STRUCTURAL-ANALYSIS SHOWS 5 GLYCOHYDROLASE FAMILIES DIVERGED FROM A COMMON ANCESTOR

We have solved the X-ray structure of barley chitinase and bacterial c hitosanase. Structural constraints predicted these would work by an in verting mechanism, which has been confirmed biochemically. The two enz ymes were compared with lysozymes from goose (GEWL), phage (T4L), and hen (HEWL). Although the proteins share no significant amino acid simi larities, they are shown to have a structurally invariant core contain ing two helices and a three-stranded beta sheet that form the substrat e binding and catalytic cleft. These enzymes represent a superfamily o f hydrolases arising from the divergent evolution of an ancient protei n. The glycohydrolase superfamily can be structurally divided into a b acterial family (chitosanase and T4L), and a eucaryotic family represe nted by chitinase, GEWL, and HEWL. Both families contain the ancestral core but differ at the amino and carboxy termini. The eucaryotes have a small N terminal domain, while the procaryotes have none. The C ter minal domain of the eucaryotic family contains a single a-helix, while the prokaryotic domain has three antiparallel helices. J. Exp. Zool. 282:127-132, 1998. (C) 1998 Wiley-Liss, Inc.