4.1 Metabolism of sucrose and fructose
4.1.1 Genetic defects in fructose assimilation
Sucrose is produced from sugar cane and sugar beet, where it is found in very high concentrations (15-18%). In our 'healthy' western diet, it may amount to as much as 20% of our dietary carbohydrate. Sucrose contains glucose and fructose joined in a 1→2–glycosidic bond (Figure 4.1-1).
Hydrolytic cleavage of sucrose, like that of of maltose, occurs in the brush border at the surface of the intestinal epithelial cells. The enzyme responsible is β-fructosidase. Both sugars are then taken up by specific transport: Glucose by the SGLT1 transporter, and fructose by the GLUT5 transporter, which is named after glucose but in fact is more active on fructose than on glucose.
Fructose degradation , sometimes called fructolysis, is carried out in the liver. In the first step, fructose is phosphorylated by fructokinase, which uses ATP as a cosubstrate. This yields fructose-1-phosphate. The latter is then cleaved by aldolase B, which is found mainly in the liver, in keeping with the liver's prominent role in fructose degradation. The products of this reaction are dihydroxyacetone phosphate, which is a metabolite in glycolysis, and glyceraldehyde. Finally, glyceraldehyde is phosphorylated (using ATP) by glyceraldehyde kinase. This yields glyceraldehyde-3-phosphate, which again is an intermediate of glycolysis (Figure 4.1-2).
Glyceraldehyde can alternatively be utilized by conversion to glycerol, and subsequently to glycerol-1-phosphate. The latter is a substrate in the synthesis of triacylglycerol, that is fat. Fructose and sucrose appear to promote obesity more strongly than equivalent amounts of starch or glucose; if that is the case, the utilization via glycerol-1-phosphate may be among the reasons.