Not all the glucose that we eat are converted to energy. Some are converted to glycogen which is a complex carbohydrate with a similar structure to starch in plants. Glycogen can then be broken down to release glucose which is metabolized for energy. Glycogenesis (production of glycogen) is controlled by insulin. When there is enough glucose available, insulin stimulates glycogenesis; a process that takes place in the liver and muscles. When there is not enough glucose, insulin stimulates glycogenolysis to breakdown glycogen and release glucose. The following diagram illustrate the steps in glycogen synthesis.
Summary of Steps in Glycogenesis
- A phosphate group is added to carbon number 6 on glucose to produce glucose-6-phosphate (G-6-P). This happens with the help of hexokinase.
- Conversion of G-6-P to glucose-1-phosphate (G-1-P) with the help of phosphoglucomutase
- Uridine triphosphate (UTP) (a high-energy molecule) reacts with G-1-P to produce UDP-Glucose. This step is facilitated by UDP-glucose pyrophoshorylase which cuts 2 phosphates off UTP to give energy
- An enzyme called glycogenin catalyzes the production of a chain of 4 glucose molecules connected by α (1, 4) bonds. This short chain acts as a primer for glycogen synthase to continue elongation
- Branching enzymes go to the end of the chains and cuts off short chains of 6-8 glucose residues. The branching enzymes then attach these short cuts as α (1, 6) side branches on the main chain