Glycogen branching enzyme

A glycogen branching enzyme is an enzyme that takes part in converting glucose to glycogen. It adds branches to the growing glycogen molecule.

Function

Glycogen is a branching polymer of large numbers of glucose units linked together. The structure is based on chains of glucose units with α(1→4) glycosidic bonds between carbon atoms 1 and 4 of each pair of units. These linkages are catalyzed by the enzyme glycogen synthase.

Every 10 to 14 glucose units, a side branch with an additional chain of glucose units occurs. The side chain attaches at carbon atom 6 of a glucose unit, and the linkage is termed an α(1→6) glycosidic bond. This connection is catalyzed by a branching enzyme, generally given the name α-glucan branching enzyme. A branching enzyme attaches a string of seven glucose units (with some minor variation to this number) to the carbon at the C6 position on the glucose unit, forming the α(1→6) glycosidic bond. The specific nature of this enzyme means that this chain of 7 carbons is usually attached to a glucose molecule that is in position three from the non-reducing end of another chain. Because the enzyme works with such specificity regarding the number of glucose units transferred and the position to which they are transferred, the enzyme creates the very characteristic; highly-branched glycogen molecule. ^[1]

Nomenclature

This enzyme belongs to the family of transferases, to be specific, those glycosyltransferases that transfer hexoses (hexosyltransferases). The systematic name of this enzyme class is 1,4-alpha-D-glucan:1,4-alpha-D-glucan 6-alpha-D-(1,4-alpha-D-glucano)-transferase. Other names in common use include branching enzyme, amylo-(1,4→1,6)-transglycosylase, Q-enzyme, alpha-glucan-branching glycosyltransferase, amylose isomerase, enzymatic branching factor, branching glycosyltransferase, enzyme Q, glucosan transglycosylase, 1,4-alpha-glucan branching enzyme, plant branching enzyme, alpha-1,4-glucan:alpha-1,4-glucan-6-glycosyltransferase, and starch branching enzyme. This enzyme participates in starch and sucrose metabolism.

Pathology

A defect in this enzyme can lead to disease; see glycogen storage disease type IV and glycogen-branching enzyme deficiency.

Structural studies

As of late 2007, only one structure has been solved for this class of enzymes, with the PDB accession code 1M7X.

References

• Barker SA, Bourne E and Peat S (Lond.). "The enzymic synthesis and degradation of starch. Part IV. The purification and storage of the Q-enzyme of the potato". J. Chem. Soc.: 1705–1711.  Check date values in: |date= (help)
• Baum H and Gilbert GA (1953). "A simple method for the preparation of crystalline potato phosphorylase and Q-enzyme". Nature 171 (4361): 983–984. doi:10.1038/171983a0. PMID 13063502. 
• Hehre EJ (1951). Enzymic Synthesis of Polysaccharides: a Biological type of Polymerization. "Advances in Enzymology and Related Areas of Molecular Biology". Adv. Enzymol. Relat. Subj. Biochem. Advances in Enzymology - and Related Areas of Molecular Biology 11: 297–337. doi:10.1002/9780470122563.ch6. ISBN 978-0-470-12256-3. 
• Handelsman DJ; Wang, Y; Jimenez, M; Marshan, B; Spaliviero, J; Illingworth, P; Handelsman, DJ (2006). "Follicle-stimulating hormone increases primordial follicle reserve in mature female hypogonadal mice". J. Endocrinol. 188 (3): 549–57. doi:10.1677/joe.1.06614. PMID 16522734. 

External links

• GeneReviews/NCBI/NIH/UW entry on Adult Polyglucosan Body Disease
• OMIM entries on Adult Polyglucosan Body Disease