Abstract
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: dihydropyrimidinase (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 hydantoinases with known metabolic
functions, such as imidase and carboxyethylhydantoinase and enzymes with
unknown metabolic function, are described in the literature and have not
yet been classified. An important question is whether the distinct
hydantoinases, which are frequently classified as L-, D-, and
non-selective hydantoinases depending on their substrate specificity and
stereoselectivity, are related to each other. In order to investigate
the evolutionary relationship, amino acid sequence data can be used for
a phylogenetic 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-dependent hydantoinases are not related, (ii)
that the urease-related amidases have evolved divergently from a common
ancestor and (iii) that they share a metal-binding motif consisting of
conserved histidine residues. The difference in enantioselectivity used
for the classification of hydantoinases on the 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.
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