Isoenzymes (or isozymes) are a group of enzymes that catalyze the same reaction but have different enzyme forms and catalytic efficiencies. Isozymes are usually distinguished by their electrophoretic mobilities.

1. ISOENZYMES
By
Mr. Abhijit Bhoyar
Department of
Child Health Nursing

2. Specific Learning Objectives
At the end of the lecture, student should be able to:
1. Define isoenzymes
2. Explain properties of isoenzyme

3. Introduction
• The multiple forms of an enzyme catalysing the same
reaction are isoenzymes or isozymes. They, however,
differ in their physical and chemical properties which
include the structure, electrophoretic and
immunological properties, K m and V max values, pH
optimum, relative susceptibility to inhibitors and degree
of denaturation.

4. Isoenzymes
Definition
• Isoenzymes is a group of enzymes that
catalyze the same reaction but have different
enzyme forms and catalytic efficiencies.

5. • In biochemistry, isozymes are enzymes that differ in
amino acid sequence but catalyze the same chemical
reaction.
• Isozymes usually have different kinetic parameters, or
are regulated differently. They permit the fine-tuning of
metabolism to meet the particular needs of a given
tissue or developmental stage.

6. Types

9. Explanation for the existence of isoenzymes
• Many possible reasons are offered to explain the
presence of isoenzymes in the living systems.
• 1. Isoenzymes synthesized from different genes e.g.
malate dehydrogenase of cytosol is different from that
found in mitochondria.
• 2. Oligomeric enzymes consisting of more than one
type of subunits e.g. lactate dehydrogenase and
creatine phosphokinase.

10. Cont..
• 3. An enzyme may be active as monomer or oligomer
e.g. glutamate dehydrogenase.
• 4. In glycoprotein enzymes, differences in carbohydrate
content may be responsible for isoenzymes e.g.
alkaline phosphatase.

11. Isoenzymes of lactate dehydrogenase (LDH)
• LDH whose systematic name is L-lactateNAD+
oxidoreductase (E.C. 1.1.1.27) catalyses the
interconversion of lactate and pyruvate

12. Structure of LDH isoenzymes :
• LDH is an oligomeric (tetrameric) enzyme made up of
four polypeptide subunits.
• Two types of subunits namely M (for muscle) and H (for
heart) are produced by different genes.
• M–subunit is basic while H subunit is acidic.
• The isoenzymes contain either one or both the
subunits giving LDH1 to LDH5.

13. Significance of differential catalytic activity :
LDH1 (H4)
• LDH1 (H4) is predominantly found in heart muscle and
is inhibited by pyruvate – the substrate. Hence,
pyruvate is not converted to lactate in cardiac muscle
but is converted to acetyl CoA which enters citric acid
cycle.
• LDH5 (M4) is mostly present in skeletal muscle and the
inhibition of this enzyme by pyruvate is minimal,hence
pyruvate is converted to lactate. Further,LDH5 has low
Km (high affinity) while LDH1 has high Km (low affinity)
for pyruvate.

14. • The differential catalytic activities of LDH1 and LDH5 in
heart and skeletal muscle, respectively, are well suited
for the aerobic (presence of oxygen) and anaerobic
(absence of oxygen) conditions, prevailing in these
tissues.

15. Diagnostic importance of LDH :
• Isoenzymes of LDH have immense value in the
diagnosis of heart and liver related disorders
• In healthy individuals, the activity of LDH2 is higher
than that of LDH1 in serum.
• In the case of myocardial infarction, LDH1 is much
greater than LDH2 and this happens within 12 to
24 hours after infarction. Increased activity of LDH

16. Isoenzymes of creatine phosphokinase
Creatine kinase (CK) or creatine phosphokinase
(CPK) catalyses the inter-conversion of
phosphocreatine (or creatine phosphate) to creat

17. Isoenzymes of alkaline phosphatase
• As many as six isoenzymes of alkaline
phosphatase (ALP) have been identified.
• ALP is a monomer, the isoenzymes are due to
the difference in the carbohydrate content (sialic
acid residues).
• The most important ALP isoenzymes are D1-
ALP, D2-heat labile ALP, D2-heat stable ALP,
pre-E ALP, J-ALP etc

18. Isoenzymes of alcohol dehydrogenase
• Alcohol dehydrogenase (ADH) has two
heterodimer isoenzymes.
• Among the white Americans and Europeans, DE1
isoenzyme is predominant whereas in Japanese
and Chinese (Orientals) DE2 is mostly present.
• The isomer DE2 more rapidly converts alcohol to
acetaldehyde.

19. Properties of isoenzymes
1. Electrophoretic- E.g Isoenzymes of Lactate
dehydrogenase have mobility different electrophoretic
mobility
2. Heat stability-E.g Alkaline phosphatase isoenzymes
are either heat labile or stable
3. Inhibitor- E.g An inhibitor can inhibit only one
isoenzyme of an enzyme eg. Acid phosphatase

20. Cont..
4. Cofactors Mitochondrial isocitrate dehydrogenase
requires NAD+ , cytosolic form requires NADP+
5. Tissue localization-Eg LDH 1 is present in heart,
LDH 5 in muscle
6. Antibodies-For creatine kinase, each isoenzyme can
be bound only by a specific antibody 3

21. Expsected Question
• Essay / Situational Question
• Describe isoenzymes with reference to definition and
properties , chemical nature and factors affecting
isoenzyme activity
• Short Question
1. Explain characteristic of isoenzyme
2. Describe Classification of isoenzyme
3. Describe factors influencing isoenzyme activity

22. THANK YOU