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Renin Angiotensin Mechanism.

Glycogenesis.

 

Definition:

Biosynthesis of Glycogen from Glucose is called Glycogenesis.

  • Glycogen is synthesized 

  • Depending on the demand for glucose and ATP (energy), insulin promotes the glucose conversion into glycogen.

  • Glycogen is the major storage form of carbohydrate in animals similar to starch in plants.

  • It is a homopolymer made up of repeated units of α- D glucose and each molecule is linked to another by 1→4 glycosidic bonds.

  • Once there is a chain consisting of 8 to 10 glycosidic residues in the glycogen fragment, branching begins by 1→6 linkages.

  • Glycogen is stored in liver and skeletal muscles.

Location:

  • Cytoplasm of cells in the muscle, liver, and adipose tissue.

Steps Involved in Glycogenesis:

  1. Glucose is converted into glucose-6-phosphate by the action of glucokinase or hexokinase  with conversion of ATP to ADP.

  2. Glucose-6-phosphate is converted into glucose-1-phosphate by the action of phosphoglucomutase.

  3. Glucose-1-phosphate is converted into UDP-glucose by the action of the enzyme UDP-glucose pyrophosphorylase. Pyrophosphate is formed, which is later hydrolysed by pyrophosphatase into two phosphate molecules.

  4. The enzyme glycogenin is needed to create initial short glycogen chains, which are then lengthened and branched by the other enzymes of glycogenesis.

  5. Once a chain of eight glucose monomers is formed, glycogen synthase binds to the growing glycogen chain and adds UDP-glucose to the 4-hydroxyl group of the glucosyl residue on the non-reducing end of the glycogen chain, forming more α(1→4) bonds in the process.

  6. Branches are made by glycogen branching enzyme (also known as amylo α(1:4)→α(1:6)transglycosylase), which transfers the end of the chain onto an earlier part via α-1:6 glycosidic bond, forming branches, which further grow by addition of more α-1:4 glycosidic units.

Energy requirement:

  •  In the synthesis of glycogen, one ATP is required per glucose incorporated into the polymeric branched structure of glycogen.

Key Enzymes:

  1. Glycogen synthase: Adds glucose units to the nonreducing ends of existing chains in α-1,4 linkages.

  2. Glucosyl (4:6) transferase : Transfers seven-glucose-residue-long pieces from the nonreducing ends of the chains to create internal branches with α-1,6 linkages.

Stimulation:

  • Insulin stimulates glycogenesis via dephosphorylation and thus activation of glycogen synthase.

  • The process is also activated during rest periods following the Cori cycle.

Inhibition:

  • Glucagon (liver) and epinephrine (liver and muscle) inhibit gluconeogenesis via the cAMP protein kinase A  phosphorylation cascade, which results in phosphorylation and thus deactivation of glycogen synthase.

Commonly Asked Questions.

  1. Write a short note on “Glycogenesis”.


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