Briefly outline the steps involved in glycogen synthesis and breakdown and explain the regulatory mechanism involved in glycogen metabolism.

Glycogen is synthesized and degraded by different pathways, which provide greater flexibility for energy production through its degradation or for storage in its synthesis and thus maintain control of both metabolic pathways.

Explanation:

Synthesis and degradation pathways:

1º.- activation of glucose units at UDP-glucose.

UDP-glucose is an activated form of glucose, which is formed in a reaction catalyzed by UDP-glucose, which is formed in a reaction catalyzed by UDP-glucose pyrophosphorylase. This reaction is an example of biosynthetic reactions that are directed by the hydrolysis of pyrophosphate (PPi-2Pi).

Many biosynthetic reactions are directed by the hydrolysis of pyrophosphate, which catalyzes pyrophosphate hydrolase or pyrophosphatase, and is very favorable thermodynamically.

2nd.- polymerization or addition of glucose units to glycogen. Two steps: addition and branching.

 UDP-glucose acts as the giver of glucose units for glycogen synthesis. Glucose transfer

from the UDP-glucose to a growing glycogen chain is catalyzed by glycogen synthase.

A branching enzyme [amyl (1,4 -> 1,6) -transglucosidase], moves a chain of about seven glucose residues, to form

bonds (α1-> 6) at the branching points.

To start glycogen synthesis, glycogen synthase is only effective when it is linked to glycogenin. Glycogenin is

an oligosaccharide carrier protein formed by glucose units with linkages (α1-> 4).

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