enzyme catalysing same reaction and examples of isoenzymes

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