The starch-binding domain from glucoamylase disrupts the structure of starch

The full-length glucoamylase from Aspergillus niger, G1, consists of an N-t erminal catalytic domain followed by a semi-rigid linker (which together co nstitute the G2 form) and a C-terminal starch-binding domain (SBD), G1 and G2 both liberate glucose from insoluble corn starch, although G2 has a rate 80 times slower than G1, Following pre-incubation of the starch with SBD, the activity of G1 is uniformly reduced with increasing concentrations of S BD because of competition for binding sites. However, increasing concentrat ions of SBD produce an initial increase in the catalytic rate of G2, follow ed by a decrease at higher SBD concentrations, The results show that SBD ha s two functions: it binds to the starch, but it also disrupts the surface, thereby enhancing the amylolytic rate. (C) 1999 Federation of European Bioc hemical Societies.