4 Catabolism of sugars other than glucose
4.1 Metabolism of sucrose and fructose
4.2 Metabolism of lactose and galactose
4.3 The polyol pathway
Starch is the most important carbohydrate in our diet. As we have seen, starch is consists of glucose, which is therefore the most important dietary monosaccharide. Other quantitatively important sugars are:
- Lactose (milk sugar) is a disaccharide of glucose and galactose and is found in milk (surprise!).
- Sucrose is a disaccharide of glucose and fructose and is found in many plants and fruit, most prominently in sugar cane and sugar beet.
- Free fructose is also found in significant amounts in the diet, both in fruit and as a sweetener.
- Sorbitol is a sugar alcohol, that is the carbonyl group found in fructose or glucose is reduced to a CHOH group. It is used as a sweetener but also occurs naturally in the diet.
- Nucleic acids contain ribose and deoxyribose.
Ribose is part of the hexose monophosphate shunt and will be covered in the corresponding chapter. Here, we will focus on the first three sugars. The main idea of metabolism of these sugars is economy: Instead of completely separate degradative pathways, there are short adapter pathways that funnel them into the main pathway of carbohydrate degradation, that is glycolysis. An overview of these pathways is given in Figure 4-1.