METABOLISMO DEL GLUCÓGENO

Metabolism of Glycogen

Overview of Glycogen Structure

• Glycogen is a branched polysaccharide composed solely of glucose units, categorized as a homopolysaccharide with alpha 1,4 and alpha 1,6 glycosidic bonds.

• The structure resembles a spiral with branches; the spiral's connections are primarily alpha 1,4 bonds while the branches utilize alpha 1,6 bonds.

• Glycogen is stored in all organs except the brain; the liver can store about 10% of its weight (approximately 200 grams), lasting between 12 to 24 hours.

Storage Capacity and Function

• Muscle tissue stores glycogen at about 1-2% of its weight. Although the liver has a higher percentage storage capacity, muscle mass contributes to overall glycogen quantity.

• Excessive glycogen can lead to fat accumulation; two metabolic pathways for glycogen are glycogenesis (formation) and glycogenolysis (breakdown).

Glycogenesis: Formation of Glycogen

• Glycogenesis is defined as synthesizing glycogen from glucose-6-phosphate, occurring mainly in the liver and skeletal muscle.

• The process consists of three stages: formation of glucose-1-phosphate, uridine diphosphate glucose synthesis, and branching formation.

Key Steps in Glycogenesis

• The first reaction involves converting glucose-6-phosphate into glucose-1-phosphate through isomerization.

• Next, UDP-glucose is formed by catalyzing the reaction between UTP and glucose-1-phosphate. This step provides energy for subsequent reactions.

Enzymatic Reactions in Glycogenesis

• In stage three, glycogen synthase catalyzes adding glucose from UDP-glucose to an existing chain via an alpha 1,4 bond.

• Branching occurs when branching enzymes transfer terminal chains to create new branches through an alpha 1,6 bond.

Regulation Mechanisms

• Regulation involves glucagon synthase being stimulated by glucose-6-phosphate levels. It’s also influenced by covalent modifications and hormones like insulin which activate phosphatases.

Glycogenolysis: Breakdown of Glycogen

• Defined as breaking down glycogen into glucose; it activates when blood sugar levels drop. Also known as degradation or lysis of glycogen.

Process Details

• This pathway occurs in both liver and muscle tissues with most enzymes located in the cytosol. It consists of two stages: shortening and branching removal during breakdown.

Shortening Stage Insights

Glucose Release and Hydrolysis Mechanisms

Mechanism of Glucose Release

• The process involves the breaking of bonds, leading to the release of glucose. This occurs through hydrolysis, where branching points containing alpha-1,6 glycosidic links are eliminated.

• Initially, alpha-1,4 bonds must be cleaved before addressing the alpha-1,6 branches to yield only glucose-6-phosphate.

Regulation of Glycolysis

• The regulation is primarily managed by an enzyme known as glycogen phosphorylase, which plays a crucial role in glycolysis.