Glykogenogeneesi

Glykogenogeneesi is the metabolic pathway that results in the synthesis of glycogen from glucose. This process primarily occurs in the liver and muscles, serving as a storage form of glucose for later use. In the liver, glycogen serves as a glucose reserve to maintain blood glucose homeostasis between meals. In muscles, glycogen acts as a readily available energy source for muscle contraction.

The process begins with glucose entering the cell and being phosphorylated to glucose-6-phosphate, catalyzed by hexokinase in muscle or glucokinase in the liver. Glucose-6-phosphate is then isomerized to glucose-1-phosphate. Subsequently, glucose-1-phosphate is activated by reacting with uridine triphosphate (UTP) to form uridine diphosphate glucose (UDP-glucose), which is the direct precursor for glycogen synthesis.

UDP-glucose is added to a growing glycogen chain by glycogen synthase, the key regulatory enzyme of this pathway. Glycogen synthase requires a pre-existing primer of glycogen, typically initiated by a protein called glycogenin. Glycogenin acts as a scaffold, adding the first few glucose units to itself, and then glycogen synthase extends this primer. The glucose units are added in alpha-1,4 glycosidic linkages. To create branches in the glycogen molecule, a branching enzyme (amylo-alpha-4,6-transglycosylase) transfers a segment of 6-7 glucose units from the end of a chain to an interior position on the same or another chain, forming an alpha-1,6 glycosidic linkage. These branches increase the solubility of glycogen and provide more free ends for both synthesis and degradation. Glycogenogeneesi is an energy-consuming process, requiring ATP for the initial phosphorylation of glucose and UTP for the activation of UDP-glucose. The pathway is hormonally regulated, primarily stimulated by insulin and inhibited by glucagon and epinephrine.