Glycogen storage disease type I similar, but Ib incurs a few additional problems (described below). Glycogen in liver and (to a lesser degree) kidneys serves as a form of stored, rapidly accessible glucose, so that the blood glucose level can be maintained between meals. For about 3 hours after a carbohydrate-containing meal, high insulin levels direct liver cells to take glucose from the blood, to convert it to glucose-6-phosphate (G6P) with the enzyme glucokinase, and to add the G6P molecules to the ends of chains of glycogen (glycogen synthesis). Excess G6P is also shunted into production of triglycerides and exported for storage in adipose tissue
Glycogen phosphorylase residues from the outer branch to the other end, and then a α1-6 glucosidase enzyme is required to break the remaining (single glucose) α1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue. The enzyme is specific to α1-4 chains, as the molecule contains a 30-angstrom-long crevice with the same radius as the helix formed by the glycogen chain; this accommodates 4-5 glucosyl residues, but is too narrow for branches. This crevice connects the glycogen storage site to the active, catalytic site. Glycogen phosphorylase has a pyridoxal phosphate (PLP, derived from Vitamin