Victor J. Chan

Functional Cloning and Expression of a Novel Endo-α-1,5-L-Arabinanase from a Metagenomic Library

A novel endo-α-L-arabinanase gene (arn2) was isolated, and expressed in E. coli in active form. The recombinant enzyme (ARN2) had optimum activity at pH 6.0 and 45-50oC with stability between pH 5.0-8.0 and at temperatures up to 40oC. The recombinant ARN2 catalyzed internal cleavage of α-1,5 glycosidic bonds of CM-arabinan, debranched arabinan, linear arabinan, and sugar beet (native) arabinan at rates of decreasing order, and was inactive on wheat arabinoxylan and p-nitrophenyl- α-L-arabinofuranoside. Kinetic analysis showed that branching in the arabinan did not significantly affect the apparent Km values, and the difference in the reaction rates was likely due to the chemical step after substrate binding. The enzyme hydrolyzed arabino-oligosaccharides of DP 6 to smaller oligomers and mostly arabinotriose. Natural and modified arabinans were cleaved to oligomers of various chain lengths, which were progressively hydrolyzed to yield arabinotriose. The pattern of degradation revealed an endo-acting mechanism with arabinotriose as the end product.

Functional Cloning and Expression of the Schizophyllum commune Glucuronoyl Esterase Gene and Characterization of the Recombinant Enzyme

The gene encoding Schizophyllum commune glucuronoyl esterase was identified in the scaffold 17 of the genome, containing two introns of 50 bp and 48 bp, with a transcript sequence of 1179 bp. The gene was synthesized and cloned into Pichia pastoris expression vector pGAPZα to achieve constitutive expression and secretion of the recombinant enzyme in soluble active form. The purified protein was 53 kD with glycosylation and had an acidic pI of 3.7. Activity analysis on several uronic acids and their derivatives suggests that the enzyme recognized only esters of 4-O-methyl-D-glucuronic acid derivatives, even with a 4-nitrophenyl aglycon but did not hydrolyze the ester of D-galacturonic acid. The kinetic values were K m 0.25 mM, V max 16.3 μM·min−1, and k cat 9.27 s−1 with 4-nitrophenyl 2-O-(methyl 4-O-methyl-α-D-glucopyranosyluronate)-β-D-xylopyranoside as the substrate.

Biochemical characterization of uronate dehydrogenases from three Pseudomonads, Chromohalobacter salixigens, and Polaromonas naphthalenivorans.

Enzyme catalysts will be vital in the development of synthetic biology approaches for converting pectinic monosaccharides from citrus and beet processing waste streams to value-added materials. We describe here the biophysical and mechanistic characterization of uronate dehydrogenases from a wide variety of bacterial sources that convert galacturonic acid, the predominate building block of pectin from these plant sources, and glucuronic acid to their corresponding dicarboxylic acids galactarate and glucarate, the latter being a DOE top value biochemical from biomass. The enzymes from Pseudomonas syringae and Polaromonas naphthalenivorans were found to have the highest reported kcat(glucuronic acid) values, on the order of 220-270 s(-1). The thermal stability of this enzyme type is described for the first time here, where it was found that the Kt((0.5)) value range was >20 °C, and the enzyme from Chromohalobacter was moderately thermostable with Kt((0.5))=62.2 °C. The binding mechanism for these bi-substrate enzymes was also investigated in initial rate experiments, where a predominately steady-state ordered binding pattern was indicated.

Cloning and Characterization of an Exo-Xylogucanase from Rumenal Microbial Metagenome

A novel exo-glucanase gene (xeg5B) was isolated from a rumenal microbial metagenome, cloned, and expressed in E. coli. The 1548 bp gene coded for a protein of 516 amino acids, which assumed an (a/b)(8) fold typical of glycoside hydrolase (GH) family 5. The protein molecule consisted of a loop segment blocking one end of the active site, which potentially provided the enzyme with exo-acting property. The recombinant enzyme showed exclusive specificity towards only xyloglucan and oligoxyloglucan substrates with no detectable activity on unsubstituted linear glucans, CMC, laminarin, and lichenan. The major end products of exhaustive hydrolysis were XX (tetrasaccharide) and XG (trisaccharide). The hydrolysis of tamarind xyloglucan followed the Michaelis-Menten kinetics, yielding K(m) and V(max) of 2.12+/-0.13 mg/ml and 0.17+/-0.01 mg/ml/min (37 degrees C, pH 6.0), respectively.

Comparative Characterization of a Bifunctional endo-1,4-β-Mannanase/ 1,3-1,4-β-glucanase and its Individual Domains

A fusion gene isolated from a microbial metagenome encodes a N-terminal endo-1,4- β-mannanase and a C-terminal 1,3-1,4- β -glucanase,. The full-length gene and the individual N- and C-domains were separately cloned and expressed in E coli. The purified whole enzyme hydrolyzed glucomannan, galactomannan, and β-glucan with Km and kcat values 2.2, 2.6, 3.6 mg/ml, and 302, 130, 337 min -1 , respectively. The hydrolysis of β-glucan by the C domain enzyme decreased significantly with added glucomannan to the reaction, suggesting inhibition effect. Analogous result was not observed with the N domain enzyme when β-glucan was added to the reaction. The whole enzyme did not show improvement of efficiency compared to the individual or additive total hydrolysis of the two domain enzymes using single or mixed substrates.

A novel method for rapid and sensitive metagenomic activity screening

Graphical abstract

Absence or presence of metal ion activation in two structurally similar GH43 β-xylosidases

Two GH43 β-xylosidases, RS223-BX from a rice straw metagenomic library, and BoXA from Bacteroides ovatus, that share similar amino acid sequences (81% identical) and 19 of 20 active-site residues, were compared by using site-directed mutagenesis of Asp and His residues implicated in metal binding. Thus, RS223-BX is strongly activated by divalent-metal cations and the previously published X-ray structure of this enzyme shows that a Ca2+ cation is chelated by an active-site Asp carboxyl group and an active-site His. Mutation to Ala causes 90% loss of activity for the Asp mutant and 98% loss of activity for the His mutant, indicating their importance to catalysis. For the other enzyme (BoXA), mutation to Ala causes 20% loss of activity for the His mutant and 40% gain of activity for the Asp mutant, indicating the lack of importance for activity of the native residues and the lack of metal-dependency, given that the Asp residue occupies the active site to secure the metal cation in known metal ion dependent GH43 xylosidases. The high activity of the BoXA mutants compared to that of the analogous RS223-BX mutants further undermines the possibility that BoXA maintains a tightly bound metal cofactor resistant to EDTA extraction. The results strengthen our conclusion that the very similar proteins differ in one being metal ion dependent and one not.