Isoenzymes

ISOENZYMES (ISOZYMES)

• Isoenzymes are the physically distinct forms of the same enzyme.
• Isoenzymes differ from each other structurally, electrophoretically and immunologically.
• Isoenzymes possess quaternary structure and are made up of two or three different subunit (mutimeric).
• Isoenzymes catalyze the same reaction and act on same substrate, but with different Km and Vmax values i.e. isozymes have different kinetics.

Plasma Enzymes-

• Functional plasma enzymes- specific function in the plasma. E.g. Coagulation factors (thrombin), lipoprotein lipase, clotting factors.
• Non functional enzymes- E.g. alkaline phosphatase, acid phosphatase, gamma glutamyl transpeptidase, LDH, Creatine kinase.

Lactate Dehydrogenase (LDH)

• LDH with two subunits- H(heart) and M(muscle)
• It has five isoenzymes.

Name of the isoenzyme	Tissue location
LDH-1	Heart muscle
LDH-2	RBC
LDH-3	Brain
LDH-4	Liver and skeletal muscle
LDH-5	Liver and skeletal muscle

• In MI, LDH1 is raised more than LDH2.
• Normal LDH pattern on electrophoresis is LDH2 > LDHI > LDH 3 > LDH 4>LDH5
• Increased in LDH level- pancreatitis.

Creatine Kinase (CK)

• Three isoenzymes.
• It is subunit M(myocardium) and B (Brain).

Name of isoenzyme	Tissue localisation
CK-1	Brain
CK-2	Heart
CK-3	Skeletal Muscle

Alkaline Phosphatase (ALP)-

Isoenzyme	Tissue Location
Alpha-1 ALP	Epithelial cells of biliary canaculi
Alpha-2 heat liable ALP	Hepatic cells
Alpha-2 heat stable ALP	Placenta (Inhibited by Phenylalanine)
Pre beta ALP	Osteoblast
Gamma ALP	Intestinal cells
Leukocyte ALP	Leukocytes

• Raised activity of alkaline phosphatase is useful in diagnosis of bone and liver pathology.

 Transaminase-

• Aspartate Transaminase- increase in myocardium.
• Mitochondrial isoenzyme present in liver.
• Alanine Transaminase (ALT)-  mainly in liver and entirely in Cytoplasm. 

Protease-

• Serine Proteases– serine residue at the active site(serine, histidine, aspirate).
• E.g.- Trypsin, chymotrypsin, elastase (catalytic traid)
• Inhibited by disopropylphosphofluridate binds covalently to serine residue.
• Activated in intestine by proteolytic activation.

Carboxyl or acid Proteases-

• Most important carboxyl proteases is Pepsin.

Don’t Forget to Solve all the previous Year Question asked on ISOENZYMES