REGULATION OF ENZYME

REGULATION OF ENZYMES

• Mechanism for regulating the enzyme activity-

1. Allosteric modulation
2. Covalent modification
3. Induction and repression of enzyme synthesis 

1. Allosteric modulation-

• Allosteric Enzymes have a catalytic site where the substrate binds and a separate site where a modifier binds.
• Examples-

Enzyme	Pathway	Stimulator	Inhibitor
Phosphofructokinase	Glycolysis	Fructose-2,6- biphosphate	Citrate
Isocitrate dehydrogenase	TCA
Carbamoyl Phosphate Synthetase I Pynrvate kinase	glycolysis	Fructose-1-6-bisphophate	N Acetyl Glutamate (NAG) ATP alanine
Acetyl CoA Carboxylase Pynrvate carboxylase	Fatty acid synthesis Glyconeogenesis	Citrate Acetl.l  CoA	Acyl CoA AI)P

2. Covalent Modification-

• The most common covalent modification is the addition or removal of a phosphate group.
• Two types-

1.  Irreversible- partial proteolysis/ zymogen activation
2. Reversible- addition/ removal of a particular group.

Methods of Reversible covalent modifications-

1.  Phosphorylation/ dephosphorylation (most common)
2. Methylation
3. Adenylation
4. ADP reibosylation
5. Acetylation

Enzyme	Active	Inactive
Glycogen Synthase	Dephosphorylated	Phosphorylated
Pyruvate dehydrogenase	Dephosphorylated	Phosphorylated
HMG- CoA- reductase	Dephosphorylated	Phosphorylated
HMG- Co reductase kinase	Phosphorylated	Dephosphorylated

• Protein kinase mostly add phosphoryl group to a serine, threonine or tyrosine.
• Specific activity- number of enzyme units per milligram of protein.
• Catalase has the highest turnover number and hence is the fastest active enzyme.

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