In humans and many other animals, glycogenesis is the process in which glycogen is produced to be stored mainly in liver and muscle cells. Short polymers of glucose are converted into long polymers. The body does not directly utilize exogenous glucose, i.e. from a carbohydrate-containing meal. Instead, the body stores glucose into glycogen within cells through the process of glycogenesis. And when the body requires metabolic energy, glycogen is broken down into glucose subunits through the process of glycogenolysis. Thus, glycogenesis is the opposite process of glycogenolysis.
Glycogenesis begins in glucose that is phosphorylated to become glucose-6-phosphate through the enzymes hexokinase in muscle and glucokinase in liver. The next steps of the process are as follows:
- Glucose-6-phosphate (by phosphoglucomutase) --> Glucose-1- phosphate
- Glucose-1-phosphate + UTP (by UDP-glucose pyrophosphorylase) --> UDP-glucose + PPi
- PPi + H2O --> 2Pi
- UDP-glucose + Glycogenn (by glycogen synthase) --> Glycogenn+1 + UDP
- UDP + ATP --> UTP + ADP
When the chain lengthens to a minimum of 11 residues, the branching enzyme amylo-1,4 to 1,6-transglucosidase transfers a part of the chain to a neighboring chain, thus, establishes a branching point in the molecule.1
1 Naik, P. (2015). Biochemistry. JP Medical Ltd. p.182-183.