Glycogen is a branched polysaccharide of high molecular mass that is used as a storage carbohydrate. In S. cerevisiae, glycogen is typically catabolized to glucose-1-phosphate and glucose by the Gph1p glycogen phosphorylase and the Gdb1p debranching enzyme (4). However, during sporulation, glycogen is rapidly catabolized to glucose by the Sga1p glucoamylase (4).

The Gph1p glycogen phosphorylase (EC 2.4 1.1) catalyzes the sequential phosphorolysis of alpha-1,4-linked glucose units in glycogen until two or three glucose units remain before an alpha-1,6-branch point (1, 5). At that point, Gph1p is not able to degrade glycogen further until the branch is resolved into a chain of linear alpha (1,4)-glucosidic bonds by debranching enzyme Gdb1p. Gph1p is then able to resume degrading the glycogen molecule into glucose-1-phosphate (4).

The N-terminus of Gph1p is involved in determining the quaternary structure, and possibly the activity, of the enzyme (6). The activity of Gph1p is also regulated by cyclic AMP-mediated phosphorylation (1). GPH1 expression is regulated by stress-response elements and the Hog1p-mitogen-activated protein (MAP) kinase-dependent pathway (2), and is induced during late exponential growth phase (1). gph1 null mutants are viable, but display increased accumulation of glycogen (1). No phenotypes corresponding to the mammalian glycogen storage disease VI, which is associated with mutations in human glycogen phosphorylase (PYGL), have been found for S. cerevisiae gph1 null mutants (4).

Last updated: 2005-08-25