A B C D Product
• One enzyme regulates the rate of overall metabolic sequence
as per the cell demands.
• Such regulatory enzymes are capable of increasing or
decreasing their catalytic activity in response to certain
• Usually the first enzyme of the sequence is the regulatory
• Four major classes of enzymes:
- Allosteric or noncovalently regulated
- Covalently regulated
- Regulation by induction and repression of enzyme synthesis.
- Regulation by proteolytic cleavage
• Allosteric enzymes have one or more allosteric sites
• Allosteric sites are binding sites distinct from an enzyme’s active site or
• Molecules that bind to allosteric sites are called effectors or modulators
• Binding to allosteric sites alters the activity of the enzyme. This is called
• Effectors may be positive or negative
• Effectors may be homotropic or heterotropic
• Regulatory enzymes of metabolic pathways are allosteric enzymes (eg: feedback
Allosteric enzymes tend to be
multi-sub unit proteins.
The reversible binding of an
allosteric modulator (here a
positive modulator M) affects
the substrate binding site.
Feedback allosteric inhibition
Glycine + Succinyl-CoA
δ- Aminolevulenic acid synthase
δ- Aminolevulenic acid
Homotropic effectors: Substrate itself acts as an effector
Heterotopic effectors : Metabolites other than substrate acts as modulator
Covalently regulated enzymes
Phosphorylation or Dephosphorylation from specific serine, threonine or tyrosine residues of enzyme
Ser Thr Tyr (His)
Glycogen phosphorylase b Glycogen phosphorylase a
Enzymes regulated by induction and repression of
• Induction and repression leads to alteration in the total
population of active sites.
Enzymes regulated by specific proteolytic cleavage
Cascade regulatory mechanism