9.1 Reactions in the hexose monophosphate shunt
9.1.1 Reactions in the oxidative stage
9.1.2 Reactions in the sugar juggling phase
9.1.3 Mechanisms of transketolase and transaldolase
The reactions occurring in the hexose monophosphate shunt can be divided into two phases (Figure 9-2a):
- Oxidation of glucose-6-phosphate to ribulose-5-phosphate, and
- regeneration of glucose-6-phosphate from ribulose-5-phosphate.
These phases may operate at the same time, or independently from each other. While glucose oxidation is irreversible, the interconversion of glucose-6-phosphate to pentose phosphates is reversible (Figure 9-2b). With a typical diet, reasonably rich in starch, the net flow of sugar conversion will be from hexoses to pentoses. However, when eating meat only, our intake of ribose (as RNA) will be a very significant fraction of the total carbohydrates, and the net flow in the hexose monophosphate shunt may actually go the other way (Figure 9-2b, bottom).