Stereochemical Course of Hydrolysis Catalysed by α- l-Rhamnosyl and α- d-Galacturonosyl Hydrolases from Aspergillus aculeatus

The stereochemical course of hydrolysis catalysed by four Aspergillus aculeatusenzymes acting on α- l-rhamnosyl and α- d-galacturonosyl linkages in the hairy regions of pectins has been determined using 1H-NMR. Exogalacturonase acts with inversion of anomeric configuration ( e→ a), shown by the initial release of β- d-Gal pA from the non-reducing end of polygalacturonic acid. Similarly, rhamnogalacturonan (RG) hydrolase also acts with inversion of anomeric configuration ( e→ a) during hydrolysis of α- d-Gal pA-(1→2)-α- l-Rha plinkages in RG, initially releasing oligosaccharides with β- d-Gal pA at the reducing end. This result is consistent with the recently solved crystal structure of this enzyme, as well as its classification based on amino acid sequence similarity into glycosyl hydrolase family 28. α- l-Rhamnosidase and RG-rhamnohydrolase also act with inversion of configuration ( a→ e), initially releasing β- l-Rha pfrom p-nitrophenyl α- l-rhamnopyranoside and RG oligosaccharides, respectively. Thus, all four enzymes examined are inverting hydrolases which probably catalyse hydrolysis via single displacement mechanisms.