allosteric

Meaning: "Allosteric" means "other site." An effector binds away from the active site and shifts the protein between a low-affinity tense (T) state and a high-affinity relaxed (R) state. Activators stabilize the R state; inhibitors stabilize the T state. Many allosteric proteins have several subunits and show cooperativity: binding at one subunit changes the affinity of the others. Hemoglobin is the standard example, with H⁺, CO₂ and 2,3-BPG acting as effectors that favor the T state and release O₂. Feedback inhibition is another common use, where a pathway's end product inhibits an enzyme early in that pathway.

On the MCAT: A passage shows rate versus [S] or saturation versus partial pressure. A sigmoidal curve points to cooperativity and allosteric regulation, whereas a hyperbola fits simple Michaelis–Menten behavior. An activator typically shifts the curve left (higher apparent affinity) and an inhibitor shifts it right. You may also be asked to identify a feedback loop from a pathway diagram.

Common confusion: Allosteric regulation is not the same as competitive inhibition. Competitive inhibitors bind the active site and usually resemble the substrate. Allosteric effectors bind elsewhere and usually look nothing like the substrate. Allosteric does not mean inhibitory, since activators are allosteric too.