THE PARALLEL (ALPHA BETA)(8)-BARREL - PERHAPS THE MOST UNIVERSAL AND THE MOST PUZZLING PROTEIN-FOLDING MOTIF/

The presented review deals with the family of proteins adopting the (a lpha/beta)(8)-barrel structural fold. Special emphasis is given to the groups of (alpha/beta)(8)-barrel enzymes for which there is good evid ence of evolutionary divergence from a common ancestor, firstly from s equence homology, such as starch hydrolases and related enzymes and fl avin oxidase/dehydrogenase enzymes, and secondly from functional and s tructural data, such as enzymes involved in tryptophan biosynthesis. T he presence of a common phosphate binding site in at least twelve diff erent (alpha/beta)(8)-barrel enzymes is reviewed. Two other groups of (alpha/beta)(8)-barrel are described, the so-called transition metal b inding superfamily and the 4/7 superfamily. Finally, recent work using so-called ''hidden homologies'' is discussed which may suggest that a significant proportion of (alpha/beta)(8)-barrels may be diverged fro m a very distant common ancestor. In general, however, the question of evolution of these enzymes still remains open.