New microbial mannan catabolic pathway that involves a novel mannosylglucose phosphorylase

► We performed the functional analysis of an operon in the genome sequence of Bacteroides fragilis. ► We propose a new mannan catabolic pathway that involves mannobiose-forming 1,4-β-mannanase, mannobiose and/or sugar transporter, mannobiose 2-epimerase, and a novel mannosylglucose phosphorylase. ► The novel enzyme plays a vital role in the mannan-degrading enzyme system in some microbes.