Download Clinical biochemistry (9) Enzymes and isoenzymes

Enzymes and isoenzymes
Enzymes
General properties of the enzymes
Coenzymes
Zymogens or proenzymes
Factors affecting enzyme activity
The Michaelis-Menten equation
Feedback inhibition
Enzyme inhibition
Isoenzymes or isozymes
Clinical significance of enzymes
Enzymes
 Enzymes are biological catalysts (biocatalysts) which bring
about chemical reactions in the living cells.
 They accelerate the rate of the reaction and usually present in
very small amount in various cells.
 They are organic compounds and number of them have been
obtained in crystalline form
 Today, more than 840 enzymes are known.
General properties of the enzymes
1) They are proteinous in nature.
2) They accelerate the rate of the reaction by: (a) not
altering the reaction equilibrium (b) being required
in minute quantity (c) being not consumed in the
overall reaction.
3) They act as catalysts.
4) They are very specific for their substrates.
5) They possess active sites at which interaction with
substrate occurs.
6) They are responsible for lowering activation energy.
7) Some enzymes are regulatory in function.
Coenzymes
 In addition to the
protein component,
may enzymes require
non-protein
constituents for their
proper functioning.
 When small organic
molecules are bound
tightly either by
covalent or
coordinate bonds
they are called as
prosthetic group.
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They are stable towards heat.
Generally derived from vitamins.
Function as co-substrates
They participate in (a) electron transfer
reaction such as NAD+ , NADH, FMN,
FAD, etc..(b) group transfer reaction
such as, CoA, TPP, pyridoxal phosphate.
Zymogens or proenzymes
Zymogen
Activator
Active enzyme
Pepsinogen
H+, pepsin
pepsin
Trypsinogen
trypsin,
enterokinase
trypsin
Chymotrypsinogen
trypsin
chymotrypsin
Procarboxypeptidase
trypsin
carboxypeptidase
Proelastase
trypsin
elastase
Prorennin
H+, rennin
rennin
Factors affecting enzyme activity
Major factors responsible to affect the enzmic
activity are the following:
1) Concentration of substrate.
2) Concentration of enzyme.
3) Concentration of reaction products.
4) Effect of pH.
5) Effect of temperature.
6) Effect of time.
The Michaelis-Menten equation
The Michaelis constant
(Km) corresponds to the
substrate
concentration that
produces half-maximal
velocity. Km is
independent of
enzyme concentration.
Derivation of the
Michaelis-Menten
equation
A mathematical analysis of the
kinetics of an enzyme catalyzed
reaction was proposed by
Michaelis and Menten.
 K1, K-1 and K2 are rate of
constant for various
reactions.
 Reaction is in steady state,
with concentration ES
constant.
 Reverse reaction is also
negligible. then
Enzymes catalyze with
two or more substrate
In most enzymatic reactions
,however, two (and some
times more) different
substrate molecules bind to
the enzyme and participate
in reaction.
(a) 2 products formed
together.
(b) Products formed one
after another.
Feedback inhibition
or
Feedback control
As seen in the diagram the product (P) acts to inhibit some
early step in the pathway, but not necessarily the first step
in the pathway.
By feedback control it is possible not only to inhibit the
production of (P), but also to divert the flow of (B) from one
pathway to another.
Enzyme inhibition
The rate of an enzyme catalyzed
reaction can be deceased by
specific inhibitor (I) compound
that combine with the enzyme
and prevent normal ES
interactions in the active site,
thus diminishing the rate of the
reaction.
1) Competitive inhibition.
2) Non-competitive inhibition.
3) Mixed inhibition.
Isoenzymes or
isozymes
The term isoenzymes or isozymes
may be define as the enzymes
existing in multiple forms having
Examples of isoenzymes are: similar catalytic activity but
Lactic dehydrogenase (LDH) different biochemical and
immunologic properties ;they can
Alkaline phosphatase (ALP)
be separated by electrophoresis.
Creatine phosphokinase or
creatine kinase (CPK or CK)
Acid phosphatase (ACP)
Amylase
Cholinesterase
Hexokinase
Nearly 100 enzymes have
been detected to exist as
isoenzymes
It has been shown that the relative
amounts of the isoenzymes of
particular enzyme differ in different
organs so that in diseases different
enzyme patterns are found
according to the organ from which
they have come.