No abstract

The catalytic mechanism of the retaining beta-glucosidase (Abg) from Agrobacterium faecalis involves a double-displacement process in which an alpha-glucosyl-enzyme intermediate is formed with general acid catalytic assistance and then hydrolyzed with general base assistance. Glu170 was identified as an important residue, possibly the acid/base catalyst, on the basis of sequence alignments. This glutamate is conserved in almost all enzymes in family 1 of glycoside hydrolases. Detailed pre-steady-state and steady-state kinetic analyses of the mutant E170G suggested very strongly that Glu170 is the acid/base catalyst. First, kcat values were invariant with pH over the range of 5.0-9.0. Secondly, rates of formation of the glycosyl-enzyme, calculated from kcat/Km and k2, were similar to those of wild-type enzyme for substrates not requiring protonic assistance but dramatically reduced for those needing acid catalysis. Thirdly, addition of azide as a competitive nucleophile increased kcat values 100-300-fold for substrates whose rate-limiting step is deglycosylation, yielding beta-glucosyl azide, but had no effect on the wild-type enzyme. Other anionic nucleophiles had similar, but less dramatic effects. Previous results [Gebler, J.C., et al. (1995) 34, 14547-14553] had indicated that Tyr298F is important for catalysis. The kinetic consequences of the mutations in the double mutant E170G-Y298F are additive, resulting in a 10(6)-fold reduction in kcat values and allowing the accumulation of a stable (t1/2 > 7 h) glucosyl-enzyme intermediate. Thus, Glu170 and Tyr298 function independently, and a possible role for Tyr298 in modulating the pKa of the catalytic nucleophile is proposed.

The catalytic domains of beta-1,4-glucanases can be grouped into families of related amino acid sequences. The endoglucanase CenA from Cellulomonas fimi is a member of family B. All enzymes from this family are believed to hydrolyze beta-1,4-glucosidic bonds using a general acid-base catalytic mechanism resulting in inversion of anomeric configuration at the scissile bond. Three-dimensional structures for two cellulases from family B have been determined by X-ray crystallographic analysis. These structures show that there are four Asp residues which are in a position to function as acid catalyst, base catalyst, and/or transition state stabilizers. These aspartates are conserved in all members of family B. The roles of Asp216, Asp252, Asp287, and Asp392, the corresponding amino acids in CenA, were determined. These aspartates have been systematically replaced with alanine and glutamate via site-directed mutagenesis, and the resulting effect on activity, substrate specificity, and overall structure has been determined. Changes in overall structure were monitored using circular dichroism spectroscopy, and no significant differences between the wild-type and mutant proteins were found. Active site structure was also found to be intact as all proteins bound to a cellobiose affinity column. The Michaelis-Menten parameters of the enzyme were determined on 2,4-dinitrophenyl cellobioside as well as (carboxymethyl)-cellulose and phosphoric acid-swollen cellulose. Initial characterization of mutant proteins indicates that Asp252 and Asp392 are the acid and base catalysts, respectively, in CenA. Residue Asp287 appears to aid Asp252 in acid catalysis, and Asp216 is not absolutely required for catalysis.

The successful syntheses of 5-azido-2,3,4,6-tetra-O-benzyl-5-deoxy-~-glucononitrile and 2,3,4,6-tetra-0-benzyl-5-deoxy-5-trifluoroacetarnido-D-glucononitrile starting from D-glucose are described. Unsuccessful attempts were made to convert these two compounds into a protected 5-amino-5-deoxy-D-glucononitrile and to subsequently cyclize them to an amidine analogue of glucose as a possible glycosidase inhibitor. On dCcrit des synthkses rCalisables des 5-azido-2,3,4,6-tCtra-0-benzyl-5-dCsoxy-et 2,3,4,6-tttra-0-benzyl-5-dCsoxy-5-trifluoroacCtamido-D-glucononitriles ?I partir du D-glucose. On a essay6 sans succks de transformer ces deux compos6s en un ~-amino-5-d6soxy-~-glucononitrile et Cventuellement de les cycliser en une amidine analogue du glucose qui aurait pu &tre tvaluCe cornrne inhibiteur de la glycosidase.Mots cle's : synthkse, amino-sucres, inhibiteurs de la glycosidase.[Traduit par la revue]

Summary ― Four naturally born lambs were placed in sterile isolators 24 h after birth, before the natural establishment of the cellulolytic microorganisms. At the age of 4 weeks, a cellulolytic bacterial population of approximately 10 a cells g-1 of rumen contents was established by inoculation with a 10-6 dilution of ruminal contents taken from an adult sheep. A pure culture of Neocallimastix frontalis MCH 3 and Piromyces communis FL was inoculated into the rumen 5 months after birth and a stable population of 10 3 -10 4 zoospores g-1 developed; the cellulolytic bacteria and fungi established populations in the 4 lambs that were similar to those observed in conventional animals. The presence of fungi led to an increase in the activity of most of the glycoside and polysaccharide hydrolases of the particle-associated microbial populations. However, this effect was not acompanied by an increase in the in sacco degradation of wheat straw or an increase in the volatile fatty acid concentration in the rumen contents.