Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis—the phosphorolytic cleavage of the 1→ 4 link ages of glycogen to yield glucose-1-phosphate (Figure 1). There are different isoenzymes of glycogen phosphorylase in liver, muscle, and brain, encoded by different genes. Glycogen phosphorylase requires pyridoxal phosphate as its coenzyme. Unlike the reactions of amino acid metabolism, in which the aldehyde group of the coenzyme is the reactive group, in phosphorylase the phosphate group is catalytically active.
Fig1. steps in glycogenolysis.
The terminal glucosyl residues from the outermost chains of the glycogen molecule are removed sequentially until approximately four glucose residues remain on either side of a 1 → 6 branch (see Figure 1). The debranching enzyme has two catalytic sites in a single polypeptide chain. One is a glucan transferase that transfers a trisaccharide unit from one branch to the other, exposing the 1 → 6 branch point. The other is a 1,6-glycosidase that catalyzes hydrolysis of the 1 →6-glycoside bond to liberate free glucose. Further phosphorylase action can then proceed. The combined action of phosphorylase and these other enzymes leads to the complete breakdown of glycogen.
The reaction catalyzed by phosphoglucomutase is reversible, so that glucose-1-phosphate derived from hydrolysis of glycogen can form glucose-6-phosphate. In liver, but not muscle, glucose-6-phosphatase catalyzes hydrolysis of glucose-6-phosphate, yielding glucose that is exported, leading to an increase in the blood glucose concentration. Glucose 6-phosphatase is in the lumen of the smooth endoplasmic reticulum, and genetic defects of the glucose-6-phosphate transporter can cause a variant of type I glycogen storage disease (Table 1).
Table1. Glycogen Storage Diseases
Glycogen granules can also be engulfed by lysosomes, where acid maltase catalyzes the hydrolysis of glycogen to glucose. This may be especially important in glucose homeostasis in neonates. Genetic lack of lysosomal acid maltase causes type II glycogen storage disease (Pompe disease; see Table 1). The lysosomal catabolism of glycogen is under hormonal control.
