Glycogenin Explained
| glycogenin glucosyltransferase |
| Ec Number: | 2.4.1.186 |
| Cas Number: | 117590-73-5 |
Glycogenin is an enzyme involved in converting glucose to glycogen. It acts as a primer, by polymerizing the first few glucose molecules, after which other enzymes take over. It is a homodimer of 37-kDa subunits and is classified as a glycosyltransferase.
It catalyzes the chemical reactions:
UDP-alpha-D-glucose + glycogenin UDP + alpha-D-glucosylglycogenin
UDP-alpha-D-glucose + a glucosyl-glycogenin (1,4-alpha-D-glucosyl)n-glucosyl glucogenin + UDP + H+
Thus, the two substrates of this enzyme are UDP-alpha-D-glucose and glycogenin, whereas its two products are UDP and alpha-D-glucosylglycogenin.[1] [2]
Nomenclature
This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-alpha-D-glucose:glycogenin alpha-D-glucosyltransferase. Other names in common use include:
- glycogenin,
- priming glucosyltransferase, and
- UDP-glucose:glycogenin glucosyltransferase.
One may also notice that the naming of glycogenin hints at its function, with the glyco prefix referring to a carbohydrate and the genin suffix derived from the Latin genesis meaning novel, source, or beginning. This hints at the role of glycogenin to simply start glycogen synthesis before glycogen synthase takes over.
Discovery
Glycogenin was discovered in 1984 by Dr. William J. Whelan, a fellow of the Royal Society of London and former professor of Biochemistry at the University of Miami.[3]
Function
The main enzyme involved in glycogen polymerisation, glycogen synthase in the liver and in the muscle glycogen synthesis is initiated by UDP-Glucose, can only add to an existing chain of at least 3 glucose residues. Glycogenin acts as the primer, to which further glucose monomers may be added. It achieves this by catalyzing the addition of glucose to itself (autocatalysis) by first binding glucose from UDP-glucose to the hydroxyl group of Tyr-194. Seven more glucoses can be added, each derived from UDP-glucose, by glycogenin's glucosyltransferase activity. Once sufficient residues have been added, glycogen synthase takes over extending the chain. Glycogenin remains covalently attached to the reducing end of the glycogen molecule.
Evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general; the molecular details of mammalian glycogen biogenesis may serve as a useful model for other systems.
Glycogenin is able to use the other two pyrimidine nucleotides as well, namely CDP-glucose and TDP-glucose, in addition to its native substrate, UDP-glucose.[4]
Structure
Isozymes
In humans, there are two isoforms of glycogenin — glycogenin-1, encoded by GYG1, and expressed in muscle; and glycogenin-2, encoded by GYG2, and expressed in the liver and cardiac muscle, but not skeletal muscle. Patients have been found with defective GYG1, resulting in muscle cells with the inability to store glycogen, and consequential weakness and heart disease.[5]
| glycogenin 1 | | Hgncid: | 4699 | | Symbol: | GYG1 | | Altsymbols: | GYG | | Entrezgene: | 2992 | | Omim: | 603942 | | Refseq: | NM_004130 | | Uniprot: | P46976 | | Ecnumber: | 2.4.1.186 | | Chromosome: | 3 | | Arm: | q | | Band: | 24 | | Locussupplementarydata: | -q25.1 |
| | glycogenin 2 | | Hgncid: | 4700 | | Symbol: | GYG2 | | Entrezgene: | 8908 | | Omim: | 300198 | | Refseq: | NM_003918 | | Uniprot: | O15488 | | Ecnumber: | 2.4.1.186 | | Chromosome: | X | | Arm: | p | | Band: | 22.3 |
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Further reading
- Krisman CR, Barengo R . 1975 . A precursor of glycogen biosynthesis: alpha-1,4-glucan-protein . Eur. J. Biochem. . 52 . 117 - 23 . 809265 . 10.1111/j.1432-1033.1975.tb03979.x . 1 . free .
- Pitcher J, Smythe C, Campbell DG, Cohen P . 1987 . Identification of the 38-kDa subunit of rabbit skeletal muscle glycogen synthase as glycogenin . Eur. J. Biochem. . 169 . 497 - 502 . 3121316 . 10.1111/j.1432-1033.1987.tb13637.x . 3 . free .
- Pitcher J, Smythe C, Cohen P . 1988 . Glycogenin is the priming glucosyltransferase required for the initiation of glycogen biogenesis in rabbit skeletal muscle . Eur. J. Biochem. . 176 . 391 - 5 . 2970965 . 10.1111/j.1432-1033.1988.tb14294.x . 2 . free .
- Berman, M.C. and Opie, L.A. (Eds.), Membranes and Muscle, ICSU Press/IRL Press, Oxford, 1985, p. 65-84.
- Rodriguez IR, Whelan WJ . 1985 . A novel glycosyl-amino acid linkage: rabbit-muscle glycogen is covalently linked to a protein via tyrosine . Biochem. Biophys. Res. Commun. . 132 . 829 - 36 . 4062948 . 10.1016/0006-291X(85)91206-9 . 2 .
- Lomako J, Lomako WM, Whelan WJ . 1988 . A self-glucosylating protein is the primer for rabbit muscle glycogen biosynthesis . FASEB J. . 2 . 3097 - 103 . 2973423 . 15 . 10.1096/fasebj.2.15.2973423. free . 24083688 .
- Alonso MD, Lomako J, Lomako WM, Whelan WJ . 1995 . Catalytic activities of glycogenin additional to autocatalytic self-glucosylation . J. Biol. Chem. . 270 . 15315 - 9 . 7797519 . 10.1074/jbc.270.25.15315 . 25 . free .
- Alonso MD, Lomako J, Lomako WM, Whelan WJ . 1995 . A new look at the biogenesis of glycogen . FASEB J. . 9 . 1126 - 37 . 7672505 . 12 . 10.1096/fasebj.9.12.7672505. free . 40281321 .
- Mu J, Roach PJ . 1998 . Characterization of human glycogenin-2, a self-glucosylating initiator of liver glycogen metabolism . J. Biol. Chem. . 273 . 34850 - 6 . 9857012 . 10.1074/jbc.273.52.34850 . 52 . free .
External links
Notes and References
- Barengo R, Krisman CR . Initiation of glycogen biosynthesis in Escherichia coli. Studies of the properties of the enzymes involved . Biochimica et Biophysica Acta (BBA) - General Subjects . 540 . 2 . 190–6 . May 1978 . 418819 . 10.1016/0304-4165(78)90131-9 .
- Ernest Y. C. Lee . Butler NA, Lee EY, Whelan WJ . A protein-bound glycogen component of rat liver . Carbohydrate Research . 55 . 73–82 . May 1977 . 861979 . 10.1016/s0008-6215(00)84444-4 .
- Whelan WJ . Pride and prejudice: the discovery of the primer for glycogen synthesis . Protein Sci. . 7 . 9 . 2038–41 . September 1998 . 9761486 . 2144155 . 10.1002/pro.5560070921 .
- Alonso . Miriam D. . Lagzdins . Erik J. . Lomako . Joseph . Lomako . Wieslawa M. . Whelan . William J. . 1995-02-13 . New and specific nucleoside diphosphate glucose substrates for glycogenin . FEBS Letters . en . 359 . 2-3 . 110–112 . 10.1016/0014-5793(95)00018-5 . 0014-5793.
- Moslemi AR, Lindberg C, Nilsson J, Tajsharghi H, Andersson B, Oldfors A . Glycogenin-1 deficiency and inactivated priming of glycogen synthesis . N. Engl. J. Med. . 362 . 13 . 1203–10 . April 2010 . 20357282 . 10.1056/NEJMoa0900661 . free .
