<p>Members of this group are predicted, based on distant sequence similarity, to be hydrolases with an alpha/beta hydrolase fold. This prediction is strengthened by several observations: (1) the proteins fit a three-dimensional motif that summarises the structure and sequence variations of the active sites of known alpha/beta hydrolases; (2) members align with structurally known hydrolases using a threading fold-prediction algorithm; and (3) members fit a conservation profile produced by a multiple sequence alignment of alpha/beta hydrolases [<cite idref="PUB00016121"/>].</p> <p>The plant members of this group have been detected as embryogenesis-associated, late embryonic-abundance proteins [<cite idref="PUB00016035"/>].</p> <p>The alpha/beta hydrolase fold is found in a wide range of enzymes with different catalytic functions [<cite idref="PUB00004484"/>]. These enzymes operate on different substrates, and may lack detectable sequence similarity. Nevertheless, they share structural similarity (an open twisted beta sheet surrounded on both sides by alpha helices) and the arrangement of three catalytic residues (the "catalytic triad"): a nucleophile (usually Ser), an acid (usually Asp), and histidine [<cite idref="PUB00016005"/>, <cite idref="PUB00004958"/>]. The functional roles of the triad residues are as follows: the side chain group at the Ser position serves as a nucleophilic centre, the His side chain acts as a general base and is hydrogen bonded to the carboxylic group of the Asp side chain. Together, His and Asp form a charge relay system [<cite idref="PUB00016121"/>].</p> AB-hydrolase YheT, putative