Donor substrates

Monosaccharides are chemically inert molecules, thus they need to be activated to high-energy donor substrates for glycosyltransferase reactions. Typically, phosphate is used as reactive group, either alone or in the context of a nucleotide. Phosphate (P) acts as a versatile leaving group in the nucleophilic attacks involved in glycosylation reactions. At the endoplasmic reticulum (ER) membrane of eukaryotic cells, Man and Glc are also linked to the polyprenol dolichol-P (Dol-P). These Dol-P linked sugars are used as donor substrates for hydrophobic glycosyltransferases that are embedded in the ER membrane.

The biosynthesis of donor substrates usually begins by the phosphorylation of hexoses at C1 and C6 mediated by hexokinase and phosphomutase enzymes. Hexokinase is an ubiquitous activity found from bacteria to mammals. By phosphorylating Glc at C6, hexokinase prevents the diffusion of Glc out the cytosol, where it is further processed. Hexokinase is not specific for Glc, since Man is also phosphorylated at C6 by the enzyme. The liver has an additional enzyme, glucokinase, which is specific for Glc and is important for glycogen formation. The sugar Gal is directly phosphorylated at C1 by the galactokinase enzyme. Deficiency of galactokinase leads to a form of galactosemia, which is characterized by the development of cataract due to the accumulation of galactitol in the lens.

Phosphoglucomutase catalyzes the transition of phosphate in both C1 to C6 and C6 to C1 ways. The enzyme is important for the breakdown of glycogen where Glc-1-P is converted to Glc-6-P. In the liver, a Glc-6-phosphatase leads to the release of Glc, which drives the equilibrium of the phosphoglucomutase reaction towards Glc-6-P. This phosphatase is only found in the liver, which is the main organ regulating the levels of blood sugar. The deficiency of the main phosphoglucomutase isoform (PGM1) leads to a rare form of glycogen storage disease (GSD type XIV). Defects of glycogen breakdown mainly lead to hypoglycemia and muscle weakness, whereas some forms are severe and lead to death in infancy.

The phosphoglucomutase isoform PGM3 also acts on GlcNAc-6-P and a dedicated phosphomannomutase (PMM2) catalyzes the formation of Man-1-P from Man-6-P. Mutations in the PMM2 gene are the main cause of congenital disorders of glycosylation (CDG).

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