BIOCHEMISTRY -
Questions and Answers
units. Isoenzymes of lactate dehydrogenase differ in quaternary structure or sub unit composition. Sub units present in lactate dehydrogenase are H and M type. Different isoenzymes of lactate dehydrogenase and their composition is given below. Name of isoenzyme
Sub unit composition
LDHI
H4 o H H H H
LDH2
H3M or H H H
LDH3
H2 M2 or H H M M
LDH4
HM3 or H MMM
LDH5
M4 or MMMM
Separation of isoenzymes: Electrophoresis is used for separation of lactate dehydrogenase isoenzymes. When serum is subjected to electrophoresis at PH8. 6 the five isoenzymes of lactate dehydrogenase separates into 5 bands. The five isoenzymes bands corresponds to LDH1, LDH2 LDH3, LDH4, and LDH5. Alkaline phosphatase Isoenzymes: Iso enzymes of alkaline phosphatase are tissue or organ specific. Four organ specific isoenzymes are known. They can be separated on electrophoresis. The four organ specific isoenzymes are derived from bone, intestine, liver and placenta. These isoenzymes of alkaline phosphatase differ in composition. They are glycoproteins. The carbohydrate content of isoenzymes is different. Creatine phosphokinase Isoenzymes: Three isoenzymes exist for creatine phosphokinase. Creatine phosphpokinase is a oligomeric protein contain two sub units. Isoenzymes of creatine phosphokinase differ in quaternary structure. Sub unit composition varies among isoenzymes of creatine phosphokinase. Two types of sub units are found in isoenzymes. They an M and B. The subunit composition of three isoenzymes is given below.
Isoenzyme
Subunit Composition
Ck1
BB
Ck2
MB
Ck3
MM
9. Write about regulation enzyme activity by covalent modification. A. 1. By covalently attaching group to enzyme molecule its activity is regulated. 2. Phosphate and nucleotide are groups used to regulate enzyme by covalent attachment. 3. Serine residue of enzyme molecule is site of phosphorylation. Tyrosine residue of enzyme molecule is site of nucleotide attachment. 4. Phosphorylation is catalyzed by protein kinase and adenyl transferase catalyzes nucleotide attachment.
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