Microbial Strategies for the Depolymerization of Glucuronoxylan: Leads to Biotechnological Applications of Endoxylanases

Abstract

The hemicellulose fractions of crop residues and hardwoods are underutilized resources for conversion to alternative fuels and bio-based products. The predominant hemicellulosic polymer in these resources is 4-O-methylglucuronoxylan in which 4-O-methyl-D-glucuronopyranosyl (MeGA) residues are linked α -1,2- to 5 to 15% of the xylose residues comprising a nearly linear β -1,4-xylan backbone. Enzymatic processing of methylglucuronoxylan to fermentable xylose utilizes microbial endoxylanases, β -xylosidases and α -glucuronidases. Endoxylanases have been assigned to glycohydrolase families, GH5, GH10 and GH11, each with unique sequences and structural motifs. Based upon the detailed characterization of selected members, the GH10 and GH11 xylanases respectively generate aldotetrauronic (MeGAXXX) and aldopentauronic (XMeGAXXX) acids, along with xylooligosaccharides, as predominant limit products (Biely, P.; Vrsanka, M; Tenkanen, M; Klupful, D.; J. Biotechnol. 1997, 57, 151-166). The GH5 xylanase from Erwinia chrysanthemi cleaves β -1,4-xylosidic linkages at a residue adjacent to a xylose residue that is substituted with