Isoenzymes are multiple forms of enzymes that catalyze the same reaction but differ in physical and chemical properties. Lactate dehydrogenase is an example of an isoenzyme that is made up of different combinations of H and M subunits, resulting in five isoenzyme forms. The isoenzyme patterns in tissue and serum can be used for clinical diagnosis, such as an increased level of the LDH5 isoenzyme indicating liver disease, or higher levels of CPK2 signaling myocardial infarction.

1. Isoenzyme
Multiple forms of enzymes catalysing the same reaction
are isoenzymes or isozymes.
ex: GLUCOKINASE & HEXOKINASE, both cataylse
conversion of glucose to glucose-6-phosphate.
Isoenzymes differ in their physical and chemical
properties like structure, immunologic prop.., Kₘ &
Vₘₐₓ, pH optimum, inhibitors & degree of denaturation.

2. isoenzymes existence’s explanation:-
• Isoenzymes are synthesized from diff. genes, eg:
malate dehydrogenase of cytosol is synthesized from
diff. genes while same enzyme is synthesized from
diff. genes in mitochondria.
• oligomeric enzymes(more than 1 subunits) consist
more than 1 type os subunits. i.e. lactate
dehydrogenase (it’s isoenzymes have diff. subunits
along with similar units.
• Isoenzymes may be active as monomer or oligomer,
e.g. glutamate dehydrogenase.
• difference in carbohydrate content may be also
responsible for isoenzymes: alkaline phosphate.

3. • lactate dehydrogenase catalyses conversion of
pyruvate to lactate.
• Made of 2 units H&M
• H = acidic, M = basic.
• 5 isoenzymes LDH (1 to 5).

4. Isoenzyme Subunit
consti…
origin Destruction
at 60⁰ C
% normal
serum in
humans
Elecrophoret
ic mobility
LDH₁ H₄ Heart and
RBC
No 25 Fastest
LDH₂ HM₃ Heart and
RBC
No 35 Faster
LDH₃ H₂M₂ Brain and
kidney
Partially 27 Fast
LDH₄ H₃M Liver and
skeletal
muscle
Yes 8 Slow
LDH₅ M₄ Skeletal
muscle and
liver
Yes 5 Slowest

5. • LDH₁ CATALYSES IN HEART CONVERSION OF
PYRUVATE TO ACETYL CO .A & INHIBITED BY
PYRUVATE
• LDH₅ CATALYSES CONVERSION OF PYRUVATE
TO LACTATE IN LIVER AND NOT INHIBITED Y
PYRUVATE.
• DIAGNOSIS: LDH₂ IS MORE ACTIVE THAN LDH₁,
IN REVERSE CONDITION MYOCARDIAL
INFARCTION ID DIGNOSIED
• WHILE INCREASE ACTIVITY OF LDH₅
INDICATED LIVER DISEASE.
• THIS ENZYME IS 800-1000 TIMES MORE ACTIVE
IN RBC THAN SERUM.

6. CREATININE PHOSPHokINASE
• THIS ENZYME HAVE 3 ISOENZYME
• MADE OF 2 UNITS M&B

7. CPK₂ is less than or 2% in serum, thus its
increase(20%) indiactes myocardial infarction
while during muscle disease CPK₃ is not
elevated.
ISOENZYME SUBUNIT TISSUE OF ORIGIN
CPK₁ BB BRAIN
CPK₂ MB HEART
CPK₃ MM SKELETAL MUSCLE