If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped 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:
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 22.214.171.124) 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 126.96.36.199) [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].
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).
Variants: glycogen degradation I
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
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