Evolution of parallel β/α-barrel enzyme family lightened by structural data on starch-processing enzymes

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Abstract
The parallel β/α-barrel domain consisting of eight parallel β-sheets surrounded by eight α-helices has been currently identified in crystal structures of more than 20 enzymes. This type of protein folding motif makes it possible to catalyze various biochemical reactions on a variety of substrates (i.e., it seems to be robust enough so that different enzymatic functionalities could be designed on it). In spite of many efforts aimed at elucidation of evolutionary history of the present-day β/α-barrels, a challenging question remains unanswered: How has the parallel β/α-barrel fold arisen? Although the complete sequence comparison of all β/α-barrel amino acid sequences is not yet available, several sequence similarities have been revealed by using the highly conserved regions of α-amylase as structural templates. Since many starch-processing enzymes adopt the parallel β/α-barrel structure these enzymes might be useful in the search for evolutionary relationships of the whole parallel eight-folded β/α-barrel enzyme family.