branching enzyme

Meaning: Glycogen synthase adds glucose from UDP-glucose to the nonreducing end of a chain by α-1,4 bonds, but it cannot make a branch. Branching enzyme (a transglycosylase) cuts an α-1,4 bond near the end of a growing chain and reattaches that fragment, about seven residues, to a C6 hydroxyl on a glucose farther inside the chain. This forms an α-1,6 bond, which is the branch point. It moves existing residues and does not add new glucose.

Branching gives many nonreducing ends, so synthase (in building) and glycogen phosphorylase (in breakdown) can work at many ends at once. It also makes the polymer more compact and more soluble.

On the MCAT: Expect a passage describing a mutation or deficiency, then a question on the result: fewer branches, longer unbranched chains, fewer ends, and slower glucose addition or release. Another common question asks which enzyme forms α-1,6 linkages. Questions may also compare glycogen with amylopectin, the branched component of plant starch, which is also branched with α-1,6 links.

Common confusion: Students mix up branching enzyme with debranching enzyme. Debranching enzyme works in breakdown. It moves a few residues from a branch to another chain end, then an α-1,6-glucosidase activity releases the last branch glucose as free glucose. Phosphorylase, not branching enzyme, cleaves α-1,4 bonds using phosphate to give glucose-1-phosphate.