Allosteric enzymes

Allosteric enzymes are those that are controlled by the binding of an allosteric effector. The effector may be positive or negative in activating or inactivating the enzyme, respectively, and creates a conformational change of the enzyme. The product may be the allosteric effector in itself producing feedback or feedforward control.

There are two classes of allosteric enzymes based on the effect of the effector on Km and Vmax. If the effector alters Km it is in K class and if the effector alters Vmax it is in V class. There are also enzymes that have both Km and Vmax affected.
K class allosteric enzymes are affected by negative effector binding because it affects the affinity of the binding site for the substrate. V class allosteric enzymes are affective positively or negatively by effectors that increase or decrease rate of enzyme-substrate complex breakdown to products.

Given the conformational change and resulting activation and inactivation of an enzyme, catalysis of reactions would be unidirectional. Examples are hexokinase (or glucokinase), phosphofructokinase, and pyruvate kinase in the regulation of glycolysis. Glucokinase is activated by fructose-6-phosphate and inhibited by fructose-1-phosphate. Phosphofructokinase is activated by fructose 2,6 biphosphate and inhibited by ATP. Pyruvate kinase is activated by fructose-1-6-biphosphate (feedforward) and inhibited by ATP and alanine.

Reference List

1. Devlin TM. Textbook of Biochemistry with Clinical Correlations. Philadelphia: Wiley-Liss, 2002, pp893-4. p401-4; 587-89; 863-5
2. Stanford. Regulation of glycolysis. Available at: http://cmgm.stanford.edu/biochem200/regulation/