyase
Copyright © Cengage Learning. All rights reserved
20
Section 21.4
Models of Enzyme Action
Enzyme Active Site
•
•
Explanations of how enzymes
function as catalysts in biochemical
systems are based on the concepts
of an enzyme active site and
enzyme-substrate complex
formation.
The active site: relatively small part
of an enzyme’s structure that is
actually involved in catalysis:
–
–
–
–
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
Copyright © Cengage Learning. All rights reserved
21
Section 21.4
Models of Enzyme Action
Enzyme Substrate Complex
• Intermediate
reaction species
formed when
substrate binds
with the active site
• Needed for the
activity of enzyme
• Orientation and
proximity is
favorable and
reaction is fast
Copyright © Cengage Learning. All rights reserved
22
Section 21.4
Models of Enzyme Action
Two Models for Substrate Binding to Enzyme
• Lock-and-Key model:
– In this model, the active site in the enzyme has a fixed, rigid
geometrical conformation
– only substrate of specific shape can bind with active site; a substrate
whose shape and chemical nature are complementary to those of the
active site can interact with the enzyme.
– fails to take into account proteins’ conformational changes to
accommodate a substrate molecule
Copyright © Cengage Learning. All rights reserved
23
Section 21.4
Models of Enzyme Action
Two Models for Substrate Binding to Enzyme
• 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)
– the enzyme active site, although not exactly complementary in
shape to that of the substrate, is flexible enough that it can
adapt to the shape of the substrate.
Copyright © Cengage Learning. All rights reserved
24
Section 21.4
Models of Enzyme Action
Two Models for Substrate Binding to Enzyme
Copyright © Cengage Learning. All rights reserved
25
Section 21.4
Models of Enzyme Action
Forces That Determine Substrate Binding
• H-bonding
• Hydrophobic interactions
• Electrostatic interactions
Copyright © Cengage Learning. All rights reserved
26
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., catalase is an enzyme with absolute specificity for
hydrogen peroxide (H2O2)
– urease absolute specificity for urea
• 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.
Copyright © Cengage Learning. All rights reserved
27
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
Copyright © Cengage Learning. All rights reserved
28
Section 21.6
Factors That Affect Enzyme Activity
Enzyme Activity
• A measure of the rate at which enzyme converts
substrate to products in a biochemical reaction
• Four factors affect enzyme activity:
– Temperature
– pH
– Substrate concentration
– Enzyme concentration
Copyright © Cengage Learning. All rights reserved
29
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
Copyright © Cengage Learning. All rights reserved
30
Section 21.6
Factors That Affect Enzyme Activity
pH
•
•
•
•
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
Copyright © Cengage Learning. All rights reserved
31
Section 21.6
Factors That Affect Enzyme Activity
Substrate Concentration
• At a constant enzyme
concentration, the enzyme activity
increases with increased
substrate concentration.
• Enzyme 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
Copyright © Cengage Learning. All rights reserved
32
Section 21.6
Factors That Affect Enzyme Activity
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.
Copyright © Cengage Learning. All rights reserved
33
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
Copyright © Cengage Learning. All rights reserved
34
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
Copyright © Cengage Learning. All rights reserved
35
Section 21.6
Factors That Affect Enzyme Activity
Copyright © Cengage Learning. All rights reserved
36
Section 21.7
Extremozymes
Extremeophiles
• Organisms that thrive in extreme environments.
– Hydrothermophiles - thrive at 80o-122oC and high pressure.
– Acidophiles - optimal growth pH <3.0.
– Alkaliphiles – optimal growth pH >9.0.
– Halophiles – live in highly saline conditions (>0.2 M NaCl).
– Piezophiles – grow under high hydrostatic pressure.
– Cryophiles – grow at temps <15oC.
Extremozyme
• A microbial enzyme that is active at conditions that would inactivate human
enzymes as well as enzymes present in most other organisms.
• Etremozymes are of high interest for industrial chemists
– enzymes are heavily used in industrial processes
– industrial processes require extremes of temp, pressure, and pH.
Copyright © Cengage Learning. All rights reserved
37
Section 21.7
Extremozymes
Extremozyme Applications
• Biotechnology industry – production
of enzymes for industrial
applications.
• Petroleum industry – oil well drilling
operations
• Environmental scavenging and
removal of heavy metals
• Environmental clean-up using
genetically engineered
extremophiles.
• Laundry detergents used in cold
wash cycles.
Copyright © Cengage Learning. All rights reserved
38
Section 21.8
Enzyme Inhibition
• Enzyme Inhibitor: a substance that slows down or stops
the normal catalytic function of an enzyme by binding to
it.
• Two types of enzyme inhibitors:
– 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
Copyright © Cengage Learning. All rights reserved
39
Section 21.8
Enzyme Inhibition
Reversible Competitive Inhibition
•
•
•
•
A competitive enzyme
inhibitor decreases enzyme
activity by binding to the
same active site as the
substrate.
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.
Copyright © Cengage Learning. All rights reserved
40
Section 21.8
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+.
Copyright © Cengage Learning. All rights reserved
41
Section 21.8
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
Copyright © Cengage Learning. All rights reserved
42
Section 21.8
Enzyme Inhibition
Copyright © Cengage Learning. All rights reserved
43
Section 21.9
Regulation of Enzyme Activity
• Enzyme activity is often regulated by the cell to conserve
energy. If the cell runs out of chemical energy, it will die
• 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 zymogens
– Covalent modification of enzymes
– Feedback control regulation of enzyme activity by
various substances produced within a cell
• The enzymes regulated are allosteric enzymes
Copyright © Cengage Learning. All rights reserved
44
Section 21.9
Regulation of Enzyme Activity
Properties of Allosteric Enzymes
•
All allosteric enzymes have
quaternary structure:
• Have at least two binding sites:
1. active site - where the substrate
binds lock-and-key
2. allosteric site (meaning “another
site”) - where the regulator
binds; distorts active site
• some regulators speed up
enzyme action (positive
allosterism); activators
• some regulators slow
enzyme action (negative
allosterism); inhibitors
Copyright © Cengage Learning. All rights reserved
45
Section 21.9
Regulation of Enzyme Activity
Copyright © Cengage Learning. All rights reserved
46
Section 21.9
Regulation of Enzyme Activity
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)
Feedback Control
Enzyme 1 inhibited by product D
A
Enzyme 1
Copyright © Cengage Learning. All rights reserved
B
Enzyme 2
C
Enzyme 3
D
47
Section 21.9
Regulation of Enzyme Activity
Proteolytic Enzymes and Zymogens
• Mechanism of regulation
by production of enzymes
in an inactive forms
(zymogens).
• Zymogens, also known
as pro-enzymes, are
“turned on” at the
appropriate time and
place
– example: proteolytic
enzymes: hydrolyze
peptide bonds in proteins
Copyright © Cengage Learning. All rights reserved
48
Section 21.9
Regulation of Enzyme Activity
Covalent Modification of Enzymes
•
•
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.
Copyright © Cengage Learning. All rights reserved
49
Section 21.9
Regulation of Enzyme Activity
• Many common prescription drugs exert their mode of
action by inhibiting enzymes
• Examples:
– Angiotensin Converting Enzyme (ACE) inhibitors
• Management of blood pressure and other heart
conditions
– Sulfa drugs – antibiotics (antimetabolites)
– Penicillins – antibiotics
• Antibiotic: a substance that kills bacteria or inhibits its
growth
Copyright © Cengage Learning. All rights reserved
50
Section 21.9
Regulation of Enzyme Activity
ACE Inhibitors
•
•
•
Angiotensin II is an octapeptide
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu
hormone that increases blood pressure
via constriction of blood vessels.
Angiotensin I
ACE converts Angiotensin I to
ACE
angiotensin II in the blood.
inhibitors
ACE inhibitors block ACE reaction and
ACE
block this
thus reduce blood pressure.
reaction
– Lisinopril is an example of a ACE
inhibitor
Angiotensin II
Asp-Arg-Val-Tyr-Ile-His-Pro-Phe
+
His-Leu
Copyright © Cengage Learning. All rights reserved
51
Section 21.9
Regulation of Enzyme Activity
Sulfa Drugs
• Derivatives of sulfanilamide
• Sulfa drugs exhibit antimetabolite activities
– sulfanilamide is structurally similar to
PABA (p-aminobenzoic acid) which
bacteria need 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 get folic acid pre-formed from food
Copyright © Cengage Learning. All rights reserved
52
Section 21.9
Regulation of Enzyme Activity
Penicillins
•
•
•
•
•
•
•
Bacteria have one structural feature not found in
animal cells – a cell wall.
The bacterial cell wall precursor is a polymer of a
repeating disaccharide unit with attached polypeptide
side chains that end with a D-alanyl-D-alanine unit.
Transpeptidase catalyzes the formation of peptide
cross links between polysaccharide strands in
bacterial cell walls
Penicillin acts by complexing with the enzyme
transpeptidase that is responsible for cell wall
synthesis
Selectively inhibits transpeptidase by covalent
modification of serine residue
The structural similarity between the penicillins and
D-alanyl-D-alanine allows the antibiotic to act as
inhibitory substrates for the transpeptidase enzyme.
Since animal cells do not have cell walls, there are
no such enzymes to be affected and penicillin has no
effect on animal cells.
Copyright © Cengage Learning. All rights reserved
53
Section 21.9
Regulation of Enzyme Activity
Enzyme Kinetics: Michaelis – Menten
E
+
Kinetics of Enzyme Action
k1
k3
S ↔ ES ↔ E + P
k2
k4
Michaelis- Menten Equation::
υ = (vmax) (S)
Km + (S)
Vmax is the turnover number
When υ = ½ vmax:
Km = (S)
Copyright © Cengage Learning. All rights reserved
54
Section 21.9
Regulation of Enzyme Activity
Enzyme Kinetics: Lineweaver –
Burke Plots
•
•
•
•
an alternative linear
transformation of the M-M
equation
estimation of the value of Km is
inconvenient from Michaelis
Equation plot and several more
convenient forms of the
equation have been developed.
The reciprocal of the equation, a
linear form called the
Lineweaver – Burke plot is
used.
1/υ = Km + (S) / vmax (S) =
Km / vmax (S) + (S) / vmax (S) =
Km / vmax x 1 / (S) + 1 /
vmax (eqn for st. line)
Copyright © Cengage Learning. All rights reserved
55
Section 21.9
Regulation of Enzyme Activity
Enzyme Kinetics:
Copyright © Cengage Learning. All rights reserved
56
Section 21.9
Regulation of Enzyme Activity
Enzyme Kinetics:
Competitive inhibitor:
Noncompetitive inhibitor: Uncompetitive inhibitor:
- binds free E
- reversible
-Vmax the same
- binds free E & ES complex - binds ES complex
- Km increases
Copyright © Cengage Learning. All rights reserved
- reversible; irriversible
- Vmax decreases
- Km constant
- irriversible
-V
decreases
max
-
Km decreases
57
Section 21.10
Prescription Drugs That Inhibit Enzyme Activity
Clinical Applications of Enzymes
– Different cells in the body produce enzymes for the same type of
reactions.
– Enzymes that catalyze the same reactions but vary slightly in structure
are called isoenzymes.
– For example, there are five isoenzymes for lactate dehydrogenase
(LDH), an enzyme that converts lactic acid to pyruvic acid.
Isoenzyme
Subunits
Abundant in
LDH1
H4
Heart
kidneys
LDH2
H3M
Heart
kidneys,
brain, rbc
Copyright © Cengage Learning. All rights reserved
LDH3
LDH4
H2M2
HM3
Kidneys Spleen
brain
LDH5
M4
Liver, skeletal muscle
58
Section 21.10
Prescription Drugs That Inhibit Enzyme Activity
Clinical Applications of Enzymes
• Enzymes produced in certain organ/tissues if found in blood serum
may indicate certain damage to that organ/tissue
Serum Enzymes used in diagnosis of tissue damage
Organ
Condition
Heart
Myocardial infarction
Lactate dehydrogenase (LDH ) ; Creatine
kinase (CK ) ; Glutamic1 oxaloacetic
2 (GOT)
transaminase
Liver
Cirrhosis, carcinoma,
Hepatitis
Glutamic pyruvic transaminase (GPT) ;
Lactate dehydrogenase (LDH ) ;
Alkaline phosphatase (ALP) ; GOT5
Bone
Rickets, carcinoma
Alkaline phosphatase (ALP)
Pancreas
Pancreatic diseases
Amylase ; Cholinesterase ; Lipase (LPS)
Prostate
Carcinoma
Acid phosphatase (ACP)
Copyright © Cengage Learning. All rights reserved
Diagnostic Enzymes
59
Section 21.10
Prescription Drugs That Inhibit Enzyme Activity
Clinical Applications of Enzymes
Copyright © Cengage Learning. All rights reserved
60
Section 21.2
Enzyme Structure
Coenzymes / Cofactors
•
•
•
the water-soluble vitamins, which include all B-vitamins and Vitamin C,
act as coenzymes or coenzyme precursors
cofactors are bound to the enzyme for it to maintain the correct
configuration at the active site
provide additional chemically reactive functional group
Copyright © Cengage Learning. All rights reserved
61
Section 21.2
Enzyme Structure
Coenzymes / Cofactors
Copyright © Cengage Learning. All rights reserved
62
Section 21.2
Enzyme Structure
Coenzymes / Cofactors
Cofactors
=============================================================
Metal Ion
Enzymes
------------------------------------------------------------------------------------------------------------------------Ca 2+
Thromboplastin
Cu2+
Tyrosinase, cytochrome oxidase
Fe2+ ; Fe3+
Cytochrome oxidase, catalase, dehydrogenase
Mg2+
Pyruvate kinase
Mn2+
Arginase, pyruvate carboxylase, phosphatase, succinic dehydrogenase,
glycosyl transferases, cholinesterase
K+
Pyruvate kinase
Zn2+
Carbonic anhydrase, carboxypeptidase, lactic dehydrogenase, alcohol
dehydrogenase
========================================================================
Copyright © Cengage Learning. All rights reserved
63
Section 21.12
General Characteristics of Vitamins
•
•
•
•
•
•
•
•
Vitamin: An organic compound essential for proper functioning of the body
Must be obtained from dietary sources because human body can’t
synthesize them in enough amounts
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 of vitamins
– Water-Soluble and Fat-Soluble
Synthetic and natural vitamins have the same function
– 13 Known vitamins
Copyright © Cengage Learning. All rights reserved
64
Section 21.12
General Characteristics of Vitamins
Copyright © Cengage Learning. All rights reserved
65
Section 21.12
General Characteristics of Vitamins
Copyright © Cengage Learning. All rights reserved
66
Section 21.12
General Characteristics of Vitamins
Vitamin C
• Humans, monkeys, apes and guinea pigs need dietary vitamins
• Co-substrate in the formation of structural protein collagen
- collagen also contains hydroxylysine and hydroxylproline.
- hydroxylation of lysine and proline in collagen formation are
catalyzed by enzymes that require ascorbic acid (Vit. C) and
iron.
- in Vit. C deficiency, hydroxylation is impaired, and the triple helix of
the collagen is not assembled properly.
- persons deprived of Vit. C develops scurvy, a disease whose
symptoms include skin lesions, fragile blood vessels, loose
teeth, and bleeding gums
• Involved in metabolism of certain amino acids
Copyright © Cengage Learning. All rights reserved
67
Section 21.14
Water-Soluble Vitamins: The B Vitamins
•
•
•
Major function: B Vitamins are
components of many coenzymes
Serve as temporary carriers of
atoms or functional groups in
redox and group transfer
reactions associated with
metabolism
The preferred and alternative
names for the B vitamins
– Thiamin (vitamin B1)
– Riboflavin (vitamin B2)
– Niacin (nicotinic acid,
nicotinamide, vitamin B3)
– Pantothenic acid (vitamin
B5)
– Vitamin B6 (pyridoxine,
pyridoxal, pyridoxamine)
– Folate (folic acid)
– Vitamin B12 (cobalamin)
– Biotin
•Copyright © Cengage Learning. All rights reserved
68
Section 21.14
Water-Soluble Vitamins: The B Vitamins
Copyright © Cengage Learning. All rights reserved
69
Section 21.15
Fat-Soluble Vitamins
Vitamins A, D, E, K
•
•
•
•
Involved in plasma membrane processes
More hydrocarbon like with fewer functional groups
Occur in the lipid fractions of their sources
Their molecules have double bonds or phenol rings, so oxidizing
agents readily attack them
• Destroyed by prolonged exposures to air or to the organic peroxides
that develop in fats and oils turning rancid.
• Because the fat-soluble vitamins are easily oxidized, they destroy
oxidizing agents (which are involved in the development of coronary
heart disease, genetic mutations, and cancer)
Copyright © Cengage Learning. All rights reserved
70
Section 21.15
Fat-Soluble Vitamins
Vitamin A
• a primary alcohol of molecular
formula C20H30O; occur only in
the animal world, where the
best sources are cod-liver oil
and other fish-liver oils, animal
liver and dairy products
• provitamin A is found in the
plant world in the form of
carotenes. Provitamins have no
vitamin activity; however, after
ingestion in the diet, -carotene
is cleaved at the central
carbon-carbon double bond to
give 2 molecules of Vit. A.
Copyright © Cengage Learning. All rights reserved
71
Section 21.15
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: a process in which immature cells
change to specialized cells with function.
– example: differentiation of bone marrow cells white blood cells
and red blood cells.
• Maintenance of the health of epithelial tissues via epithelial tissue
differentiation.
– lack of vitamin A causes skin surface 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.
Copyright © Cengage Learning. All rights reserved
72
Section 21.15
Fat-Soluble Vitamins
Vitamin D - Sunshine Vitamin
•
•
•
•
•
•
The antirachitic vitamin
Necessary for the normal
calcification of bone tissue
It controls correct ratio of Ca and
P for bone mineralization
(hardening)
Two forms active in the body:
Vitamin D2 and D3
Pigment in the skin, 7dehydrocholesterol, is a
provitamin D; when irradiated by
the sun becomes converted to
Vit. D3
humans exposed to sunlight yearround do not require dietary Vit. D
Copyright © Cengage Learning. All rights reserved
73
Section 21.15
Fat-Soluble Vitamins
Vitamin E - Antisterility vitamin
• Alpha-tocopherol is the most active
biological active form of Vitamin E
• tocopherol  Greek, promoter of childbirth
• functions in the body as an antioxidant in
that it inhibits the oxidation of unsat’d fatty
acids by O2
• Primary function: Antioxidant – protects
against oxidation of other compounds
Copyright © Cengage Learning. All rights reserved
74
Section 21.15
Fat-Soluble Vitamins
Vitamin K - Antihemorrhagic vitamin
•
•
•
•
Vit K is synthesized by
bacteria that grow in colon
Active in the formation of
proteins involved in
regulating blood clotting
Deficiency may occur
during the first few days
after birth, because
newborns lack the intestinal
bacteria that produce Vit. K
and because they have no
store of Vit. K (it does not
cross the placenta)
Deficiency may also occur
following antibiotic therapy
that sterilizes the gut
Copyright © Cengage Learning. All rights reserved
75