By Dr. Dipendra Raj Pandeya, PhD Assistant Professor Department of Clinical Laboratory Science College of Applied Medical Sciences Al Jouf University

Glycogen Metabolism

Utilization of Glucose Glucose is utilized in several pathways; Glycogen synthesis Pentose phosphate pathway Lipogenesis Amino acid synthesis Energy metabolism: both anaerobic & aerobic.

Glycogen Glycogen is a homopolysaccharide of D- glucose residues linked by: α(1  4) glycosidic bonds, mainly α(1  6) glycosidic bonds, at branch points. MW 10 8 D. Liver glycogen fluctuates during well fed and fasting state.

Role of glycogen Excess dietary glucose is stored as glycogen Glucose can be rapidly and easily mobilized from glycogen when the need arises. A constant supply of glucose is essential for life because it is the main fuel of the brain and the only energy source that can be used by cells lacking mitochondria or by contracting skeletal muscle.

LOCATION & FUNCTIONS OF GLYCOGEN

Note :- Glycogen in muscles cannot get converted to glucose, instead it gets converted to glucose 6- p due to absence of glucose-6 phosphatase, In liver glycogen is converted to glucose due to presence of that enzyme

Function of Glycogen: Liver glycogen: It maintains normal blood glucose concentration especially during the early stage of fast (between meals). After 12 ‐ 18 hours fasting, liver glycogen is depleted. Muscle glycogen: It acts as a source of energy within the muscle itself especially during muscle contractions. Muscle glycogen is depleted after prolonged exercise.

The enzymes of glycogen metabolism are under hormonal control. Insulin promote Glycogen synthesis Glucagon promote Glycogen breakdown

The synthesis and degradation of glycogen occur by different metabolic pathways allowing for reciprocal regulation. Glycogen synthesis- takes place in well fed condition Glycogen breakdown- takes place in fasting condition

Glycogenesis: Glycogenesis is the formation of glycogen in liver and muscles It occurs in the cytosol Substrates for glycogen synthesis: In liver Blood glucose Other hexoses: fructose & galactose Non-carbohydrate sources: glycerol & lactate glucose, then to glycogen In muscles Blood glucose ONLY gluconeogenesis

STRUCTURE OF GLYCOGEN what is glycogen? It is a highly branched-chain homopolysaccharide made exclusively from a- D-glucose.

Structure of Glycogen

GLYCOGENESIS Glycogenesis is the synthesis of glycogen from glucose. Glycogenesis mainly occurs in muscle and liver. Muscle glycogen provides a readily available source of glucose for glycolysis within the muscle itself. Liver glycogen functions to store and export glucose to maintain blood glucose between meals.

GLYCOGENOLYSIS (BREAKDOWN OF GLYCOGEN) Glycogen breakdown or, it is otherwise known as glycogenolysis. It takes place in the cell cytosol. There are two stages of glycogenolysis: Stage I: Shortening of the glycogen chain Stage II: Removal of branches

Blood glucose levels rise after ingestion of carbohydrates, leading to glycogen synthesis. Inactivation of phosphorylase and an activation ofglycogen synthase. Liver

A Take Home Lesson! Glucogon = starved state; stimulates glycogen breakdown, inhibits glycogen synthesis. High blood glucose levels = fed state; insulin stimulates glycogen synthesis and inhibits glycogen breakdown.

Glycogen storage diseases (GSD) GSD are a group of genetic disorders due to inherited deficiencies in enzymes of glycogen synthesis or degradation pathway in both liver and muscle. The defects in liver enzymes generally cause hepatomegaly (enlarged liver) and hypoglycemia whereas those in muscle enzymes generally cause muscle cramps.

The defect in glycogen metabolism is characterized by impaired conversion of glycogen to free glucose resulting in the accumulation of normal or abnormal glycogen in tissues. Disruption of glycogen metabolism also lead to accumulation of abnormal metabolic by-products can damage other organs.

Types of glycogen storage diseases (GSD) 1.Hepatic type Symptoms: Hepatomegaly, Hypoglycemia 2. Myopathic type Symptoms: Muscle weakness, Cramps following exercise