Evolution of parallel beta/alpha-barrel enzyme family lightened by structural data on starch-processing enzymes.

The parallel beta/alpha-barrel domain consisting of eight parallel beta-sheets surrounded by eight alpha-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 beta/alpha-barrels, a challenging question remains unanswered: How has the parallel beta/alpha-barrel fold arisen? Although the complete sequence comparison of all beta/alpha-barrel amino acid sequences is not yet available, several sequence similarities have been revealed by using the highly conserved regions of alpha-amylase as structural templates. Since many starch-processing enzymes adopt the parallel beta/alpha-barrel structure these enzymes might be useful in the search for evolutionary relationships of the whole parallel eight-folded beta/alpha-barrel enzyme family.