Glycogen synthase versus branching enzyme
Glycogen synthase (the enzyme that adds glucose to the end of a chain) lengthens glycogen with alpha-1,4 bonds, and the branching enzyme starts new branches with alpha-1,6 bonds.
Remember
Synthase builds the straight chains with alpha-1,4 bonds and the branching enzyme builds the branches with alpha-1,6 bonds, and the branches give glycogen many free ends for rapid synthesis and breakdown.
Memory aid
synthase makes the stalk, branching enzyme makes the branches
Glycogen synthase makes the long stalk, and the branching enzyme makes the branches that come off it.
Glycogen is a molecule that stores glucose in cells, mostly in skeletal muscle and the liver. Its glucose units are joined into straight chains by α1-4 glycosidic bonds (a glycosidic bond is the link between two sugar units, and the numbers say which carbon atoms are joined). Branches start at α1-6 glycosidic bonds.
Making glycogen from glucose is called glycogenesis, and two enzymes extend the chains and make the branches, working from a short starter chain made by glycogenin (a small protein that primes the process). Glycogen synthase takes glucose from UDP-glucose (uridine diphosphate glucose, the activated form of glucose that acts as the donor) and attaches it to the end of an existing chain through an α1-4 bond, so the straight chain gets longer. It is the rate-limiting enzyme of glycogen synthesis (the slowest, controlled step, which sets the pace). Insulin switches it on, and glucagon and adrenaline switch it off.
The branching enzyme then forms an α1-6 bond to start a new branch, so it changes the shape of the molecule and does not lengthen a chain. Every branch has its own free end, so glycogen can be built and broken down very quickly. Without the branching enzyme, synthase still lengthens the chains, but they stay long and poorly branched, and glucose is released more slowly.
Glycogen phosphorylase (the enzyme that breaks glycogen down) and the debranching enzyme (which removes the α1-6 links during breakdown) take the molecule apart again.
Key facts
- Chain lengthening
- glycogen synthase adds glucose from UDP-glucose to the end of an existing chain through α1-4 bonds.
- New branches
- the branching enzyme forms α1-6 bonds, which create new branch points (where a side chain is attached to the main chain).
- Control
- glycogen synthase is the rate-limiting enzyme of glycogen synthesis; insulin switches it on, and glucagon and adrenaline switch it off.
- Why branch
- every branch has its own free end, so glycogen can be built and broken down very quickly.
- Blocked branching
- without the branching enzyme, glycogen has long, poorly branched chains that release glucose slowly.
- Disease link
- abnormal, poorly branched glycogen builds up in the liver in glycogen storage disease type IV.
Exam traps
- Glycogen synthase does not start new branches, because it adds glucose units to the ends of existing chains and cannot create branches.
- The branching enzyme does not lengthen a chain, because it starts a new branch by forming an α1-6 bond and so adds branch points.
- Glycogen phosphorylase does not extend a chain, because it works in the opposite direction, in glycogenolysis (glycogen breakdown), and removes glucose units from the straight chains.
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