A unique thermostable lichenase from<i style=""> Thermotoga maritima </i>MSB8 with divergent substrate specificity

Abstract

A putative endoglucanase gene corresponding to locus TM 1752 (AAD36817.1, Q9X274) of <i style="">Thermotoga maritima</i> MSB8 was cloned and expressed in <i style="">Escherichia coli</i>. The enzyme contains a NEP (<i style="">Asn-Glu-Pro</i>) motif but lacks a PCD (proposed catalytic domain) block. This endoglucanase is grouped under family 5 of the glycosyl hydrolases members of which predominantly hydrolyze β-1,4-linkages of carbohydrate polymers. The enzyme efficiently hydrolyzes the glycosidic bonds of mixed β-1,3-1,4 linkages in lichenan and barley β-glucan, but did not display any activity on crystalline cellulose or laminarin. However, negligible amount of hydrolysis was observed with CM-cellulose. This enzyme produced glucosyl β-1,3 glucosyl β-1,4 glucose as the major end product and glucosyl β-1,4 glucosyl β-1,3 glucose was not detected. The putative endoglucanase of <i style="">T. maritima</i> appears to be a unique endoglucanase being the first lichenase producing glucosyl β-1,3 glucosyl β-1,4 glucose to be placed into family 5 of the glucosyl hydrolases.