Sqadia.com offer medical students a series of lectures on the basics of biochemistry. In this video lecture, Dr. Muhammad Daniyal explains that highly branched structure of glycogen provides a large number of sites for glycogenolysis. Glycogen is synthesized in four steps.
- Activation of glucose
Glycogenin catalyses the addition of eight glucose units into partner in the glycogenin dimer. In elongation, UDP is displaced by the terminal hydroxyl group of the growing glycogen molecule. Branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase.
Glycogenolysis takes place in three steps.
- Glucose 1-Phosphate release
- Remodeling of Glucogen Substrate
- Conversion of Glucose 1-Phosphate to Glucose 6-Phosphate
Cleavage of a bond by the addition of orthophosphate is referred to as phosphorolysis. Remodeling of glycogen substrate takes place using debranching enzyme. Phosphoroylytic cleavage of glycogen is energetically advantageous because the released sugar is already phosphorylated. Glucose 6-phosphatase is absent from most of other tissues. Pyridoxal phosphate (PLP) is held at the active site by a Schiff base linkage.
UDP-Glucose is used in nucleotide sugar metabolism, as precursor of glycogen, sucrose lipopolysaccharides and glycosphingolipids. Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, by allosteric effectors and by phosphorylation.
Glycogenesis and Glycogenolysis is regulated by covalent modification. Insulin plays many roles in glycogenesis. Glycogen Synthase is allosterically activated by glucose-6-Phosphate. Glycogen Phosphorylase in muscle is subject to allosteric regulation by AMP, ATP, and glucose-6-phosphate. cAMP and Ca+2 play several roles in glycogen degradation. Glycogen storage diseases are very common and can be divided into eight types.
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