Mannose is a sugar monomer of the aldohexose series of carbohydrates and is a C-2 epimer of glucose. It is a key monosaccharide for protein and lipid glycosylation . The majority of mannose metabolism takes place in the cytosol. There are two routes to form mannose 6-phosphate. The first subpathway involves using beta-D-fructose 6-phosphate from glycolysis. The enzyme mannose-6-phosphate isomerase catalyzes the interconversion of beta-D-fructose 6-phosphate and D-mannose 6-phosphate. It requires a zinc ion as a cofactor. The second subpathway involves using the secreted enzyme, mannan endo-1,4-beta-mannosidase to catalyze the random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans to form D-mannose residues. These D-mannose residues are then imported into the cell cytoplasm via a sugar transport protein (a sugar/hydrogen symporter). Once inside the cell, hexokinase catalyzes the conversion of D-mannose into D-mannose 6-phosphate. Next, phosphomannomutase catalyzes the interconversion of D-mannose 6-phosphate and D-mannose 1-phosphate. However, D-mannose 1-phosphate can also be synthesized from ADP-mannose in the chloroplast via nudix hydrolase 14 and a magnesium or manganese ion cofactor. D-mannose 1-phosphate is then transported into the cytosol by a predicted D-mannose 1-phosphate transporter. Next, mannose-1-phosphate guanylyltransferase uses GTP to catalyze the conversion of D-mannose 1-phosphate into GDP-mannose. This is followed by GDP-mannose 4,6 dehydratase catalyzing the conversion of GDP-mannose into GDP-4-dehydro-6-deoxy-D-mannose. It requires NADP as a cofactor. Last, GDP-L-fucose synthase catalyzes the conversion of GDP-4-dehydro-6-deoxy-D-mannose into GDP-L-fucose.

Mannose Metabolism References