Stereochemical course of hydrolysis catalysed by alpha-L-rhamnosyl and alpha-D-galacturonosyl hydrolases from Aspergillus aculeatus.

The stereochemical course of hydrolysis catalysed by four Aspergillus aculeatus enzymes acting on alpha-L-rhamnosyl and alpha-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 beta-D-GalpA from the non-reducing end of polygalacturonic acid. Similarly, rhamnogalacturonan (RG) hydrolase also acts with inversion of anomeric configuration (e-->a) during hydrolysis of alpha-D-GalpA-(1-->2)-alpha-L-Rhap linkages in RG, initially releasing oligosaccharides with beta-D-GalpA 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. alpha-L-Rhamnosidase and RG-rhamnohydrolase also act with inversion of configuration (a-->e), initially releasing beta-L-Rhap from p-nitrophenyl alpha-L-rhamnopyranoside and RG oligosaccharides, respectively. Thus, all four enzymes examined are inverting hydrolases which probably catalyse hydrolysis via single displacement mechanisms.