Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2006
Publication Date: February 1, 2007
Citation: Jordan, D.B., Li, X., Dunlap, C.A., Whitehead, T.R., Cotta, M.A. 2007. Beta-D-xylosidase from Selenomonas ruminantium of glycoside hydrolase family 43. Applied Biochemistry and Biotechnology. 136-140:93-104. Interpretive Summary: Agricultural biomass like crop residues, grain processing byproducts, dedicated energy crops (e.g., switchgrass), etc., represent abundant, renewable feedstocks 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 becomes unstable at low pH (<4.3) or high temperature (>50 deg C), but it is protected from destabilization by the presence of monosaccharides. Our results will help us and other researchers in the development of new bioconversion strategies to produce fuel ethanol economically.
Technical Abstract: Beta-D-xylosidase from the ruminal anaerobic bacterium, Selenomonas ruminantium (SXA), catalyzes the hydrolysis of beta-1,4-xylooligosacharides and has potential utility in industrial saccharification processes. The enzyme, heterologously produced in Escherichia coli and purified to homogeneity, has an isoelectric point of ~4.4, an intact N terminus, and a Stokes radius that defines a homotetramer. SXA denatures between pH 4.0 and 4.3 and between 50 and 60 deg C. Following heat or acid treatment, partially inactivated SXA exhibits lower kcat values but similar Km values as untreated SXA, consistent with denaturation of a portion of enzyme and the remainder retaining catalytic properties of the native enzyme. Individually, D-glucose and D-xylose protect SXA from inactivation at high temperature and low pH; Ki values for slowing first order inactivation rates of SXA by the two monosaccharides are similar to their respective Ki values for competitive inhibition of SXA catalysis. Affinities of D-glucose and D-xylose for SXA decrease with decreasing pH.