Biochemical characterization of plasmodium falciparum glutamine amidotransferase and adenosine triphosphate pyrophosphatase : domains of GMP synthetase

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.