Glikogenesis

Glikogenesis, also known as glycogenesis, is the metabolic process by which glucose is converted into glycogen, a branched polysaccharide stored mainly in the liver and skeletal muscle. Its primary purpose is to store excess glucose for later use and to help regulate blood glucose levels in the liver, while in muscle it provides a rapidly mobilizable energy reserve for contraction.

The pathway begins with the uptake of glucose into cells and its phosphorylation by hexokinase (in most tissues) or glucokinase (in the liver) to form glucose-6-phosphate. Glucose-6-phosphate is isomerized to glucose-1-phosphate by phosphoglucomutase, and then activated to UDP-glucose by UDP-glucose pyrophosphorylase. Glycogen synthase catalyzes the addition of glucose from UDP-glucose to the non-reducing ends of a preexisting glycogen chain, forming alpha-1,4 glycosidic bonds. In many cells, glycogenin serves as a protein primer to initiate the initial glucose chain; glycogen synthase then extends the chain. Branching enzyme (amylo-1,4→1,6-transglycosylase) introduces branch points by transferring segments of seven or more glucose residues to form alpha-1,6 linkages, increasing solubility and rapid synthesis.

Regulation of glycogenesis is tightly controlled. Glycogen synthase is activated when dephosphorylated by protein phosphatase 1, a process promoted by insulin. It is inhibited when phosphorylated by kinases such as glycogen synthase kinase 3 and others in response to glucagon and epinephrine, which simultaneously promote glycogen breakdown.

Glikogenesis operates in coordination with glycogenolysis and other metabolic pathways to maintain energy homeostasis. Defects in glycogen synthesis or branching enzymes can lead to glycogen storage diseases.