active site 1 the general region of an enzyme molecule containing the catalytic residues identified with the binding and reaction of substrate( s). It includes those amino-acid residues that are, in the enzyme– substrate complex, either contact amino acids, i.e. those that at some point are only one bond distance removed from some point of the substrate molecule, or auxiliary amino acids, i.e. those that are not in such intimate physical contact with the substrate but nonetheless play a definite role in the action of the enzyme. See also contributing amino acid. 2 (sometimes) the portion of a peptide hormone responsible for its biological activity. Isolated fragments containing the active site may show some activity, but may function less efficiently than the intact hormone.
