A regulatory enzyme responsive to alterations in the metabolic state of a cell or tissue. Its catalytic activity is modified by the noncovalent binding of a specific metabolite at a site (see allosteric site) other than the active site. The most common type of allosteric enzyme is one found at the beginning of a multienzyme sequence that is inhibited specifically by the end product of the reaction sequence (*feedback inhibition
). However, the activity of an allosteric enzyme is not always decreased by the action of a modulator; some allosteric modulators increase the enzyme's activity. Allosteric enzymes may also have more than one modulator.
Reactions involving allosteric enzymes are always irreversible in the cell, as it would be impossible to regulate a reversible reaction. Allosteric enzymes are often more complex structurally than other enzymes; all known allosteric enzymes have at least two protein subunits.