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Feeder Pathways for Glycolysis:- Glycogen and Starch Are Degraded by Phosphorolysis

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p534-535

2026-06-01

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Feeder Pathways for Glycolysis:- Glycogen and Starch Are Degraded by Phosphorolysis

Glycogen in animal tissues and in microorganisms (and starch in plants) can be mobilized for use within the same cell by a phosphorolytic reaction catalyzed by glycogen phosphorylase (starch phosphorylasein plants). These enzymes catalyze an attack by Pion the (α1→4) glycosidic linkage that joins the last two glucose residues at a nonreducing end, generating glucose 1-phosphate and a polymer one glucose unit shorter (Fig. 14–10). Phosphorolysis preserves some of the en ergy of the glycosidic bond in the phosphate ester glucose 1-phosphate. Glycogen phosphorylase (or starch phosphorylase) acts repetitively until it approaches an (α1→6) branch point (see Fig. 7–15), where its action stops. A debranching enzyme removes the branches. The mechanisms and control of glycogen degradation are described in detail in Chapter 15. Glucose 1-phosphate produced by glycogen phos phorylase is converted to glucose 6-phosphate by phosphoglucomutase, which catalyzes the reversible reaction

Glucose 1-phosphate⇌glucose 6-phosphate

The glucose 6-phosphate thus formed can enter glycolysis or another pathway such as the pentose phosphate pathway, described in Section 14.5. Phosphoglucomu tase employs essentially the same mechanism as phos phoglycerate mutase. The general name mutaseis given to enzymes that catalyze the transfer of a functional group from one position to another in the same molecule. Mutases are a subclass of isomerases, enzymes that interconvert stereoisomers or structural or positional isomers.

FIGURE 14–9 Entry of glycogen, starch, disaccharides, and hexoses into the preparatory stage of glycolysis.

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