Microbial hydantoinases - industrial enzymes from the origin of life?

Hydantoinases are valuable enzymes for the production of optically pure D- and L-amino acids. They catalyse the reversible hydrolytic ring cleavage of hydantoin or 5'-monosubstituted hydantoins and are therefore classified in the EC nomenclature as cyclic amidases (EC 3.5.2.). In the EC nomenclature , four different hydantoin-cleaving enzymes are described: dihydropyrimidin ase (3,5.2.2), allantoinase (EC 3.5.2.5), carboxymethylhydantoinase (EC 3.5 .2.4), and N-methylhydantoinase (EC 3.5.2.14). Beside these, other hydantoi nases with known metabolic functions, such as imidase and carboxyethylhydan toinase and enzymes with unknown metabolic function, are described in the l iterature and have not yet been classified. An important question is whethe r the distinct hydantoinases, which are frequently classified as L-, D-, an d non-selective hydantoinases depending on their substrate specificity and stereoselectivity, are related to each other. In order to investigate the e volutionary relationship, amino acid sequence data can be used for a phylog enetic analysis. Although most of these enzymes only share limited sequence homology (identity <15%) and therefore are only distantly related, it can be shown (i) that most of them are members of a broad set of amidases with similarities to ureases and build a protein superfamily, whereas ATP-depend ent hydantoinases are not related, (ii) that the urease-related amidases ha ve evolved divergently from a common ancestor and (iii) that they share a m etal-binding motif consisting of conserved histidine residues. The differen ce in enantioselectivity used for the classification of hydantoinases on th e basis of their biotechnological value does not reflect their evolutionary relationship, which is to a more diverse group of enzymes than was assumed earlier. This protein superfamily probably has its origin in the prebiotic conditions of the primitive earth.