Expansion
Glycogen synthase builds; glycogen phosphorylase breaks down
Expansion
Synthesis
- Glycogen synthase adds glucose in alpha-1,4 linkages
- Branching enzyme creates alpha-1,6 branches every 8 to 12 residues, increasing solubility and the number of ends available for rapid mobilisation
Breakdown
- Glycogen phosphorylase cleaves alpha-1,4 bonds, releasing glucose-1-phosphate
- Debranching enzyme handles the alpha-1,6 branch points
Stores: about 100 g in liver (for blood glucose, lasting roughly 24 hours) and 400 g in muscle (for local use only).
Control is reciprocal and by phosphorylation: glucagon and adrenaline raise cyclic AMP, activating phosphorylase and inhibiting synthase; insulin does the opposite through protein phosphatase-1.
Glycogen storage diseases map onto the enzymes:
- Type I (von Gierke): glucose-6-phosphatase. Severe fasting hypoglycaemia
- Type II (Pompe): lysosomal acid maltase. Cardiomyopathy; a lysosomal rather than cytoplasmic defect
- Type III (Cori): debranching enzyme
- Type V (McArdle): muscle phosphorylase. Exercise intolerance with cramps, a second wind phenomenon and myoglobinuria, but normal blood glucose