The pentose phosphate pathway an alternative to glycolysis

There is an alternative pathway for the conversion of glucose 6-phosphate to fructose 6-phosphate, the pentose phosphate pathway (sometimes known as the hexose monophosphate shunt), shown in Figure 5.14.

Overall, the pentose phosphate pathway produces 2 mol of fructose 6-phosphate, 1 mol of glyceraldehyde 3-phosphate and 3 mol of carbon dioxide from 3 mol of glucose 6-phosphate, linked to the reduction of 6 mol of NADP+ to NADPH. The sequence of reactions is as follows:

• Three mol of glucose are oxidized to yield 3 mol of the five-carbon sugar ribulose 5-phosphate + 3 mol of carbon dioxide.
• Two mol of ribulose 5-phosphate are isomerized to yield 2 mol of xylulose 5-phosphate.
• One mol of ribulose 5-phosphate is isomerized to ribose 5-phosphate.
• One mol of xylulose 5-phosphate reacts with the ribose 5-phosphate, yielding (ultimately) fructose 6-phosphate and erythrose 4-phosphate.
• The other mol of xylulose-5-phosphate reacts with the erythrose 4-phosphate, yielding fructose 6-phosphate and glyceraldehyde 3-phosphate.

This is the pathway for the synthesis of ribose for nucleotide synthesis (section

Figure 5.14 The pentose phosphate pathway (also known as the hexose monophosphate shunt).

9.2.2); more importantly, it is the source of about half the NADPH required for fatty acid synthesis (section 5.6.1); tissues that synthesize large amounts of fatty acids have a high activity of the pentose phosphate pathway. As discussed in section 7.4.2.2, the pentose phosphate pathway is also important in the respiratory burst of macrophages that are activated in response to infection.

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