allosteric regulation

Meaning: An allosteric effector binds a regulatory site separate from the active site and shifts the protein between a low-activity form (T, tense) and a high-activity form (R, relaxed). Activators stabilize R; inhibitors stabilize T. Many allosteric proteins have several subunits and show cooperativity: binding at one subunit changes affinity at the others, so rate versus [S] is sigmoidal rather than hyperbolic. Feedback inhibition often works this way, with a pathway's end product inhibiting an early enzyme. Hemoglobin and O₂ is the classic nonenzyme example.

On the MCAT: Passages give rate versus [S] curves with and without an effector. An activator typically shifts the curve left (less [S] needed for half-maximal rate); an inhibitor shifts it right. Expect pathway diagrams where you predict how an end product, ATP, or AMP affects an enzyme such as phosphofructokinase-1, and hemoglobin questions on how H⁺, CO₂, or 2,3-BPG shift the O₂ binding curve to the right.

Common confusion: Allosteric does not mean inhibitory, and it is not competitive inhibition. A competitive inhibitor binds the active site and can be outcompeted by high [S]; an allosteric inhibitor binds elsewhere, so added substrate does not fully reverse it.