Substrate specificity of family 5, 6, 7, 9, 12, and 45 endoglucanases

Endoglucanases are important enzymes for biomass conversion and other industrial processes. Determining the specificity of endoglucanases from various glycoside hydrolase families is of interest for bioinformatic functional prediction and substrate-tailored enzyme development. To do so, we characterized ∼30 endoglucanases from six glycoside hydrolase families. For p-nitrophenyl cellobioside and lactoside, only family 7 enzymes showed significant activity. For xyloglucan, both family 7 and 12 enzymes showed significant activity. For xylan and arabinoxylan, only family 7 enzymes showed significant activity. For mannan and galactomannan, both family 5 and 9 enzymes showed significant activity. The difference in specificity was preliminarily attributed mainly to the structural difference of the enzymes’ active sites. For family 7 endoglucanases, difference in thermal stability might affect their performance in hydrolyzing various (hemi)cellulose substrates. Phylogenetic analysis on the subfamily distribution of family 5 endoglucanases (in relation with mannanases) suggested that their mannanase side-activity might be the remnant of an ancestral multi-function enzyme. Similar analysis was also made with the xyloglucanase or arabionxylans side-activity of family 12 and 7 endoglucanases. The apparent dependence of the specificity on family (primary/tertiary structure) might assist us in better understanding the structure–function relationship of the enzymes, and developing more versatile biocatalysts for the utilization of biomass.