title: X-ray crystallography and QM/MM investigation on the oligosaccharide synthesis mechanism of rice BGlu1 glycosynthases.
abstract: Nucleophile mutants of retaining β-glycosidase can act as glycosynthases to efficiently catalyze the synthesis of oligosaccharides. Previous studies proved that rice BGlu1 mutants E386G, E386S and E386A catalyze the oligosaccharide synthesis with different rates. The E386G mutant gave the fastest transglucosylation rate, which was approximately 3- and 19-fold faster than those of E386S and E386A. To account for the differences of their activities, in this paper, the X-ray crystal structures of BGlu1 mutants E386S and E386A were solved and compared with that of E386G mutant. However, they show quite similar active sites, which implies that their activities cannot be elucidated from the crystal structures alone. Therefore, a combined quantum mechanical/molecular mechanical (QM/MM) calculations were further performed. Our calculations reveal that the catalytic reaction follows a single-step mechanism, i.e., the extraction of proton by the acid/base, E176, and the formation of glycosidic bond are concerted. The energy barriers are calculated to be 19.9, 21.5 and 21.9kcal/mol for the mutants of E386G, E386S and E386A, respectively, which is consistent with the order of their experimental relative activities. But based on the calculated activation energies, 1.1kcal/mol energy difference may translate to nearly 100 fold rate difference. Although the rate limiting step in these mutants has not been established, considering the size of the product and the nature of the active site, it is likely that the product release, rather than chemistry, is rate limiting in these oligosaccharides synthesis catalyzed by BGlu1 mutants.
authors.affiliation: Key Lab of Theoretical and Computational Chemistry in University of Shandong, School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China.
authors.forename: Jinhu
authors.initials: J
authors.lastname: Wang
authors.forename: Salila
authors.initials: S
authors.lastname: Pengthaisong
authors.forename: James R Ketudat
authors.initials: JR
authors.lastname: Cairns
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Liu
mesh_terms: D019943:Amino Acid Substitution; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D016695:Glycosyltransferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D009844:Oligosaccharides / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics; D012275:Oryza / Q000201:enzymology* / Q000235:genetics; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D009844:Oligosaccharides; D010940:Plant Proteins; D016695:Glycosyltransferases