Glycogen Metabolism (Glycogenesis)

Learning Objectives To learn about the storage form of Glucose To describe how glucose is converted into its storage form i.e. Glycogen To discuss the regulation of this process

Glycogen Also known as Animal Starch Storage form of glucose (liver & muscle) Readily available source of hexose units. Average glycogen molecule contains 10,000 – 60,000 glucose molecules.

Glycogen Glycogen is stored in the cytosol & endoplasmic reticulum as granules (-particles) (as in muscles). Each -particle is an individual glycogen molecule. -particles aggregate to form a larger particle (-particle) (as in liver) Storage granules contain enzymes both for glycogenesis and glycogenolysis

Glycogen Liver Glycogen: (Up to 6%  70-80g) is used to maintain the blood glucose level between meals. After 12-18 hours fasting, liver is depleted of glycogen.

Glycogen Muscle Glycogen: (Up to 1%  250g) is used as source of glucose for muscle itself, and cannot directly contribute to blood glucose level.

Metabolism of Glycogen Two processes Glycogenesis Synthesis of glycogen from glucose Glycogenolysis Breakdown / degradation of glycogen into glucose Both processes are separate metabolic pathways having only one enzyme in common namely, phosphoglucomutase Both these processes are reciprocally regulated Site – liver, muscle (cytosol)

Metabolism of Glycogen Glycogen Storage Diseases (Glycogenoses): Inherited disorders due to deficiency or absence of enzyme(s) involved in glycogenesis or glycogenolysis – leading to deficient metabolism or deposition of abnormal form of glycogen  muscular weakness  even death.

Glycogenesis Requirements Glucose molecules Glycogen primer and glycogenin Enzymes ATP, UTP & Mg++ Glycogen Primer Synthesized by glycosylation (by UDPGlc) of tyrosine residue on the backbone of protein primer, glycogenin.

Reactions of Glycogenesis

Also known as Amylo [1 4]  [1 6]-transglucosidase