Chapter 21, Problem 1P

Interpretation Introduction

Interpretation:

The three steps in glycogen degradation and the enzymes needed for the same should be determined.

Concept introduction:

The storage form of glucose is glycogen, a polymer of glucose having branches at every ten residues. It is stored in the skeletal muscle cells and liver within the granules of cytoplasm. Glycogen is used for regulation of blood glucose level by the liver and as a form of energy by the skeletal muscles.

Answer to Problem 1P

The breakdown of glycogen is divided into three steps-(i) release of glucose from glycogen catalyzed by glycogen phosphorylase (ii) glycogen remodeling by enzymes transferase and a 1,6 glycosidase and (iii) conversion of glucose-1-phosphate to glucose-6-phosphate with the help of enzyme phosphoglucomutase.

Explanation of Solution

The breakdown of glycogen is divided into three steps-(i) release of glucose from glycogen (ii) glycogen remodeling and (iii) conversion of glucose-1-phosphate to glucose-6-phosphate.

The glucose-6-phosphate can enter the glycolytic pathway or can be transformed into glucose to be released in plasma of blood or enter pentose phosphate pathway. For the synthesis of glycogen from glucose, the monomers of glucose are required to be activated as UDP glucose or uridine diphosphate glucose.

In the first step of glycogen breakdown, phosphorolysis catalyzed by glycogen phosphorylase cleaves an a 1,4 glycosidic bond between a terminal glucose containing a free hydroxyl group on the fourth carbon and its adjacent glucose molecule using orthophosphate. The enzyme glycogen phosphorylase cannot cleave the a 1,6 glycosidic bond that form the branching points.

During the second stage, enzymes transferase and a 1,6 glycosidase helps in modification of glycogen for glycogen phosphorylase to continue the degradation process. A group of three glucose molecules are removed and transferred to another branch of glycogen. The remaining glucose molecule is removed by cleaving the a 1,6 glycosidic bond by the enzyme a 1,6 glycosidase.

In the final stage, conversion of glucose-1-phosphate to glucose-6-phosphate takes place with the help of an enzyme known as phosphoglucomutase.

Conclusion

The breakdown of glycogen is divided into three steps-(i) release of glucose from glycogen catalyzed by glycogen phosphorylase (ii) glycogen remodeling by enzymes transferase and a 1,6 glycosidase and (iii) conversion of glucose-1-phosphate to glucose-6-phosphate with the help of enzyme phosphoglucomutase.