What is Glycogenesis?
Glycogenesis can be defined as the process through which glycogen is synthesized and glucose molecules are added to the glycogen chains for storage purposes. In the human body, the process of glycogenesis is activated post the Cori cycle when the body is in a rest period. The process usually occurs in the liver. It is important to note that the process of glycogenesis can also be activated by the peptide hormone insulin in order to respond to relatively high glucose levels in the body.
The biological mechanism of producing glycogen from glucose (which is the simplest cellular sugar) is generally referred to as glycogenesis. Via the glycogenesis phase, the body is known to generate glycogen in order to preserve these molecules for later use (for a time when the body does not have glucose readily accessible). It is important to note that glycogen is not the same as fat that is processed for energy in the long term. It is not uncommon for glycogen stores to be used by the body during meals, especially when the concentration of blood glucose has decreased. In this situation, the body’s cells are known to resort to their glycogen reserves, undergoing a process that is the reverse of glycogenesis. This reverse process is generally referred to as glycogenolysis.
What is the Process of Glycogenesis?
It is important for the cell to have an excess of glucose in order to commence the process of glycogenesis. Glucose is known to be the starting molecule that, via the glycogenesis process, is modified. The ability to be stored in long chains is imparted to the glucose through these changes. The process of glycogenesis is known to begin when a signal from the body to commence glycogenesis is received by the cell. It is important to note that these signals could come from a variety of different routes.
Initially, in the process of glycogenesis, the glucose molecule is known to interact with the glucokinase enzyme (which is an enzyme that adds to the glucose a group of phosphates). The phosphate group is moved to the other side of the molecule, with the help of the enzyme phosphoglucomutase, in the next step of the glycogenesis process. UDP-glucose pyrophosphorylase, which is another enzyme that is involved in this process, takes this molecule and produces glucose uracil-diphosphate. There are two phosphate groups in this glucose form, along with the nucleic acid uracil. Such additions help build a chain of molecules, which is vital for the next step of the glycogenesis process.
In the final stage of the glycogenesis process, a very important enzyme known as glycogenin plays a vital role. By attaching itself to this specific molecule, the UDP-diphosphate glucose tends to form relatively short chains. More enzymes facilitate the completion of the process after approximately 8 of these molecules form a chain together. This chain is then added to glycogen synthase. Simultaneously, the enzyme responsible for glycogen branching helps to build branches in the chains. This results in a fairly compact macromolecule that is quite efficient in storing energy.
Regulation of Glycogenesis
Via the process of phosphorylation, glycogen phosphorylase is known to be activated while glycogen synthase is known to be inhibited. Glycogen phosphorylase is generally transformed by the enzyme known as phosphorylase kinase from its relatively less reactive “b” form to a relatively more reactive “a” form. Phosphorylase kinase is also known to be activated by the protein kinase A. Furthermore, phosphoprotein phosphatase-1 is known to be deactivated by the same protein.
The hormone adrenaline acts to stimulate the protein kinase A. Furthermore, the hormone epinephrine binds itself to an adenylate cyclase-activating receptor protein. This enzyme also allows ATP to form cyclic AMP. Two cyclic AMP molecules tend to bind to the kinase A regulatory subunit, which activates it. This allows the protein kinase A catalytic subunit to dissociate from the assembly and to subject other proteins to phosphorylation.
Not only does epinephrine activate glycogen phosphorylase, it also contributes towards the inhibition of glycogen synthase. The effect of activating glycogen phosphorylase is amplified by this. A similar mechanism accomplishes this inhibition. This occurs as the protein kinase A acts to subject the enzyme to phosphorylation (which decreases its activity). This is commonly referred to as coordinate reciprocal control. To learn more about glycogenesis and other important concepts related to glucose, such as the structure of glucose and fructose, register with BYJU’S and download our app.