Microbial glucuronoyl esterases – ten years after discovery

One of the cross-links in plant cell walls contributing to their recalcitrance is the ester linkage between 4-O-methyl-D-glucuronic acid (MeGlcA) residues of xylans and hydroxyl groups of lignin alcohols. Ten years ago we published the first evidence for existence of an esterase that could cleave such linkages. An enzyme found in the wood-rotting fungus Schizophyllum commune was capable of hydrolyzing various artificial MeGlcA acid esters with alkyl and aryl alcohols and was named glucuronoyl esterase (GE). The sequence of the first gene coding for GE was identified in Trichoderma reeseias cip2, originally described by Genencor as a gene of unknown function inducible by cellulose and sophorose. Occurrence of similar genes in fungal and bacterial genomes led to foundation of carbohydrate esterase family CE15. Surprisingly, the physiological role of GEs still has not been demonstrated on natural substrates, such as plant cell walls or lignin-carbohydrate complexes. The evidence for possible role in plant cell wall destruction has so far been obtained indirectly by expressing exogenous GE genes in plants. Patent literature on similarly genetically modified plants claims a decrease of ester linkages in cell walls, facilitating their enzymatic saccharification. New GE substrates were recently developed for screening of genomic libraries and selection and development of industrially relevant GEs. We have introduced also an NMR assay of GEs on artificial polymeric substrate, glucuronoxylan methyl ester.

Acknowledgments. This work was supported by the Slovak Research and Development Agency under contract No. APVV-0602-12, and by Scientific Grant Agency under contract No. 2/0037/14.