STRUCTURE-FUNCTION-RELATIONSHIPS IN THE CATALYTIC AND STARCH BINDING DOMAINS OF GLUCOAMYLASE

Sixteen primary sequences from five sub-families of fungal, yeast and bacterial glucoamylases were related to structural information from th e model of the catalytic domain of Aspergillus awamori var. X100 gluco amylase obtained by protein crystallography. This domain is composed o f thirteen cr-helices, with five conserved regions defining the active site. Interactions between methyl or-maltoside and active site residu es were modelled, and the importance of these residues on the catalyti c action of different glucoamylases was shown by their presence in eac h primary sequence. The overall structure of the starch binding domain of some fungal glucoamylases was determined based on homology to the C-terminal domains of Bacillus cyclodextrin glucosyltransferases. Crys tallography indicated that this domain contains 6-8 beta-strands and h omology allowed the attribution of a disulfide bridge in the glucoamyl ase starch binding domain. Glucoamylase residues Thr525, Asn530 and Tr p560, homologous to Bacillus stearothemophilus cyclodextrin glucosyltr ansferase residues binding to maltose in the C-terminal domain, could be involved in raw-starch binding. The structure and length of the lin ker region between the catalytic and starch binding domains in fungal glucoamylases can vary substantially, a further indication of the func tional independence of the two domains.