GlgB

glgB is the gene that encodes the glycogen branching enzyme in many bacteria. The enzyme is commonly referred to as the branching enzyme or α-1,4→α-1,6-transglucosidase and is essential for constructing branch points in glycogen. Its catalytic activity transfers a fragment of a growing α-1,4-glucan chain to a new glucose residue, creating an α-1,6 glycosidic linkage (EC 2.4.1.18). This branching increases glycogen’s solubility and optimizes its degradation, enabling rapid mobilization of glucose.

Genetic and cellular context. glgB is typically part of the glycogen biosynthesis cluster, often alongside glgA (glycogen synthase) and glgC (ADP-glucose pyrophosphorylase). In many bacteria, the enzyme operates in the cytosol as part of the glycogen biosynthetic pathway, and its expression is coordinated with cellular energy status and carbon availability. The structure of GlgB is generally modular, with a catalytic core and auxiliary regions that aid substrate recognition; many GlgB proteins form dimers or higher-order oligomers in solution.

Functional consequences of glgB disruption. Mutants lacking glgB tend to accumulate glycogen with fewer branches and longer linear segments, resulting in altered glucan structure and changes in glycogen metabolism and turnover under certain conditions. Such phenotypes underscore the importance of branching for glycogen architecture and accessibility to degradative enzymes.

Distribution and related enzymes. GlgB homologs are widespread among bacteria and are central to glycogen biosynthesis in these organisms. In plants and animals, analogous activities are carried out by starch- or glycogen-branching enzymes, which serve a similar biochemical role but belong to different gene families. glgB thus represents a key component of the conserved mechanism to generate highly branched glucose polymers.