24 Glycogenesis

Dr. Chirantan Rawal

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GLYCOGENESIS

 

Objectives

  1. To understand the synthesis of glycogen.
  2. Role of glycogenin in glycogen synthesis
  3. Glycogen branching

   

Introduction

  • Glycogen is a highly branched, large polymer of glucose molecules linked along its main line by α-1, 4 glycosidic linkages; branches arise by α-1,6 glycosidic bond at about every tenth residues.

Fig: 25.1 STRUCTURE OF GLYCOGEN

 

  • It occurs in the cytoplasm as granules. Granules also contain the enzymes and regulatory proteins which is required for its synthesis and degradation.
  • It acts as an important energy reserve for the body.
  • It is stored in the liver and skeletal muscle. Glycogen stored in the muscles will be utilized for the energy requirement of muscles only, while glycogen stored in the liver will be used for the energy requirement of the rest of the body.
  • Glycogen metabolism is very important because it facilitate the blood glucose level to be maintained between meals (liver glycogen) and also act as an energy reserve for muscular activity. The maintenance of blood glucose is essential in order to supply energy to tissues.
  • Blood is closely monitored and regulated by hormones. If blood glucose level rises, our body hormones came into action to reduce the excess sugar. This is done by increasing the process of glycogenesis. It means synthesis and storage of glycogen. The suffix ‘genesis’ means creation; so, glycogenesis is the creation of glycogen.

Fig 25.2: Glycogen is synthesized from glucose

 

STEPS OF GLYCOGENESIS

 

Glycogen synthesis requires 3 main enzymes. Glycogenesis occurs by a different pathway from glycogenolysis.

  1. UDP-glucose formation by UDP-glucose pyrophosphorylase
  2. Glycogen synthesis by glycogen synthase
  3. Glycogen Branching

 

Fig: 25.3 OVERVIEW OF GLYCOGENESIS

 

    UDP-glucose formation by UDP-glucose pyrophosphorylase

  • Glucose is activated before or glucokinase enzyme (in phosphate. polymerisation in to glycogen. Hexokinase enzyme (In muscles) liver) catalyzes the phosphorylation of glucose to glucose-6-
  • For further anabolic polymerisation reaction, it is converted in to glucose-1-phosphate by the reversible action of Phosphoglucomutase.
  • Glucose-1- phosphate is then attached to UTP by the action of UDP-Glucose pyrophosphorylase form UDP-Glucose and pyrophosphate. The C-1 carbon of the glucosyl unit is esterified to the diphosphate group of UDP. The ΔG° of this reaction is very less . Pyrophosphates (PPi) formed is hydrolyzed to 2 Pi (orthophosphate). This reaction is highly exergonic reaction. Conversion of PPi to orthophosphate is catalysed by Inorganic pyrophosphatase.
  • UDP-glucose is the activated form of glucose used for metabolism of galactose and biosynthesis. It is a high energy compound. It can donate glucose units to the growing glycogen chain. No further energy is required for synthesis of glycogen.

 

Fig: 25.4 STRUCTURE OF UDP- GLUCOSE

 

Glycogen synthesis by glycogen synthase

  • Glycogen synthase transfers the glucosyl residue from UDP-glucose to the non reducing terminal residues of glycogen. It is transferred to hydroxyl terminal of C4 end of glycogen to form an α-1–4 glycosidic bond.
  • This reaction is catalysed by glycogen synthase. Glycogen synthase is the regulatory enzyme in synthesis of glycogen.
  • Glycogen synthesis requires a primer. It can add glucosyl residues to the glycogen chain if it contains more than four residues. It means that glycogen synthase can only extend an existing chain. Priming function is carried out by glycogenin.

 

 

Glycogenin:

  • It is a primer for glycogen synthesis. It is a protein which composed of two identical subunits. It is a 37 kDa protein that is glycosylated on a specific tyrosine residue. Glycogenin contains eight glucose units linked by α- 1–4 linkages. These glucose molecules are added to the protein by autocatalysis.
  • Formation of glycogenin primer: The first glucose molecule is attached to the hydroxyl group on Tyr-194 of glycogenin, which is catalysed by the subunit of glycogenin. Then it is auto catalytically extends the glucose chain by up to seven residues long. This glucose molecule is donated by UDPG. In this form, glycogenin can act as a primer.

Fig: 25.5 FORMATION OF GLYCOGENIN PRIMER

 

  • New glucose residue is attached to the primer by Glycogen synthase. Glycogen Synthase catalyzes the transfer of glucose from UDP-glucose. New glucose molecule will transfer to the C-4 hydroxyl group at the non reducing end of the growing glycogen molecule.

   

Glycogen Branching

  • Glycogen synthase catalyzes only α- 1–4 glycosidic bonds. It results in to the formation of α- amylose.
  • Branching is catalysed by separate enzyme called Branching enzyme.
  • It is also known as amylo-(1–4→1–6) transglycosylase
  • After a number of glucose units have been linked as a straight chain with α1–4 linkages, branching enzyme breaks α 1–4 bonds.
  • It breaks a 7 unit segment of α 1–4 residues from a glycogen chain and transfers to a C-6 hydroxyl group of a glucosyl residue that is four residues away from an existing branch.

        Reattachment is done by creating an α1–6 bond.

  • Branches are very important in a growing chain of glycogen. Enzymes involved in glycogen synthesis (glycogen synthase) and degradation (glycogen phosphorylase) works only at the ends of the glycogen molecules. More terminal ends increases the rate of synthesis and degradation of glycogen.

Fig: 25.6 Glycogen Branching

 

 

you can view video on Glycogenesis

 

References

  • Peter J. Roach, Anna A. Depaoli-Roach, Thomas D. Hurley, Vincent S. Tagliabracci (2012) Glycogen and its metabolism: some new developments and old themes . Biochemical Journal, 441 (3) 763-787; DOI: 10.1042/BJ20111416
  • Roach PJ, Depaoli-Roach AA, Hurley TD, Tagliabracci VS (2012) Glycogen and its metabolism: some new developments and old themes. Biochem J 441: 763–787. doi: 10.1042/BJ20111416
  • Wilson WA, Roach PJ, Montero M, Baroja-Fernandez E, Munoz FJ, Eydallin G, et al. Regulation of glycogen metabolism in yeast and bacteria. Fems Microbiol Rev. 2010;34(6):952–985.

 

Books

  • Harper’sIllustrated Biochemistry by Robert K.. Murray, Daryl K. Granner, Peter A. Mayes, 26th Edition (2003)
  • Lehninger’S Principle of Biochemistry David L. Nelson and Michael M. Cox,5th Edition (2008)
  • Instant Notes: Biochemistry, 2nd Edition, B.D. Hames & N. M. Hooper (2005)
  • Textbook of Biochemistry, 4th Edition Donald Voet, Judith G. Voet (2011)

Web site

  • http://www.wiley.com/college/fob/quiz/quiz15/15-20.swf
  • https://www.tamu.edu/faculty/bmiles/lectures/Glycogen%20Metabolism.pdf
  • http://themedicalbiochemistrypage.org/glycogen.php
  • https://en.wikibooks.org/wiki/Principles_of_Biochemistry/Gluconeogenesis_and_Glycogenesis