CoachAn allosteric enzyme has several subunits and, besides its active sites, regulatory sites where effectors bind: activators shift it toward its active conformation, inhibitors toward the inactive one. Its rate against is sigmoid, not hyperbolic (cooperativity among the active sites, as for haemoglobin, Chapter 12), so that a small change of substrate near the steep part changes the rate greatly; effectors shift the curve sideways (changing the substrate concentration needed) or up and down (changing the maximal rate). Allosteric enzymes stand at the branch points of metabolism, and the effectors are the pathway’s own products and the cell’s energy signals (ATP, ADP, AMP).
![An allosteric enzyme. The sigmoid curve makes the rate sensitive to substrate near the midpoint; an activator shifts it left (more active at a given [S]), an inhibitor right. Near [S] = 3 the rate can swing from a tenth to nine tenths of maximum.](/images/onecourse/chapters/biology-3/b1-enzymes/fig-0bb8ee6cbc43.svg)
Examples
Example 13.13 (Isoenzymes)
Lactate dehydrogenase exists in five forms, tetramers of two subunit types in all combinations; the heart’s form has a low for lactate and is inhibited by pyruvate (it oxidises lactate to feed the Krebs cycle), the muscle’s form has a high and tolerates pyruvate (it makes lactate in a sprint). The pattern of forms in the blood reveals which organ has been damaged — a heart attack releases the heart’s.