Glycogenesis enzymes

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

The hormonal mechanisms of action to regulate glycogenesis and glycogenol-ysis involve reversible phosphorylation of the critical enzymes glycogen synthase and phosphorylase, respectively (Figure 6—6). 

The glycogenesis and glycogenolysis enzymes subject to hormonal control are glycogen synthase and phosphorylase, respectively. Briefly, glycogen synthase is inhibited by high cellular cAMP and Ca2+ levels, whereas phorphorylase is stimulated. The mechanisms for accomplishing this are quite complex. 

Figure 2.6 GLycogenesis. The addition of glucose to glycogen depends upon the presence of a pre-existing glycogen primer glucose monomers are arranged and added to the primer by glycogen synthase, a key regulatory enzyme that is subject to control by covalent phosphorylation.

The second step in glycogenesis is conversion of glucose-6-phosphate to glucose-1-phosphate by phosphoglucomutase in a reaction similar to that catalyzed by phospho-glyceromutase. The phosphoryl group of the enzyme participates in this reversible reaction in which glucose-1,6-bisphosphate serves as an intermediate ... 

When occupied, the insulin receptor becomes an active tyrosine kinase enzyme that causes a phosphorylation cascade that ultimately has the opposite effect of the glucagon/cAMP system the enzymes of glycogenolysis are inhibited and the enzymes of glycogenesis are activated. Insulin also increases the rate of glucose uptake into several types of target cells, but not liver or brain cells. 

The second reaction of glycogenesis is the reverse of one of the reactions of glycogenolysis. The glucose-6-phosphate formed in the first step is isomerized to glucose-l-phosphate. The enzyme that catalyzes this step is phosphoglucomutase ... 

Glucose 6-phosphate has one of two major fates either glycogenesis or glycolysis. The particular fate is determined by regulation of the fluxes via the enzymes in the two pathways. Even though both of these pathways contain multiple enzyme-catalyzed steps, each pathway has specific controls on the constituent enzymes. 

Phosphorolysis entails the depolymerisation of polysaccharides. In this process (which is the reverse of glycogenesis and uses a different enzyme, namely glycogen phosphorylase), polysaccharides are successfully depolymerised by the action of orthophosphoric acid to glucose-l-phosphate (11.61). This is the form required for the process of glycolysis described below. 

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