Glycogenin

glycogenin glucosyltransferase
Glycogenin structure (from rabbit). [1]
Identifiers
EC no.	2.4.1.186
CAS no.	117590-73-5
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
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.

Contents

• Nomenclature
• Discovery
• Function
• Structure
• Isozymes
• References
• Further reading
• External links

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. ^{[2] [3]}

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. ^[4]

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. ^[5]

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. ^[7]

Related Research Articles

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1,4-alpha-glucan-branching enzyme, also known as brancher enzyme or glycogen-branching enzyme is an enzyme that in humans is encoded by the GBE1 gene.

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Sucrose-phosphate synthase (SPS) is a plant enzyme involved in sucrose biosynthesis. Specifically, this enzyme catalyzes the transfer of a hexosyl group from uridine diphosphate glucose (UDP-glucose) to D-fructose 6-phosphate to form UDP and D-sucrose-6-phosphate. This reversible step acts as the key regulatory control point in sucrose biosynthesis, and is an excellent example of various key enzyme regulation strategies such as allosteric control and reversible phosphorylation.

In enzymology, a sucrose synthase is an enzyme that catalyzes the chemical reaction

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Glycogenin-1 is an enzyme that is involved in the biosynthesis of glycogen. It is capable of self-glucosylation, forming an oligosaccharide primer that serves as a substrate for glycogen synthase. This is done through an inter-subunit mechanism. It also plays a role in glycogen metabolism regulation. Recombinant human glycogenin-1 was expressed in E. coli and purified using conventional chromatography techniques.

Glucosyl-3-phosphoglycerate synthase is an enzyme with systematic name NDP-glucose:3-phospho-D-glycerate 2-alpha-D-glucosyltransferase. This enzyme catalyses the following chemical reaction

References

Further reading

External links

Wikimedia Commons has media related to Glycogenin .

• glycogenin at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• molbio.med.miami.edu