Isoenzymes are multiple forms of the same enzyme that catalyze the same reaction but differ in physical and chemical properties. Isoenzymes exist for several reasons, including being synthesized from different genes, consisting of different subunit combinations, or having differences in post-translational modifications. Creatine kinase exists as three main isoenzymes (CK-BB, CK-MB, CK-MM) that are dimers of M and B subunits. The CK-MB isoenzyme is useful for detecting myocardial infarction as its levels are elevated in blood after a heart attack.

1. ISOENZYMES
The multiple forms of an enzyme catalyzing the same
reaction are isoenzymes or isozymes.
They, however, differ in their physical and chemical
properties which include the structure, electrophoretic
and immunological properties, Km and Vmax values,
pH optimum, relative susceptibility to inhibitors and
degree of denaturation.
Although isoenzymes of a given enzyme catalyse the
same reaction, they differ in Km, Vmax or both. e.g.
isoenzymes of LDH and CPK.

2. 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.
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

3. TYPES OF ISOENZYMES
1) Lactate Dehydrogenase (LDH)
2) Creatine Kinase (CK)
3) Alkaline Phosphatase (ALP)
4) Alcohol Dehydrogenase (ADH)
5) Troponin
6) Aldolases
7) Amylases
8) Glycosyl transferases II (GT II), etc.,
In this presentation, we are going to learn about
isoenzymes of Creatine Kinase (CK)

4. Isoenzymes of creatine phosphokinase (CK)
Creatine kinase (CK) or creatine phosphokinase (CPK)
catalyses the inter-conversion of phosphocreatine (or
creatine phosphate) to creatine.
CPK exists as three isoenzymes.
Each isoenzyme is a dimer composed of two subunits—M
(muscle) or B (brain) or both.

5. Types of CK isoenzymes

6. CK isoenzymes can be separated by:
1.Electrophoresis.
2.Ion exchange chromatography techniques.

8. CK-BB

9. CK-MB
In myocardial infarction:
MB accounts for 4.5 to 20 per cent of the total CK activity in plasma
of patients with a recent myocardial infarction and the total MB
isoenzyme level is elevated up to 20-fold above normal.
In healthy individuals, the isoenzyme CPK2 (MB) is almost
undetectable in serum with less than 2% of total CPK.
After the myocardial infarction (MI), within the first 6-18 hours,
CPK2 increases in the serum to as high as 20% (against 2% normal).
CPK2 isoenzyme is not elevated in skeletal muscle disorders.
Therefore, estimation of the enzyme CPK2 (MB) is the earliest
reliable indication of myocardial infarction.

12. CK-MM

13. Total serum CK

14. -R. PRIYA
THIRD YEAR
B.SC. BIOCHEMISTRY

15. Reference:
1. Biochemistry, 4th edition, book by U. Satyanarayana and U.
Chakrapani.
2. www.slideshare.com ppts about isoenzymes are referred.