Glycogenin is an enzyme involved in converting

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body. Glycogen functions as one of three regularly used forms ...

. It acts as a

primer Primer may refer to: Arts, entertainment, and media Films * ''Primer'' (film), a 2004 feature film written and directed by Shane Carruth * ''Primer'' (video), a documentary about the funk band Living Colour Literature * Primer (textbook), a te ...

, by polymerizing the first few glucose molecules, after which other enzymes take over. It is a

homodimer In biochemistry, a protein dimer is a macromolecular complex or protein multimer, multimer formed by two protein monomers, or single proteins, which are usually Non-covalent interaction, non-covalently bound. Many macromolecules, such as proteins ...

of 37-

kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...

subunits and is classified as a

glycosyltransferase Glycosyltransferases (GTFs, Gtfs) are enzymes ( EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the "glycosyl donor") to a nucleophilic gl ...

. It

catalyzes Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

the

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s: :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 Product may refer to: Business * Product (business), an item that can be offered to a market to satisfy the desire or need of a customer. * Product (project management), a deliverable or set of deliverables that contribute to a business solution ...

are UDP and alpha-D-glucosylglycogenin.

Nomenclature

This enzyme belongs to the family of

s, specifically the hexosyltransferases. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivi ...

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 Genesis may refer to: Religion * Book of Genesis, the first book of the biblical scriptures of both Judaism and Christianity, describing the creation of the Earth and of humankind * Genesis creation narrative, the first several chapters of the Bo ...

'' meaning ''novel, source, or beginning.'' This hints at the role of glycogenin to simply start glycogen synthesis before

glycogen synthase Glycogen synthase (UDP-glucose-glycogen glucosyltransferase) is a key enzyme in glycogenesis, the conversion of glucose into glycogen. It is a glycosyltransferase () that catalyses the reaction of UDP-glucose and (1,4--D-glucosyl)n to yield UD ...

takes over.

Discovery

Glycogenin was discovered in 1984 by Dr. William J. Whelan, a fellow of the

Royal Society of London The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...

and former professor of Biochemistry at the

University of Miami The University of Miami (UM, UMiami, Miami, U of M, and The U) is a private university, private research university in Coral Gables, Florida, United States. , the university enrolled 19,852 students in two colleges and ten schools across over ...

Function

The main enzyme involved in glycogen

polymerisation In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many form ...

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

residues. Glycogenin acts as the

, to which further glucose

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

s may be added. It achieves this by catalyzing the addition of glucose to itself (

autocatalysis In chemistry, a chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same reaction. Many forms of autocatalysis are recognized.Steinfeld J.I., Francisco J.S. and Hase W.L. ''Chemical Kinetics and ...

) by first binding glucose from

UDP-glucose Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism. Functions UDP-glucose is used in nucleotide sugar metabolism as an activated form of g ...

to the

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

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 A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

. 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 Cytidine diphosphate glucose, often abbreviated CDP-glucose, is a nucleotide-linked sugar consisting of cytidine diphosphate and glucose. This nucleotide saccharide participates in the synthesis of deoxy sugars such as paratose and tyvelose. ...

and

TDP-glucose Thymidine diphosphate glucose (often abbreviated dTDP-glucose or TDP-glucose) is a nucleotide-linked sugar consisting of deoxythymidine diphosphate linked to glucose. It is the starting compound for the syntheses of many deoxysugars. Biosynthes ...

, in addition to its native substrate, UDP-glucose.

Structure

Isozymes

In humans, there are two

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...

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.

References

External links

*
molbio.med.miami.edu
{{Portal bar, Biology, border=no Carbohydrate metabolism EC 2.4.1 Enzymes of known structure