ENZYMES: KINETICS, INHIBITION, REGULATION

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ENZYMES KINETICS, INHIBITION, REGULATION
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Kinetic properties of enzymes
Study of the effect of substrate concentration on
the rate of reaction
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Rate of Catalysis

• At a fixed enzyme concentration E, the initial
  velocity Vo is almost linearly proportional to
  substrate concentration S when S is small but
  is nearly independent of S when S is large
• - Rate rises linearly as S increases and then
  levels off at high S (saturated)

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Leonor Michaelis and Maud Menten first
researchers who explained the shape of the rate
curve (1913)
During reaction enzyme molecules, E, and
substrate molecules, S, combine in a reversible
step to form an intermediate enzyme-substrate
(ES) complex
k1, k-1, k2, k-2 - rate constant - indicate the
speed or efficiency of a reaction
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The Michaelis-Menten Equation
The basic equation derived by Michaelis and
Menten to explain enzyme-catalyzed reactions is
Km - Michaelis constant
Vo initial
velocity caused by substrate concentration, S

Vmax maximum velocity
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Effect of enzyme concentration E on velocity
(v)
In fixed, saturating S, the higher the
concentration of enzyme, the greater the initial
reaction rate This relationship will hold as long
as there is enough substrate present
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Enzyme inhibition
In a tissue and cell different chemical agents
(metabolites, substrate analogs, toxins, drugs,
metal complexes etc) can inhibit the enzyme
activity
Inhibitor (I) binds to an enzyme and prevents the
formation of ES complex or breakdown it to E P
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Reversible and irreversible inhibitors
Reversible inhibitors after combining with
enzyme (EI complex is formed) can rapidly
dissociate
Enzyme is
inactive only when bound to inhibitor EI complex
is held together by weak, noncovalent interaction
Three basic types of reversible inhibition
Competitive, Uncompetitive, Noncompetitive
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Reversible inhibition
Competitive inhibition Inhibitor has a
structure similar to the substrate thus can bind
to the same active site The enzyme cannot
differentiate between the two compounds When
inhibitor binds, prevents the substrate from
binding Inhibitor can be released by increasing
substrate concentration
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Competitive inhibition
Example of competitive inhibition
Benzamidine competes with arginine for binding to
trypsin
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Noncompetitive inhibition
Binds to an enzyme site different from the
active site Inhibitor and substrate can bind
enzyme at the same time Cannot be overcome by
increasing the substrate concentration
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Uncompetitive inhibition

• Uncompetitive inhibitors bind to ES not to free E
• This type of inhibition usually only occurs in
  multisubstrate reactions

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Irreversible Enzyme Inhibition
very slow dissociation of EI complex Tightly
bound through covalent or noncovalent interactions
Irreversible inhibitors group-specific
reagents substrate analogs suicide inhibitors
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Group-specific reagents react with specific R
groups of amino acids
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Substrate analogs structurally similar to the
substrate for the enzyme -covalently modify
active site residues
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Suicide inhibitors
Inhibitor binds as a substrate and is initially
processed by the normal catalytic mechanism

It then
generates a chemically reactive intermediate that
inactivates the enzyme through covalent
modification Suicide because enzyme
participates in its own irreversible inhibition

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Regulation of enzyme activity
Methods of regulation of enzyme activity
Allosteric control Reversible covalent
modification Isozymes (isoenzymes)
Proteolytic activation
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Allosteric enzymes
Allosteric enzymes have a second regulatory site
(allosteric site) distinct from the active
site Allosteric enzymes contain more than one
polypeptide chain (have quaternary
structure). Allosteric modulators bind
noncovalently to allosteric site and regulate
enzyme activity via conformational changes
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2 types of modulators (inhibitors or activators)
Negative modulator (inhibitor) binds to the
allosteric site and inhibits the action of the
enzyme usually it is the end product of a
biosynthetic pathway - end-product (feedback)
inhibition Positive modulator
(activator) binds to the allosteric site and
stimulates activity usually it is the substrate
of the reaction
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Example of allosteric enzyme - phosphofructokinase
-1 (PFK-1)

• PFK-1 catalyzes an early step in glycolysis
• Phosphoenol pyruvate (PEP), an intermediate near
  the end of the pathway is an allosteric inhibitor
  of PFK-1

PEP
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Regulation of enzyme activity by covalent
modification
Covalent attachment of a molecule to an amino
acid side chain of a protein can modify activity
of enzyme
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Phosphorylation reaction
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Dephosphorylation reaction
Usually phosphorylated enzymes are active, but
there are exceptions (glycogen synthase)
Enzymes taking part in phospho-rylation are
called protein kinases Enzymes taking part in
dephosphorylation are called phosphatases
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Isoenzymes (isozymes)
Some metabolic processes are regulated by enzymes
that exist in different molecular forms -
isoenzymes

• Isoenzymes - multiple forms of an enzyme which
  differ in amino acid sequence but catalyze the
  same reaction
• Isoenzymes can differ in
• kinetics,
• regulatory properties,
• the form of coenzyme they prefer and
• distribution in cell and tissues
• Isoenzymes are coded by different genes

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Lactate dehydrogenase tetramer (four subunits)
composed of two types of polypeptide chains, M
and H

• There are 5 Isozymes of LDG
• H4 heart
• HM3
• H2M2
• H3M
• M4 liver, muscle
  

H4 highest affinity best in aerobic
environment M4 lowest affinity best in
anaerobic environment
Isoenzymes are important for diagnosis of
different diseases
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Activation by proteolytic cleavage
Many enzymes are synthesized as inactive
precursors (zymogens) that are activated by
proteolytic cleavage Proteolytic activation
only occurs once in the life of an enzyme molecule
Examples of specific proteolysis Digestive
enzymes Synthesized as zymogens in stomach and
pancreas Blood clotting enzymes Cascade of
proteolytic activations Protein hormones
Proinsulin to insulin by removal of a peptide
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Multienzyme Complexes and Multifunctional Enzymes

• Multienzyme complexes - different enzymes that
  catalyze sequential reactions in the same pathway
  are bound together
• Multifunctional enzymes - different activities
  may be found on a single, multifunctional
  polypeptide chain

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Metabolite channeling

• Metabolite channeling - channeling of
  reactants between active sites
• Occurs when the product of one reaction is
  transferred directly to the next active site
  without entering the bulk solvent
• Can greatly increase rate of a reactions
• Channeling is possible in multienzyme complexes
  and multifunctional enzymes