Abstract:
Allostery refers to a phenomenon wherein binding of a ligand molecule at a
site distinct from the active site brings about changes at the active site. Allostery may
reveal itself as either inhibition or activation of the enzyme activity. Allostery can be
either homotropic, wherein the same ligand binds at the two distinct sites, or
heterotropic, wherein different ligand molecules bind at the allosteric site and the
active site. Heterotropic allosteric activation might either lead to an increase in the
affinity of the substrate at the active site or an increase in the velocity of the reaction.
On the other hand, allosteric inhibition (either non-competitive or uncompetitive)
leads to decrease in both the affinity and velocity of the enzyme for the substrate.
Cooperativity, which is homotropic allostery exhibited exclusively by oligomeric
enzymes, is a special case wherein binding of the substrate at one active site regulates
the affinity of the substrates for the other active sites in the oligomer. This can either
lead to an increase (positive cooperativity) or decrease (negative cooperativity) in the
affinity of substrate binding to each subsequent active site. Most multifunctional
enzymes and multienzyme complexes are regulated by allosteric communication
between active sites. Specialized cases of allostery also include co-ordination of
catalysis across active sites, domain-domain interactions and substrate channeling.
