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Endo–1,4-β-mannanases (β-mannanases) randomly hydrolyse the mannosidic bonds within the main chain of various mannans and heteromannans. Some of these polysaccharides are hemicelluloses, a major part of the plant cell-wall. The β-mannanases have been assigned to family 5 and 26 of the glycoside hydrolase clan A. This work presents a detailed kinetic analysis of the family 26 β-mannanase CfMan26A from the soil-bacterium Cellulomonas fimi. The full-length enzyme consists of five modules: a family 26 catalytic module, an immunoglobulin-like module, a mannan-binding module, a surface layer homology-module and a module of unknown function. A truncated variant consisting of the catalytic module and the immunoglobulin-like module was used in these studies. The degradation of mannotriose, mannotetraose and mannopentaose was studied by 1H-NMR. First, the mutarotation of one of the hydrolysis products (mannose) was determined to be 1.7 10−5s−1 at 5°C and pH 5.0. As expected for a family 26 glycoside hydrolase, the ...

The chemical mechanism of a retaining beta-mannosidase from Cellulomonas fimi has been characterized through steady-state kinetic analyses with a range of substrates, coupled with chemical rescue studies on both the wild-type enzyme and mutants in which active site carboxyl groups have been replaced. Studies with a series of aryl beta-mannosides of vastly different reactivities (pK(a)(lg) = 4-10) allowed kinetic isolation of the glycosylation and deglycosylation steps. Substrate inhibition was observed for all but the least reactive of these substrates. Brønsted analysis of k(cat) revealed a downward breaking plot (beta(lg) = -0.54 +/- 0.05) that is consistent with a change in rate-determining step (glycosylation to deglycosylation), and this was confirmed by partitioning studies with ethylene glycol. The pH dependence of k(cat)/K(m) follows an apparent single ionization of a group of pK(a) = 7.65 that must be protonated for catalysis. The tentative assignment of E429 as the acid-base catalyst of Man2A on the basis of sequence alignments with other family 2 glycosidases was confirmed by the increased turnover rate observed for the mutant E429A in the presence of azide and fluoride, leading to the production of beta-mannosyl azide and beta-mannosyl fluoride, respectively. A pH-dependent chemical rescue of E429A activity is also observed with citrate. Substantial oxocarbenium ion character at the transition state was demonstrated by the alpha-deuterium kinetic isotope effect for Man2A E429A of alpha-D(V) = 1.12 +/- 0.01. Surprisingly, this isotope effect was substantially greater in the presence of azide (alpha-D(V) = 1.166 +/- 0.009). Likely involvement of acid/base catalysis was revealed by the pH dependence of k(cat) for Man2A E429A, which follows a bell-shaped profile described by pK(a) values of 6.1 and 8.4, substantially different from that of the wild-type enzyme. The glycosidic bond cleaving activity of Man2A E519A and E519S nucleophile mutants is restored with azide and fluoride and appears to correlate with the corresponding "glycosynthase" activities. The contribution of the substrate 2-hydroxyl to stabilization of the Man2A glycosylation transition state (DeltaDeltaG() = 5.1 kcal mol(-1)) was probed using a 2-deoxymannose substrate. This value, surprisingly, is comparable to that found from equivalent studies with beta-glucosidases despite the geometric differences at C-2 and the importance of hydrogen bonding at that position. Modes of stabilizing the mannosidase transition state are discussed.

Engineering enzymes: The glutamic acid nucleophile of a retaining β-mannosidase has been replaced with a serine residue to form a β-mannosynthase. When the new enzyme is provided with an α-mannosyl fluoride donor and an appropriate acceptor, β-mannoside linkages are synthesized. Remarkably, α-mannosyl fluoride can be generated in situ by providing the mannosynthase with excess fluoride ion.

Incubation of the beta-mannosidase Man2A from Cellulomonas fimi with 2-deoxy-2-fluoro-beta-D-mannosyl fluoride (2FMan beta F) resulted in time-dependent inactivation of the enzyme (inactivation rate constant k(i)=0.57 min(-1), dissociation constant for the inactivator K(i)=0.41 mM) through the accumulation of a covalent 2-deoxy-2-fluoro-alpha-D-mannosyl-beta-mannosidase 2A (2FMan-Man2A) enzyme intermediate, as observed by electrospray ionization mass spectrometry. The stoichiometry of inactivation was 1:1. Removal of excess inactivator and regeneration of active enzyme by transglycosylation of the covalently attached inhibitor to gentiobiose [Glc beta(1-6)Glc] demonstrated that the covalent intermediate was catalytically competent. Comparison by MS of the peptic digests of 2FMan-Man2A with peptic digests of native Man2A revealed a peptide of m/z 1520 that was unique to 2FMan-Man2A, and one of m/z 1036.5 that was unique to a Man2A peptide. Their sequences, determined by collision-induced fragmentation, were CSEFGFQGPPTW and FGFQGPPTW, corresponding to residues 517-528 and 520-528 of Man2A respectively. The difference in mass of 483.5 between the two peptides equals the sum of the masses of the tripeptide CSE plus that of 2-fluoromannose. It was concluded that in 2FMan-Man2A, the 2-fluoromannose esterified to Glu-519 blocks hydrolysis of the Glu-519-Phe-520 peptide bond, and that Glu-519 is the catalytic nucleophile in this enzyme. This residue is conserved in all members of family 2 of the glycosyl hydrolases. This represents the first ever labelling and identification of an active-site nucleophile in a beta-mannosidase.

Mutant endo-mannanases, in which the catalytic nucleophile has been replaced, function as glycosynthases in the synthesis of manno-oligosaccharides of defined lengths.

The endo-beta-1,4-mannanase from the soil bacterium Cellulomonas fimi is a modular plant cell wall degrading enzyme involved in the hydrolysis of the backbone of mannan, one of the most abundant polysaccharides of the hemicellulosic network in the plant cell wall. The crystal structure of a recombinant truncated endo-beta-1,4-mannanase from C. fimi (CfMan26A-50K) was determined by X-ray crystallography to 2.25 A resolution using the molecular replacement technique. The overall structure of the enzyme consists of a core (beta/alpha)8-barrel catalytic module characteristic of clan GH-A, connected via a linker to an immunoglobulin-like module of unknown function. A complex with the oligosaccharide mannotriose to 2.9 A resolution has also been obtained. Both the native structure and the complex show a cacodylate ion bound at the -1 subsite, while subsites -2, -3, and -4 are occupied by mannotriose in the complex. Enzyme kinetic analysis and the analysis of hydrolysis products from manno-oligosaccharides and mannopentitol suggest five important active-site cleft subsites. CfMan26A-50K has a high affinity -3 subsite with Phe325 as an aromatic platform, which explains the mannose releasing property of the enzyme. Structural differences with the homologous Cellvibrio japonicus beta-1,4-mannanase (CjMan26A) at the -2 and -3 subsites may explain the poor performance of CfMan26A mutants as "glycosynthases".

Engineering enzymes: The glutamic acid nucleophile of a retaining β-mannosidase has been replaced with a serine residue to form a β-mannosynthase. When the new enzyme is provided with an α-mannosyl fluoride donor and an appropriate acceptor, β-mannoside linkages are synthesized. Remarkably, α-mannosyl fluoride can be generated in situ by providing the mannosynthase with excess fluoride ion.