Glycogenin

glycogenin glucosyltransferase
File:Rabbit muscle glycogenin structure.jpg Glycogenin structure (from rabbit).
Identifiers
EC number	2.4.1.186
CAS number	Template:CAS
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles NCBI proteins

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 reaction:

UDP-alpha-D-glucose + glycogenin $\rightleftharpoons$ UDP + alpha-D-glucosylglycogenin

Thus, the two substrates of this enzyme are UDP-alpha-D-glucose and glycogenin, whereas its two products are UDP and alpha-D-glucosylglycogenin.

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.

Discovery

Glycogenin was discovered in 1984 by Dr. William J. Whelan, a fellow of the Royal Society of London and current professor of Biochemistry at the University of Miami.

Function

The main enzyme involved in glycogen polymerisation, glycogen synthase, can only add to an existing chain of at least 4 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.

Structure

2-D cross-sectional view of glycogen. A core protein of glycogenin is surrounded by branches of glucose units. The entire globular complex may contain approximately 30.000 glucose units.

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.

glycogenin 1 Identifiers Symbol GYG1 Alt. symbols GYG Entrez 2992 HUGO 4699 OMIM 603942 RefSeq NM_004130 UniProt P46976 Other data EC number 2.4.1.186 Locus Chr. 3 q24-q25.1

glycogenin 2 Identifiers Symbol GYG2 Entrez 8908 HUGO 4700 OMIM 300198 RefSeq NM_003918 UniProt O15488 Other data EC number 2.4.1.186 Locus Chr. X p22.3

Further reading

• Berman, M.C. and Opie, L.A. (Eds.), Membranes and Muscle, ICSU Press/IRL Press, Oxford, 1985, p. 65-84.

External links

• glycogenin at the US National Library of Medicine Medical Subject Headings (MeSH)
• http://molbio.med.miami.edu/NewWebsite/Whelan.htm