Enzymes.pptx.pdf its chapter of enzyme .

Classify enzymes on the basis of mechanism of actions
in relation to medical biochemistry
• Write down the mechanism of catalysis of enzymes
• Describe the factors affecting enzyme activity
•Define Michaelis-Menten equation &
LineweaverBurk plot and its application in enzyme
kinetics (no derivation of equations)
LEARNING OBJECTIVES

maltose + water maltase glucose +
glucose

1956 - to create a systematic basis for enzyme
nomenclature
4 digit numbered code
first digit - major class
Second digit - sub class
third digit - sub sub class
final digit - specific enzyme
IUB Nomenclature

2- class name (transferase)
7- subclass name (phosphotransferase)
1- sub sub class (hydroxyl group as acceptor)
1- specific enzyme (D- glucose as phosphoryl
group acceptor
ATP: glucose
phosphotransferase
2.7.1.1

1. Oxidoreductase
Catalyze oxidation-reduction reactions by
transferring H atom or hydride ions
e.g. Alcohol Dehydrogenase
Lactate dehydrogenase
cytochrome oxidase

Oxidoreductases are further classified
according to the substrate oxidized and
to the mechanism of oxidation
1.Dehydrogenases
2.Oxidases
3.Hydroperoxidases
4.Oxygenases

Either catalyze the removal of hydrogen from
the substrate but not able to use oxygen as
hydrogen acceptor e.g. lactate dehydrogenase,
pyruvate dehydrogenase e.t.c.
Or
Remove electrons from substrate e.g.
cytochrome b ,c and c1
1.Dehydrogenases

Catalyze the addition of oxygen to H
atoms removed from substrate forming
H2O or H2O2
Example:
Xanthine oxidase
L-amino acid oxidase
Cytochrome aa3
2. OXIDASES

Two enzymes in this class
Peroxidase
H2O2 + 2H⁺+ 2e¯ 2H2O
Catalase
2H2O2 2H2O+ O2
Found in both plants and animals
3.Hydroperoxidases

Catalyze the incorporation of molecular
oxygen into the substrate
May be
Mono-oxygenase
or
Di-oxygenases
incorporate one or both atoms of molecular
oxygen into the substrate
4. OXYGENASES

Functional group is transferred from one
compound to another
e.g. kinases
Transaminase
Phosphorylase
2. Transferase

They are further classified according to the
group transferred into:
1.Transaminases (ALT,AST)
2.Phosphotransferases:kinases (HEXOKINASE)
3.Transmethylases(Conversion of noradrenaline
to adrenaline)
4.Transpeptidases(formation of hippuric acid
from benzoyl-CoA and glycine)
5.Transacylase (Choline acyltransferase)

Cleave C-C, C-O, C-N, C-S or P-O etc bonds by
adding water across the bond
e.g. lipase
acid phosphatase
(important in digestive process)
3. Hydrolase

Catalyze the addition of NH3,H2O or CO2 to
double bonds or removal of these groups leaving
behind double bonds
Cleave without adding water
Example:
▪ Fumarase
A hydratase (subclass) that add water to double
bond
▪ Carbonic anhydrase
▪ Aldolase
▪ HMG CoA Lyase
▪ ATP citrate lyase
4. Lyases

Catalyze intramolecular rearrangements
of functional groups that reversibly
interconvert to optical or geometric isomers
e.g. Triose isomerase
Phosphohexose isomerase
mutase
5. Isomerase

Catalyze condensation reactions joining two
molecules by forming C-O, C-S, C-N and C-C bonds
along with energy releasing hydrolysis or cleavage of
high energy phosphates e.g.ATP,GTPetc
Glutamine synthetase
DNA- ligase
Acetyl CoA carboxylase
6. Ligase

Synthetase (requires ATP)
Synthase (No ATP required)
Phosphatase (Use water to remove phosphate
group)
phosphorylase (Use inorganic phosphate to break a
bond and generate phosphorylated product)
Dehydrogenase (NAD or FAD ,NADPH electron
acceptor)
Oxidase (Oxygen is acceptor ,oxygen atom not
incorporated into substrate)
Oxygenase (One or both oxygen atom are
incorporated)

Two different perspectives
1. Catalysis in terms of energy changes that occur
during the reaction
2. How the active site chemically facilitates catalysis
The mechanism of enzyme action

All chemical reactions have an energy barrier
separating the reactants and the products
This barrier, called the free energy of activation, is the
energy difference between that of the reactants and a
high-energy intermediate that occurs during the
formation of product
Energy changes

1. Free energy of activation:
The peak of energy is the difference in free energy
between the reactant and T* , where the high-energy
intermediate is formed during the conversion of
reactant to product
Because of the high free energy of activation, the rates
of uncatalyzed chemical reactions are often slow

For molecules to react, they must contain sufficient energy
to overcome the energy barrier of the transition state
In the absence of an enzyme, only a small proportion of
molecules may possess enough energy to achieve the
transition state between reactant and product
The rate of reaction is determined by the number of such
energized molecules
2. Rate of reaction

In general, the lower the free energy of activation, the
more molecules have sufficient energy to pass
through the transition state, and, therefore, the faster
the rate of the reaction

An enzyme allows a reaction to proceed rapidly under
conditions prevailing in the cell by providing an alternate
reaction pathway with a lower free energy of activation
The enzyme does not change the free energies of the
reactants or products and, therefore, does not change the
equilibrium of the reaction
It does, however, accelerate the rate by which equilibrium
is reached
3. Alternate reaction pathway

Figure shows the changes in energy during the conversion of a molecule of reactant
A to product B as it proceeds through the transition state (high-energy
intermediate), T*: A T * B

BIOCHEMICAL MECHANISMS
1.Catalysis by Proximity
For molecules to interact, 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 at which they react

When an enzyme binds substrate molecules at its
active site, it creates a region of high local substrate
concentration in which the substrate molecules are
oriented in a position ideal for them to chemically
interact
This results in rate enhancements of at least a
thousand fold over the same nonenzyme-catalyzed
reaction

The ionizable functional groups of aminoacyl side
chains and
prosthetic groups, can contribute to catalysis by
acting as acids or bases
Two types of acid–base catalysis
1.Specific acid or base catalysis
2. general acid catalysis or general base catalysis
2.Acid–Base Catalysis

Specific acid or base catalysis
Refers to reactions for which the only participating acids or
bases are protons or hydroxide ions
The rate of reaction is sensitive to changes in the
concentration of protons or hydroxide ions, but is
independent of the concentrations of other acids (proton
donors) or bases (proton acceptors) present in the solution or
at the active site

Reactions whose rates are responsive to all the acids
or bases present
General acid catalysis or general base catalysis

For catalysis of lytic reactions, which involve
breaking a covalent bond, enzymes typically bind
their substrates in a conformation that weakens the
bond targeted for cleavage through physical
distortion and electronic polarization
This strained conformation mimics that of the
transition state intermediate
3.Catalysis by Strain

Knowledge of the transition state of an
enzyme-catalyzed reaction is frequently exploited by
chemists to design and synthesize more effective
enzyme inhibitors, called transition state analogs, as
potential pharmacophores

The process of covalent catalysis involves the formation
of a covalent bond between the enzyme and one or
more substrates
The modified enzyme thus becomes a reactant
Covalent catalysis provides a new reaction pathway
whose activation energy is lower—and rate of reaction
therefore faster—than the pathways available in
homogeneous solution
4. Covalent Catalysis

The chemically modified state of the enzyme is
transient
Completion of the reaction returns the enzyme to its
original, unmodified state
Its role thus remains catalytic
Covalent catalysis is particularly common among
enzymes that catalyze group transfer reaction

Residues on the enzyme that participate in covalent catalysis
generally are
cysteine or
serine, and
occasionally histidine
Covalent catalysis often follows a “ping-pong” mechanism—one
in which the first substrate bound and its product released prior
to the binding of the second substrate

1.Enzyme concentration
2. Substrate concentration
3.Temperature
4.pH
5.Product
6. presence of coenzyme and prosthetic group
7.Presence of inhibitors
Factors affecting enzyme activity

Define Michaelis-Menten equation &
LineweaverBurk plot and its application in enzyme
kinetics (no derivation of equations)
Compare & contrast different types of enzyme
inhibitions with examples & biomedical importance
LEARNING OBJECTIVES

Reaction model
Proposed by Leonor Michaelis and Maude Menten
Simple model
The enzyme reversibly combines with its substrate
to form an ES complex that subsequently yields
product, regenerating the free enzyme
MICHAELIS-MENTEN
KINETICS

The model, involving one substrate molecule
S is the substrate
E is the enzyme
ES is the enzyme–substrate complex
P is the product
k1, k-1, and k2 are rate constants

The Michaelis-Menten equation describes how
reaction velocity varies with substrate concentration
Michaelis-Menten equation

1. Relative concentrations of enzyme and substrate
The concentration of substrate (S) is much greater than
the concentration of enzyme (E), so that the
percentage of total substrate bound by the enzyme at
any one time is small
Assumptions in
Michaelis-Menten rate equation:

Effect of substrate concentration on reaction velocities for two enzymes: enzyme 1
with a small Michaelis constant (Km) and enzyme 2 with a large Km.
Vmax = maximal velocity

ES does not change with time
The rate of formation of ES is equal to that of the
breakdown of ES to E + S and E + P
An intermediate in a series of reactions is said to be in
steady state when its rate of synthesis is equal to its
rate of degradation
2. Steady-state assumption:

Initial reaction velocities (vo) are used in the analysis
of enzyme reactions
the rate of the reaction is measured as soon as
enzyme and substrate are mixed
At that time, the concentration of product is very
small, and, therefore, the rate of the back reaction
from product to substrate can be ignored
3. Initial velocity

Characteristics of Km:
Km, the Michaelis constant, is characteristic of an
enzyme and its particular substrate and reflects the
affinity of the enzyme for that substrate
Km is numerically equal to the substrate
concentration at which the reaction velocity is equal
to 1⁄2Vmax
Km does not vary with enzyme concentration
Important conclusions

A numerically small (low) Km reflects a high affinity
of the enzyme for substrate, because a low
concentration of substrate is needed to
half-saturate the enzyme to reach a velocity that is
1⁄2Vmax
Small Km

A numerically large (high) Km reflects a low affinity
of enzyme for substrate because a high
concentration of substrate is needed to
half-saturate the enzyme
Large Km:

Effect of substrate concentration on reaction velocity for an enzyme
catalyzed reaction. Vmax = maximal velocity; Km = Michaelis constant.

The rate of the reaction is directly proportional to
the enzyme concentration at all substrate
concentrations
For example, if the enzyme concentration is halved,
the initial rate of the reaction (vo), as well as that of
Vmax , are reduced to half that of the original
2. Relationship of velocity to enzyme concentration:

When S is much less than Km, the velocity of the
reaction is approximately proportional to the
substrate concentration
The rate of reaction is then said to be first order with
respect to substrate
3. Order of reaction:

When S is much greater than Km, the velocity is
constant and equal to Vmax
The rate of reaction is then independent of substrate
concentration (the enzyme is saturated with
substrate) and is said to be zero order with respect to
substrate concentration

When vo is plotted against [S], it is not always
possible to determine when Vmax has been achieved
because of the gradual upward slope of the
hyperbolic curve at high substrate concentrations
Lineweaver-Burk plot

However, if 1/v o is plotted versus 1/[S], a straight
line is obtained
This plot, the Lineweaver-Burk plot (also called a
doublereciprocal plot) can be used to calculate Km
and Vmax as well as to determine the mechanism of
action of enzyme inhibitors

1. The equation describing the Lineweaver-Burk plot is:
where the intercept on the x axis is equal to −1/Km, and
the intercept on the y axis is equal to 1/Vmax
The slope = Km/Vmax

Lineweaver-Burk plot. vo = reaction velocity; Vmax =
maximal velocity; Km = Michaelis constant; [S] = substrate
concentration

Any substance that can decrease the velocity of an
enzyme-catalyzed reaction is called an inhibitor
Two types
1. Reversible
Competitive
Noncompetitive
2.Irreversible
INHIBITION OF ENZYME
ACTIVITY

Bind to enzymes through covalent bonds
Lead forms covalent bonds with the sulfhydryl side
chain of cysteine in proteins
Ferrochelatase, an enzyme involved in heme
synthesis is irreversibly inhibited by lead
Irreversible inhibitors

An important group of irreversible inhibitors are the
mechanism-based inhibitors that are converted by
the enzyme itself to a form that covalently links to
the enzyme, thereby inhibiting it
“suicide” inhibitors

Bind to enzymes through non-covalent bonds
Dilution of the enzyme–inhibitor complex results in
dissociation of the reversibly bound inhibitor and
recovery of enzyme activity
Reversible inhibitors

Inhibitor binds reversibly to the same site that the
substrate would normally occupy
Competes with the substrate
Competitive Inhibition

The effect of a competitive inhibitor is reversed by
increasing [S]
At a sufficiently high substrate concentration, the
reaction velocity reaches the Vmax observed in the
absence of inhibitor
1. Effect on Vmax:

A competitive inhibitor increases the apparent Km
for a given substrate
This means that, in the presence of a competitive
inhibitor, more substrate is needed to achieve
1⁄2Vmax
2. Effect on Km:

Competitive inhibition shows a characteristic
Lineweaver-Burk plot in which the plots of the
inhibited and uninhibited reactions intersect on the y
axis at 1/Vmax (Vmax is unchanged)
3. Effect on the Lineweaver-Burk plot

The inhibited and uninhibited reactions show different x-axis
intercepts, indicating that the apparent Km is increased in the
presence of the competitive inhibitor because - 1/Km moves
closer to zero from a negative value

An important group of competitive inhibitors are the
transition state analogs, stable molecules that
approximate the structure of the transition state
and, therefore, bind the enzyme with a higher
affinity than the substrate

Effect of a competitive inhibitor on the
reaction velocity versus substrate ([S]) plot. B.
Lineweaver-Burk plot of competitive
inhibition of an enzyme

Antihyperlipidemic agents
competitively inhibits the rate-limiting (slowest) step
in cholesterol biosynthesis
Catalyzed by hydroxymethylglutaryl–CoA reductase
(HMG-CoA reductase)
Statin drugs as examples of competitive inhibitors:

Statins, such as atorvastatin (Lipitor) and pravastatin
(Pravachol), are structural analogs of the natural
substrate for this enzyme and compete effectively to
inhibit HMG-CoA reductase
Inhibit de novo cholesterol synthesis
lowering plasma cholesterol levels

Pravastatin competes with HMGCoA for the active site of
HMGCoA reductase. HMG-CoA =
hydroxymethylglutaryl-coenzyme A.

This type of inhibition is recognized by its
characteristic effect on Vmax
Noncompetitive inhibition occurs when the inhibitor
and substrate bind at different sites on the enzyme
The noncompetitive inhibitor can bind either free
enzyme or the enzyme-substrate complex,preventing
the reaction from occurring
Noncompetitive inhibition

Noncompetitive inhibition cannot be overcome by
increasing the concentration of substrate
Noncompetitive inhibitors decrease the apparent
Vmax of the reaction
1. Effect on Vmax:

Noncompetitive inhibitors do not interfere with the
binding of substrate to enzyme
Therefore, the enzyme shows the same Km in the
presence or absence of the noncompetitive inhibitor
2.Effect on Km:

Effect of a noncompetitive inhibitor on the reaction
velocity versus substrate ([S]) plot. B. Lineweaver-Burk
plot of noncompetitive inhibition of an enzyme.

Noncompetitive inhibition is readily differentiated from
competitive inhibition by plotting 1/vo versus 1/[S]
The apparent Vmax decreases in the presence of a
noncompetitive inhibitor, whereas Km is unchanged
Oxypurinol, a metabolite of the drug allopurinol, is a
noncompetitive inhibitor of xanthine oxidase, an enzyme
of purine degradation
Effect on Lineweaver-Burk plot:

A noncompetitive inhibitor binding to both free enzyme and
enzymesubstrate

At least half of the ten most commonly prescribed
drugs in the United States act as enzyme inhibitors
β-lactam antibiotics, such as penicillin and
amoxicillin, act by inhibiting enzymes involved in
bacterial cell wall synthesis
Enzyme inhibitors as drugs

Drugs may also act by inhibiting extracellular reactions
e.g. angiotensin-converting enzyme (ACE) inhibitors
They lower blood pressure by blocking the enzyme that
cleaves angiotensin I to form the potent vasoconstrictor,
angiotensin II
These drugs, which include captopril, enalapril, and
lisinopril, cause vasodilation and, therefore, a reduction
in blood pressure

Aspirin, irreversibly inhibits prostaglandin and
thromboxane synthesis
Irreversible acetylation of cyclooxygenase (COX)-1
and COX-2 by aspirin

Aspirin acts as an acetylating agent where an acetyl
group is covalently attached to a serine residue in
the active site of the COX enzyme
This makes aspirin different from other NSAIDs such
as diclofenac and ibuprofen, which are reversible
inhibitors; aspirin creates an allosteric change in the
structure of the COX enzyme

The rate or velocity of a reaction (v) is the number of
substrate molecules converted to product per unit time
Velocity is usually expressed as µmol of product formed
per minute
The rate of an enzyme-catalyzed reaction increases with
substrate concentration until a maximal velocity (Vmax)
is reached
Maximal velocity

Explain regulatory enzymes
Explain coenzymes, cofactors, and with their
biochemical importance
Overview of Vitamins as coenzymes I (B1, B2, B3, B6,
biotin, pantothenic)
Role of minerals as a cofactor
LEARNING OBJECTIVES

Substrate concentration
Allosteric regulation
Covalent Modification
Induction and repression of enzyme activity
REGULATION OF ENZYME ACTIVITY

Regulated by molecules called effectors
Bind non-covalently at a site other than the active
site
multiple subunits
The regulatory (allosteric) site that binds the effector
is distinct from the substrate-binding site and may be
located on a subunit that is not itself catalytic
Allosteric Regulation

Effectors that inhibit enzyme activity
Positive effectors
Effectors that increase enzyme activity
They can
affect the affinity of the enzyme for its substrate (K0.5),
modify the maximal catalytic activity of the enzyme (Vmax),
or both
Allosteric enzymes frequently catalyze the committed step
early in a pathway
Negative effectors

The both effectors can
affect the affinity of the enzyme for its substrate (K0.5)
modify the maximal catalytic activity of the enzyme
(Vmax),
or both
Allosteric enzymes frequently catalyze the committed
step early in a pathway

Effects of negative or positive effectors on an allosteric
enzyme. A. Vmax is altered. B. The substrate concentration
that gives half-maximal velocity (K0.5) is altered

When the substrate itself serves as an effector, the effect
is said to be homotropic
An allosteric substrate functions as a positive effector
The presence of a substrate molecule at one site on the
enzyme enhances the catalytic properties of the other
substrate-binding sites
That is, their binding sites exhibit cooperativity
Homotropic effectors:

These enzymes show a sigmoidal curve when
reaction velocity (vo) is plotted against substrate
concentration [S]
This contrasts with the hyperbolic curve
characteristic of enzymes following
Michaelis-Menten kinetics
Cooperativity of substrate binding is analogous to
the binding of oxygen to hemoglobin

The effector may be different from the substrate, in which
case the effect is said to be heterotropic
example is the feedback inhibition
The enzyme that converts D to E has an allosteric site that
binds the endproduct, G
If the concentration of G increases (for example, because it is
not used as rapidly as it is synthesized), the first irreversible
step unique to the pathway is typically inhibited
Heterotropic effectors:

Feedback inhibition of a metabolic pathway

Feedback inhibition provides the cell with
appropriate amounts of a product it needs by
regulating the flow of substrate molecules through
the pathway that synthesizes that product
Heterotropic effectors are very common
For example, the glycolytic enzyme
phosphofructokinase-1 is allosterically inhibited by
citrate, which is not a substrate for the enzyme

Covalent modification is by the addition or removal
of phosphate groups from specific serine, threonine,
or tyrosine residues of the enzyme
Protein phosphorylation is mediated by hormonal
signals
Regulation of enzymes by covalent
modification

Phosphorylation reactions are catalyzed by a family
of enzymes called protein kinases that use ATP as
the phosphate donor
Phosphate groups are cleaved from phosphorylated
enzymes by the action of phosphoprotein
phosphatases

Response of enzyme to phosphorylation
Depending on the specific enzyme, the
phosphorylated form may be more or less active
than the unphosphorylated enzyme
Phosphorylation of glycogen phosphorylase (an
enzyme that degrades glycogen) increases activity,
whereas phosphorylation of glycogen synthase (an
enzyme that synthesizes glycogen) decreases activity

Covalent modification by the addition and removal of
phosphate groups. [Note: HPO4 2− may be represented as Pi.]

Cells can also regulate the amount of enzyme
present by altering the rate of enzyme degradation
or the rate of enzyme synthesis
Induction: increase in enzyme synthesis
Repression: decrease of enzyme synthesis
leads to an alteration in the total population of active
sites
Induction and repression of enzyme synthesis

Enzymes subject to regulation of synthesis are often those
that are needed at only one stage of development or under
selected physiologic conditions
For example, elevated levels of insulin as a result of high
blood glucose levels cause an increase in the synthesis of key
enzymes involved in glucose metabolism

Enzymes that are in constant use are usually not
regulated by altering the rate of enzyme synthesis
Induction or repression of protein synthesis are slow
(hours to days)
Allosterically or covalently regulated changes in
enzyme activity occur in seconds to minutes

Mechanisms for regulating enzyme activity

The active enzyme with its non-protein component
Apo-enzyme
The enzyme without its non-protein moiety and is
inactive
Holo-enzyme

Facilitate the activity or regulation of enzymes
Non-protein moiety is a metal ion, such as
Magnesium,copper ,Zinc,Iron etc
Cofactors

Non-protein is a small organic molecule
Coenzymes can be :
1.Co-substrates
2. Prosthetic group
coenzyme

Coenzymes that only transiently associate with the
enzyme are called co-substrates
Co-substrates dissociate from the enzyme in an
altered state e.g. NAD+
Co-Substrates

If the coenzyme is usually permanently associated with
the enzyme and return to its original form e.g. FAD
Often attached to proteins by a covalent bond
Shuttle molecules in enzymatic reaction rather than
contributing directly to a chemical group
Heme is a prosthetic group in Hb,shuttles oxygen and
carbondioxide
Prosthetic group

Coenzymes commonly are derived from vitamins
For example, NAD+ contains niacin, and FAD contains
riboflavin
Cofactors work allosterically and are not required for enzyme
activity
Coenzymes bind to the active site of the enzyme and are
required for enzyme activity by contributing or by accepting
a chemical group necessary for enzyme to work

B1 (Thiamine)
B2 (Riboflavin)
B3 (Niacin)
B6 (Pyridoxine)
B7 ( biotin)
B5 (Pantothenic acid)
Vitamins as coenzymes

Vitamin B1
(thiamine)
Also called aneurine or anti-beriberi factor
ACTIVE FORM IS TPP(thiamine pyrophosphate)
Made up of a pyrimidine and a thiazole part linked
together by a methylene bridge

Sources and distribution
Outer layers of grains like bran and rice
Whole grains, legumes, beef, liver, nuts and
yeast
Eggs, fish and vegetables contain Vit. B1
in
small amounts
Whole white bread is a good source

Thiamine Triphosphate has a role in nerve
conduction
▪ It phosphorylates and so activates chloride
channel in
nerve membrane
Thiamine nutritional status can be assessed
by
Erythrocyte Transketolase activity

Vitamin B2
(Riboflavin)
It is a heterocyclic dimethylisoalloxazine ring
attached to the sugar alcohol D-ribitol
Dimethylisoalloxazine
D-ribitol

VITAMIN B3
(NIACIN)
Niacin or nicotinic acid is pyridine 3-carboxylic
acid and its amide derivative nicotinamide
Niacin exerts its effects in two forms
▪ NAD+
(nicotinamide adenine dinucleotide)
▪ NADP+
(nicotinamide adenine dinucleotide
phosphate)

Structure and synthesis of NAD⁺ AND NADP⁺

Reduction of oxidized nicotinamide adenine
dinucleotide (NAD+) to NADH

PANTOTHENIC ACID
● Pantothenic acid: from the word ‘pentos’ meaning
‘everywhere’
● Occurrence:
○ widespread in nature
○ yeast, liver and eggs, potatoes, cabbage,
cauliflower, broccoli, peanuts, tomatoes
○ skimmed milk, wheat bran, whole milk and
canned salmon

Structure
● Pantothenic acid (C9
H17
NO5
) is an amide of pantoic acid
and β-alanine
● Stable to oxidizing and reducing agents
● Destroyed by heating in an acidic or alkaline medium

BIOCHEMICAL ROLE
❖ Component of coenzyme A (CoA) which functions in the
transfer of acyl groups
▪ Co A contains a terminal thiol or sulfhydryl group (-SH)
that carries acyl compounds as activated thiol esters
Examples of such compounds are succinyl Co A ,Fatty
acyl Co A and acetyl Co A.
▪ -SH group is the reactive site
Hence CoA –SH is used
▪ It plays role in integrating various pathways

Enzymes requiring Co A as cofactor
Pyruvate dehydrogenase complex
α – Ktoglutarate dehydrogenase complex
FAS complex
Thiolase, HMG Co A synthase

ROLE OF SUCCINYL Co A
Succinyl CoA is formed from Propionyl CoA and α-
ketoglutarate
Propionyl CoA is formed from oxidation of odd chain
fatty acids, valine, isoleucine
Utilization of succinyl CoA is through TCA cycle
Succinate is utilized for metabolism of ketone bodies
It is also used for heme synthesis

ROLE OF ACETYL CoA
Acetyl Co A is formed from oxidation of pyruvate, Fatty
acids, aminoacids, Ketone bodies
UTILIZATION
Acetyl CoA is used to provide energy through TCA cycle
It is essential for synthesis of neurotransmitter
Acetylcholine
It is a substrate for synthesis of F.A., Cholesterol, Ketone
bodies

ROLE OF PROPIONYL CoA
Propionyl CoA is formed from oxidation of odd chain
Fatty acids, isoleucine and valine
It is converted to succinyl CoA

Pantothenic acid deficiency is not well characterized
in humans
No RDA has been established for it

Vitamin B7 (Biotin)
● Isolated in 1935 by a dutch biochemist from dried egg yolks
● Also known as anti-egg white injury factor
● Occurrence:
○ Eggs, yeast, liver, kidney, molasses, peanuts and
vegetables are rich sources
○ Cereals and dairy products are poor sources

○ biotin occurs in nature usually in combined state
as biocytin (biotin linked to ε-amino group of
amino acid lysine) which is released on
proteolysis
○ It is synthesized by intestinal flora in excess of
requirements

STRUCTURE
● It is a heterocyclic sulfur containing monocarboxylic
acid
● Biotin (C10
H16
O3
N2
S) consists of a fused imidazole and
thiophene ring with a valeric acid side chain side chain
● Biotin and thiamine are the only sulfur-containing
vitamins isolated to date

Coenzyme form
● Biocytin is coenzyme form of biotin
● Biotin is a prosthetic group of carboxylases

Biochemical Role
● Coenzyme in carboxylase reactions (carrier of activated CO2
):
○ pyruvate carboxylase
○ Acetyl CoA carboxylase
○ propionyl CoA carboxylase
○ β Methyl corotonyl CoA carboxylase

Pyruvate Carboxylase
Pyruvate carboxylase catalyzes the conversion of
pyruvate to oxaloacetate
Pyruvate
CO₂,ATP pyruvate carboxylase
Biotin
ADP+ Pᵢ
Mg+Mn
Oxaloacetate

Acetyl CoA Carboxylase
It Catalyzes the formation of malonylCoA from acetyl
CoA
The reaction provides acetate molecule for fatty acid
synthesis

Propionyl CoA carboxylase
It Catalyzes the formation of D methyl malonyl CoA
from propionylCoA (from odd chain fatty acid and
methionine)
It is required for entry of propionyl CoA into TCA
cycle via succinyl CoA

β Methyl crotonyl CoA
It catalyzes the formation of β Methylglutaconyl CoA
from β Methyl crotonyl CoA
It is essential for leucine catabolism

Few carboxylation reactions donot require biotin
Formation of Carbomyl phosphate in urea cycle and
incorporation of CO2 in pyrimidine and purine
synthesis

Biotin Antagonist
Avidin (Raw egg white injury factor)
Avidin binds to biotin and makes it unavailable for absorption
Avidin is inactivated by boiling the eggs and biotin is readily
available
One molecule of avidin can bind four molecules of biotin
Affinity is more than the usual antigen antibody reaction
This system avidin-biotin is commonly utilized for detection of
pathogens in ELISA test

❖ Six compounds have vitamin B6 activity
pyridoxine, pyridoxal, pyridoxamine and their 5′-
phosphates
❖ The active co-enzyme is Pyridoxal 5′- phosphate

● All forms are derivatives of pyridine (C5
H5
N)
● Nature of substituent at position 4 of the ring is
different
● Readily inter convertible biologically

BIOCHEMICAL FUNCTIONS
1. Transamination
2. Decarboxylation
3. Coenzyme for deamine oxidase
4. Formation of niacin from Tryptophan
5. Catabolism of tryptophan
6. Metabolism of sulphur containing amino acids
7. Coenzyme for threonine aldolase
8. Synthesis of δ ALA
9. Cofactor for glycogen phosphorylase
10. Role in active transport of amino acids and K⁺ into the cell
11. Synthesis of arachidonic acid

Biochemical Role
❖ Almost all conversion reactions involving amino acids require
pyridoxal phosphate, including
Transamination
Deamination
Decarboxylation
Condensation
Transulfuration

❖ Involved in the synthesis of neurotransmitters:
Serotonin
Dopamine
Gamma amino-butyric acid (GABA)
Norepinephrine
❖ Synthesis of sphingolipids

❖ Glycogen phosphorylase, the enzyme for glycogen
degradation also contains pyridoxal phosphate as a
cofactor
❖ Glycogen phosphorylase catalyzes the release of glucose
from glycogen
❖ In Gluconeogenesis, PLP is needed to convert amino
acid to glucose
❖ Conversion of tryptophan to niacin
❖ Conversion of homo-cysteine to cysteine

❖ PLP also functions in the synthesis of heme
1st step of heme synthesis
Helps in nucleic acid synthesis
❖ B6 is important in steroid hormone action. Pyridoxal
phosphate removes the hormone receptor complex from DNA
binding, terminating the action of hormone

1. CALCIUM
Acts as a cofactor
Calcium Calmodulin complex activates certain enzymes
by attaching with them which are :
Adenylate cyclase
Ca⁺² ATPase
Phosphorylase kinase
Myosin light chain kinase
Phosphodiestrase
Phospholipase A2
This mechanism is also required for release of acetylcholine
at neuromuscular junction
MINERALS AS A COFACTOR

Magnesium is required as a cofactor for
Phosphorylation by kinases (Mg⁺² binds the ATP
cosubstrate)
Phosphodiester bond formation by DNA and RNA
Polymerases
Peptidases
Ribonucleases
Magnesium

Cu requiring enzyme Function
Cytochrome c oxidase Transfers electrons from cytochrome c to
oxygen in the ETC
Dopamine β hydroxylase Hydroxylates dopamine to norepinephrine
Ferroxidase Oxidize Iron
Lysyl oxidase Forms cross links in collagen and elastin
Tyrosinase Synthesizes melanin
Superoxide dismutase
Non-mitochondrial form
Also requires zinc
Converts superoxide to hydrogen peroxide
Copper

Component of many proteins both catalytic and Non catalytic
Catalytic like
hydroxylases e.g.prolyl hydroxylase
Cofactor for catalases
Tryptophan pyrrolase
Non Catalytic
Linked to sulphur in the Fe-S proteins of ETC
Part of heme prosthetic group in proteins like Hb, Myoglobin, Cytochromes
Iron

Activator and Cofactor for several enzymes e.g.
Hexokinase
Arginase
Choline esterase
hydrolase
Pyruvate carboxylase
Transferase
Glutamine synthetase
Manganese

Hundreds of enzymes require Zn for activity
Examples
Alcohol dehydrogenase that Oxidizes ethanol to
acetaldehyde
Carbonic anhydrase
Porphobilinogen synthase of heme synthesis
Non mitochondrial form of superoxide dismutase
Zinc

Molybdenum is bound to unique pterin forming molybdenum
cofactor (MOCO) which is the active compund at the catalytic
site of all molybdenum containing enzymes except bacterial
molybdenum nitrogenase e.g.
Xanthine oxidase Oxidizes hypoxanthine to xanthine and
xanthine into uricacid
Liver aldehyde oxidase metabolizes drugs
Sulfite oxidase converts sulfite to sulphate in metabolism of
sulphur containing aminoacids
Molybdenum

Lippincott illustrated reviews biochemistry
Harper’s illustrated biochemistry
Essentials of medical biochemistry
Internet
Learning Resources

Se is present in almost 25 selenoproteins which
include
Glutathione peroxidase
Oxidizes glutathione in the reduction of hydrogen
peroxide to water
Thioredoxin reductase
Reduces thioredoxin ,a coenzyme of ribonucleotide
reductase
Deiodinase
Removes iodine from thyroid hormones
Selenium

Enzymes.pptx.pdf its chapter of enzyme .

Enzymes.pptx.pdf its chapter of enzyme .

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