Cocatalytic zinc binding sites are characteristic of enzyme molecules which contain two or more zinc and/or other metal atoms. In each site an aspartate, glutamate, or histidine residue simultaneously binds to two zinc atoms or a zinc and a different metal atom. In the resultant amino acid bridge, two of the cocatalytic metal atoms bind to the same amino acid. Consequently the participating metal atoms are in close proximity and function as a catalytic unit, typical of this motif. In these functional units aspartate seems to be preferred over glutamate. Serine, threonine, tryptophan, and lysine residues are encountered as zinc ligands, although they have not so far been identified as ligands in monozinc enzymes or DNA-binding zinc proteins. The resultant coordination spheres and their mechanistic implications raise interesting questions for further study.