Microbial Glucuronoyl Esterases: 10 Years after Discovery

Abstract

A carbohydrate esterase called glucuronoyl esterase (GE) was discovered 10 years ago in a cellulolytic system of the wood-rotting fungus Schizophyllum commune Genes coding for GEs were subsequently found in a number of microbial genomes, and a new family of carbohydrate esterases (CE15) has been established. The multidomain structures of GEs, together with their catalytic properties on artificial substrates and positive effect on enzymatic saccharification of plant biomass, led to the view that the esterases evolved for hydrolysis of the ester linkages between 4-O-methyl-d-glucuronic acid of plant glucuronoxylans and lignin alcohols, one of the crosslinks in the plant cell walls. This idea of the function of GEs is further supported by the effects of cloning of fungal GEs in plants and by very recently reported evidence for changes in the size of isolated lignin-carbohydrate complexes due to uronic acid de-esterification. These facts make GEs interesting candidates for biotechnological applications in plant biomass processing and genetic modification of plants. This article is a brief summary of current knowledge of these relatively recent and unexplored esterases.

FIG 1

Phylogenetic tree of confirmed and putative GEs, acetylxylan esterases, feruloyl esterases, and pectin methyl esterases, constructed by rearrangement of the published data (9). The protein sequences are marked by accession numbers given in databases.

FIG 2

3D structures of the catalytic domain of GE from T. reesei (19) (A) and GE from M. thermophila (20) (B). Both structures show the twisted β-sheet structure sandwiched between two layers of α-helixes. Strands are labeled with the letter “S” followed by a number, helices with the letter “H” followed by a number, C termini with “C,” and N termini with “N.” The catalytic triad Ser, His, and Glu, shown in balls and sticks (S278, H411, and E301 in T. reesei GE; S213, H346, and E236 in M. thermophila GE), is in both cases located on the surface of the enzyme.

FIG 3

Fragment of alkali-extracted beech wood glucuronoxylan and its methyl ester serving as the substrate of GEs (22). The site of attack is marked by an arrow.

FIG 4

Esters of GlcA (left) and MeGlcA (right) methyl glycoside with the largest aryl propane structures found to be hydrolyzed by GEs so far (23–25). The sites of attack are marked by arrows.

FIG 5

Scheme of the β-glucuronidase-coupled assay of glucuronoyl esterase on methyl esters of d-glucuronic acid β-glycosylated with chromophore aglycons. Thus far, the tested substrates have been found to contain 4-nitrophenol and 5-bromo-4-chloro-3-hydroxyindol as aglycons (38).