enzymes which contain regions to which small, regulatory molecules (cf. effector) may bind in addition to and separate from substrate binding sites. On binding the effector, the catalytic activity of the enzyme towards the substrate may be enhanced, in which case the effector is an activator, or reduced, in which case it is an inhibitor.
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 146 (https://doi.org/10.1351/pac199264010143)