section 15.2

Glycogen Metabolism

285

16G-1-P

1

1

Glycogen

phosphorylase

16 R

Debranching

enzyme

Glucose

HO

2

ch> ck> ckx5><x x x >o o o o

FIGURE 15-8

Catabolism of a small glycogen molecule by glycogen phosphorylase and debranching enzyme. P; = inorganic

phosphate; G-l-P = glucose-1-phosphate.

glycogenin and up to 60,000 glucosyl units

{ß

-particles).

In the liver, 20-40 ^-particles are aggregated into rosettes,

known as (a-particles).

Glycogen Breakdown

Glycogenolysis is catalyzed by two enzymes unique to

the pathway: glycogen phosphorylase and debranching en-

zyme. The former normally regulates the rate of glucose

release from glycogen. The progressive degradation of

glycogen is illustrated in Figure 15-8. Glycogen phospho-

rylase catalyzes the release of glucose-

1

-phosphate from

the terminal residue of a nonreducing end of a glyco-

gen branch, by means of phosphorolysis. A molecule of

inorganic phosphate attacks the Ci side of an

a(\ —>■

4) glycosidic bond, leaving a hydroxyl group on C

4

that remains in the glycogen polymer. The reaction is

analogous to hydrolysis, in which water attacks and

cleaves bonds. The phosphorolysis reaction is summarized

below.

(Glucose)« + P j ^ (glucose)«_i + glucose-1-Phosphate

The energy stored in the a (1 —> 4) glycosidic bond during

the condensation reaction in glycogen synthesis is suffi-

cient to permit the formation of a glucose-phosphate bond

without using ATP.