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Aquifex aeolicus VF5 Pathway: glycogen degradation II

Pathway diagram: glycogen degradation II

If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Locations of Mapped Genes:

Schematic showing all replicons, marked with selected genes

Synonyms: glycogenolysis II

Superclasses: Degradation/Utilization/Assimilation Carbohydrates Degradation Polysaccharides Degradation Glycogen Degradation
Degradation/Utilization/Assimilation Polymeric Compounds Degradation Polysaccharides Degradation Glycogen Degradation

Pathway Summary from MetaCyc:
General Background

Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous control in many cells. Glycogen metabolism increases in response to a wide variety of environmental stresses, including heat stress or exposure to sodium chloride, hydrogen peroxide, copper sulfate, high levels of ethanol, or weak organic acids, such as sorbate or benzoate. Glycogen metabolism also increases in response to conditions of nutrient starvation, such as limited nitrogen, carbon, phosphorous, or sulfur, and during diauxic growth on glucose [Francois01].

Glycogen is highly branched. It is formed of small chains of 8 to 12 glucose molecules linked together by α (1->4) bonds (these short linear chains are also called maltodextrins) that are in turn linked to each other by α (1->6) bonds, known as branch linkages.

About This Pathway

Glycogen is degraded in fungi and animals by two enzyme working in tandem - glycogen phosphorylase and glycogen debranching enzyme (encoded in Saccharomyces cerevisiae by GPH1 and GDB1, respectively [Francois01]). GPH1 progressively releases α-D-glucopyranose 1-phosphate from the linear α-(1,4)-glucosidic bonds [Hwang89, Lerch75], but is not able to break bonds that are close to α(1,6)-branch linkages, leaving tails of 4 α-1,4-linked residues next to branching points [Francois01]. The highly-branched, short-chained product formed by exhausetive hydrolysis by glycogen phosphorylase has been named an α-limit dextrin, since it is the limit of what this enzyme is able to achieve on its own.

The branches are resolved by the dual-functional glycogen debranching enzyme, which eliminates branch points in a two-step process. The first step is the transfer of a maltotriosyl (or maltosyl) unit from a branch to the non-reducing end of an adjacent α-1,4-glucosyl chain, resulting in formation of a α-limit dextrin with short branches, catalyzed by the 4-α-glucanotransferase activity (EC 2.4.1.25) of the enzyme. This is followed by the hydrolysis of the residual α-1,6-linked glucose residue by the amylo-α-1,6-glucosidase activity (EC 3.2.1.33) [Teste00]. Once all the branches are removed, the polymer becomes a debranched α-limit dextrin, and glycogen phosphorylase can resume its activity, resulting in complete breakdown to α-D-glucopyranose 1-phosphate units [Francois01].

Another enzyme that can act on the debranched limit dextrin is EC 3.2.1.3, glucan 1,4-α-glucosidase, which, unlike the phosphorylase, produces non-phosphorylated β-D-glucopyranose residues.

Note that many bacteria species possess a different debranching enzyme (encoded by the glgX gene) that is able to remove maltotetraose chains from the an α-limit dextrin, enabling the removal of the four-residue side-branches that are linked to the main chain by an α-1-6 linkage in a single step, as described in glycogen degradation I (bacterial).

In the last step of the pathway α-D-glucopyranose 1-phosphate is converted to β-D-glucose 6-phosphate, which can enter glycolysis.

Variants: glycogen degradation I

Pathway Evidence Glyph:

Pathway evidence glyph

Key to pathway glyph edge colors: ?

  An enzyme catalyzing this reaction is present in this organism
  No enzyme catalyzing this reaction has been identified in this organism
  The reaction and any enzyme that catalyzes it (if one has been identified) is unique to this pathway

Credits:
Created in MetaCyc 28-May-2008 by Caspi R , SRI International
Imported from MetaCyc 08-Aug-2014 by Subhraveti P , SRI International


References

Francois01: Francois J, Parrou JL (2001). "Reserve carbohydrates metabolism in the yeast Saccharomyces cerevisiae." FEMS Microbiol Rev 25(1);125-45. PMID: 11152943

Hwang89: Hwang PK, Tugendreich S, Fletterick RJ (1989). "Molecular analysis of GPH1, the gene encoding glycogen phosphorylase in Saccharomyces cerevisiae." Mol Cell Biol 9(4);1659-66. PMID: 2657401

Lerch75: Lerch K, Fischer EH (1975). "Amino acid sequence of two functional sites in yeast glycogen phosphorylase." Biochemistry 14(9);2009-14. PMID: 1092346

Teste00: Teste MA, Enjalbert B, Parrou JL, Francois JM (2000). "The Saccharomyces cerevisiae YPR184w gene encodes the glycogen debranching enzyme." FEMS Microbiol Lett 193(1);105-10. PMID: 11094287

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."


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Page generated by SRI International Pathway Tools version 19.0 on Fri Jul 31, 2015, BIOCYC13B.