Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2007
Publication Date: 1/1/2008
Citation: Jordan, D.B. 2008. Beta-D-xylosidase from Selenomonas ruminantium: catalyzed reactions with natural and artificial substrates. Applied Biochemistry and Biotechnology. 146:137-149. Interpretive Summary: Agricultural biomass like crop residues, grain processing byproducts, dedicated energy crops (e.g., switchgrass), etc., represent an abundant, renewable feedstock for production of ethanol and other valuable products, if practical conversion technologies can be developed. These materials are rich in complex carbohydrates that must first be broken down to simple sugars that can be fermented by microorganisms to ethanol and other products. A critical step in the development of new conversion processes is the discovery and development of new enzymes to convert these complex materials to simple sugars. We have discovered an enzyme involved in the final step in the hydrolysis of xylan, the second most abundant carbohydrate in plants. This enzyme produces the simple sugar, xylose, more efficiently than other enzymes described by other workers. The enzyme also catalyzes the release of arabinose from arabinosides. This work reveals pH dependencies of the enzyme acting on natural substrates and the affinities of reaction products for the enzyme. Our results will help the development of new bioconversion strategies to produce fuel ethanol economically.
Technical Abstract: Catalytically efficient beta-D-xylosidase from Selenomonas ruminantium (SXA) exhibits pKa’s 5 and 7 (assigned to catalytic base, D14, and catalytic acid, E186) for kcat/Km with 1,4-beta-D xylobiose (X2) and 1,4-beta-D-xylotriose (X3). Catalytically inactive, dianionic SXA (D14**-E186**-) has 3-fold lower affinity than catalytically-active, monoanionic SXA (D14**-E186**H) for X2 and X3, whereas D14**-E186**- has 2-fold higher affinity than D14**-E186**H for 4-nitrophenyl-beta-D-xylopyranoside (4NPX) and D14**-E186**- has no affinity for 4-nitrophenyl-alpha-L-arabinofuranoside (4NPA). Alpha-D-xylose and beta-D-xylose have similar affinity for SXA. 4-nitrophenol competitively inhibits SXA-catalyzed hydrolysis of 4NPX. SXA steady-state kinetic parameters account for complete progress curves of SXA-catalyzed hydrolysis reactions.