allosteric site

Meaning: An allosteric site is a location on a protein, distinct from the active site, where an effector molecule binds. Binding causes a conformational change that is passed through the protein and shifts it between a more active (relaxed, R) state and a less active (tense, T) state. Activators stabilize the active form; inhibitors stabilize the inactive form. Allosteric enzymes usually have several subunits, so binding at one subunit can change the others (cooperativity).

On the MCAT: Expect feedback inhibition, where the end product of a pathway binds an allosteric site on an early enzyme and slows the whole pathway. Expect velocity versus [S] graphs: allosteric enzymes give a sigmoidal curve, an activator shifts it left, and an inhibitor shifts it right or lowers it. Hemoglobin is the other standard case. H⁺, CO₂ and 2,3-BPG bind away from the heme groups and lower oxygen affinity, which shifts the saturation curve right.

Common confusion: Students equate an allosteric inhibitor with a competitive inhibitor. A competitive inhibitor binds the active site, so extra substrate can overcome it. An allosteric inhibitor binds elsewhere, so extra substrate usually cannot fully reverse it. Also, allosteric does not mean inhibitory: effectors can be activators. It is not identical to classic noncompetitive inhibition either, because allosteric effectors can change apparent substrate affinity as well as Vmax.