Dieting complex carbohydrates and fats, for energy. The first source to which the body turns is glycogen (by glycogenolysis). Glycogen is a complex carbohydrate, 65% of which is stored in skeletal muscles and the remainder in the liver (totaling about 2,000 kcal in the whole body). It is created from the excess of ingested macronutrients, mainly carbohydrates. When glycogen is nearly depleted, the body begins lipolysis, the mobilization and catabolism of fat stores for energy. In this process, fats, obtained from adipose tissue, or fat cells, are broken down into glycerol and fatty acids, which can be used to generate energy.
Glycogen phosphorylase Glycogen phosphorylase Glycogen phosphorylase is one of the phosphorylase enzymes (). Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation and allosteric effects. Glycogen phosphorylase breaks up glycogen into glucose subunits (see also figure below): (α-1,4 glycogen chain) + Pi ⇌ (α-1,4 glycogen chain) + α-D-glucose-1-phosphate. Glycogen is left with one fewer glucose molecule, and the free glucose molecule is in the form of glucose-1-phosphate. In order to be used for metabolism, it must be converted to