Glycogenolysis Is Not The Reverse Of Glycogenesis But Is A Separate Pathway Figure 181

Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis by promoting the phosphorylytic cleavage by inorganic phosphate (phosphorylysis; cf hy-

Glycogen (1^ 4 and 6 glucosyl units)x

(1^ 4 Glucosyl units)x

Glycogen primer

(1^ 4 Glucosyl units)x

Glycogen primer

Glycogenin

Glycogenin

Insulin

cAMP t

Glucagon Epinephrine

Uridine disphosphate glucose (UDPGlc)

To uronic acid pathway

INORGANIC PYROPHOSPHATASE

Uridine triphosphate (UTP)

Glucose 1-phosphate

Uridine triphosphate (UTP)

Glucose 1-phosphate

Free glucose from debranching enzyme

PHOSPHOGLUCOMUTASE

Glucose 6-phosphate

GLUCOSE-6-PHOSPHATASE

To glycolysis and pentose phosphate pathway

GLUCOKINASE

Glucose

Figure 18-1. Pathway of glycogenesis and of glycogenolysis in the liver. Two high-energy phosphates are used in the incorporation of 1 mol of glucose into glycogen. ©, stimulation; 0, inhibition. Insulin decreases the level of cAMP only after it has been raised by glucagon or epinephrine—ie, it antagonizes their action. Glucagon is active in heart muscle but not in skeletal muscle. At asterisk: Glucan transferase and debranching enzyme appear to be two separate activities of the same enzyme.

Table 18-1. Storage of carbohydrate in postabsorptive normal adult humans (70 kg).

Liver glycogen

4.0%

72 g1

Muscle glycogen

0.7%

245 g2

Extracellular glucose

0.1%

10 g3

327 g

1Liver weight 1800 g. 2Muscle mass 35 kg. 3Total volume 10 L.

1Liver weight 1800 g. 2Muscle mass 35 kg. 3Total volume 10 L.

drolysis) of the 1^4 linkages of glycogen to yield glucose 1-phosphate. 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 (Figure 18-4). Another enzyme (a-[1v4]va-[1v4] glucan transferase) transfers a trisaccharide unit from one branch to the other, exposing the 1^6 branch point. Hydrolysis of the 1^6 linkages requires the de-branching enzyme. Further phosphorylase action can

6ch2oh H J-O

kOH H

Uracil

HO OH

Uracil

Ribose

Glucose Diphosphate Uridine

Figure 18-2. Uridine diphosphate glucose (UDPGlc).

then proceed. The combined action of phosphorylase and these other enzymes leads to the complete breakdown of glycogen. The reaction catalyzed by phospho-glucomutase is reversible, so that glucose 6-phosphate can be formed from glucose 1-phosphate. In liver (and kidney), but not in muscle, there is a specific enzyme, glucose-6-phosphatase, that hydrolyzes glucose 6-phosphate, yielding glucose that is exported, leading to an increase in the blood glucose concentration.

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