Characterization of the GH13 and GH57 glycogen branching enzymes from Petrotoga mobilis SJ95 and potential role in glycogen biosynthesis

Glycogen is a highly branched alpha-glucan polymer widely used as energy and carbon reserve by many microorganisms. The branches are introduced by glycogen branching enzymes (EC 2.4.1.18), that are classified into glycoside hydrolase families 13 (GH13) and 57 (GH57). Most microorganisms have typically only a single glycogen branching enzyme (gbe) gene. Only a few microorganisms carry both GH13 and GH57 gbe genes, such as Petrotoga mobilis and Mycobacterium tuberculosis. Here we report the basic characteristics of the GH13 and GH57 GBE of P. mobilis, both heterologously expressed in E. coli. The GH13 GBE has a considerably higher branching activity towards the linear alpha-glucan amylose, and produces a highly branched alpha-glucan with a high molecular weight which is very similar to glycogen. The GH57 GBE, on the contrary, makes a much smaller branched alpha-glucan. While the GH13 GBE acts as a classical glycogen branching enzyme involved in glycogen synthesis, the role of GH57 GBE remains unclear.