MBBS Nutrition: Glycogen Synthesis and Degradation

Glycogen Synthesis and Degradation

• Glycogen is a branched polysaccharide with α-1,6 and α-1,4 glycosidic links, and its structure has physiological significance.
• The liver contains 100-120g of glycogen, which is sensitive to blood glucose concentration and plays a crucial role in maintaining constant levels of glucose in the blood.
• Muscle contains 250-300g of glycogen, which is sensitive to energy needs and is used as a rapid source of glucose for muscle contraction.

Features of Biosynthetic Pathways

• ATP, UTP, or GTP are required as cofactors to drive reactions forward.
• One or more enzyme reactions are irreversible, and alternative enzymes are used for the opposite direction.
• Enzyme reactions at the beginning or end of the pathway are tightly regulated.

Glycogen Synthesis

• Glycogen synthesis involves the conversion of glucose to glycogen, with glycogen synthetase as the key enzyme.
• The first stage of glycogen synthesis involves the formation of UDP glucose from glucose-1-P and UTP.
• UDP glucose is then added to a protein primer, glycogenin, to form a glycogen chain.
• Glycogen synthase and branching enzyme work together to produce the branched structure of glycogen.

Regulation of Glycogen Synthesis

• Glycogen synthase is regulated by phosphorylation, which inactivates the enzyme.
• Protein kinase and protein phosphatase play a crucial role in the regulation of glycogen synthase.

Glycogen Breakdown

• Glycogen breakdown involves the conversion of glycogen to glucose, with glycogen phosphorylase as the key enzyme.
• Glycogen phosphorylase breaks down glycogen into glucose-1-phosphate, which is then converted to glucose-6-phosphate.
• Glucose-6-phosphate is then converted to glucose in the liver.

Regulation of Glycogen Breakdown

• Glycogen phosphorylase is regulated by phosphorylation, which activates the enzyme.
• Protein kinase and protein phosphatase play a crucial role in the regulation of glycogen phosphorylase.
• Additional controls include insulin and glucagon, which regulate glycogen phosphorylase in the liver, and adrenaline, calcium, and AMP, which regulate glycogen phosphorylase in muscle.