Mannose-6-phosphate, glycolysis

Another simple sugar that enters glycolysis at the same point as fructose is mannose, which occurs in many glycoproteins, glycolipids, and polysaccharides (Chapter 7). Mannose is also phosphorylated from ATP by hexokinase, and the mannose-6-phosphate thus produced is converted to fructose-6-phosphate by phosphomannoisomerase. 

Mannose 6-phosphate is isomerized by phosphoman-nose isomerase to yield fructose 6-phosphate, an intermediate of glycolysis. 

A variety of D-hexoses, including fructose, galactose, and mannose, can be funneled into glycolysis. Each is phosphorylated and converted to either glucose 6-phosphate or fructose 6-phosphate. 

Hexose entry into glycolysis fructose, mannose, galactose-----> glucose 6-phosphate... 

Mannose is a monosaccharides. Containing six carbons and an aldehyde group, it is classified as an aldose and a hexose. Mannose is a constituent of glycoproteins and a few polysaccharides. Mannose is phosphorylated by hexokinase to mannose-6-phosphate, which is readily isomerized to fructose-6-phosphate for entry into glycolysis or gluconeogenesis (Figure 13.12). 

Mannose-6-phosphate is an intermediate in metabolism of mannose formed in a phosphorylation reaction catalyzed by hexokinase. Mannose-6-phosphate is readily isomerized to fructose-6-phosphate, which participates in the glycolysis, gluconeogenesis, pentose phosphate pathway, or Calvin cycle pathways. 

The biosynthesis of D-mannose involves the conversion of the D-gluco isomer via the D-fructo (as the 6-phosphates) by isomerization at C-2 differing from the conversions of other sugars (including D-galactose) which proceed via nucleoside diphospho sugars, the formation of which requires a supply of nucleoside triphosphates. However, despite the widespread distribution of D-mannose and its utilisation in glycolysis and... 

Glucose is not the only hexose used for glycolysis— fructose, mannose, and galactose can also enter the glycolytic cycle after phosphorylation. Like glucose, fructose can be used only after phosphorylation in one of three ways [33] (1) phosphorylation to fructose-6-phosphate by hexokinase, (2) phosphorylation to fructose-6-phosphate by a specific fructokinase, and (3) phosphorylation to fructose-1-phosphate by fructokinase (Fig. 1-7). It is well established that the glu-cokinase of liver and muscle can also phosphorylate fructose. Fructose can enter muscle metabolism only in the form of fructose-6-phosphate. This is strikingly different from liver metabolism in which fructose is converted to fructose-1-phosphate by a specific fructokinase. 

Therefore, glucose, mannose, and fructose can all be in equilibrium with the same enediol. This isomerization process occurs in biochemical reactions near pH 7 for several aldoses and ketoses in enzyme-catalyzed reactions. The isomerization of glucose and fructose occurs by way of their 6-ph osphate esters and is catalyzed by glucose 6-phosphate isomerase. The equilibrium constant for the formation of fructose 6-phosphate from glucose 6-phosphate is approximately 0.3. This reaction is one of the initial steps in glycolysis. 

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