Gluconeogenesis y y
FIGURE 23.39 • Large amounts of NADPH can be produced by the pentose phosphate pathway without significant net production of ribose-5-P. In this version of the pathway, ribose-5-P is recycled to produce glycolytic intermediates.
tions to convert ribulose-5-P to fructose-6-P and glyceraldehyde-3-P, which can be recycled to glucose-6-P via gluconeogenesis. The net reaction for this process is
6 ribulose-5-P + 6 CO2 + 12 NADPH + 12 H+ 6 Ribulose-5-P-> 5-glucose-6-P + Pi
Note that in this scheme, the six hexose sugars have been converted to six pentose sugars with release of six molecules of CO2, and the six pentoses are reconverted to five glucose molecules.
(4) BOTH NADPH AND ATP ARE NEEDED BY THE CELL, BUT RIBOSE-5-P IS NOT Under some conditions, both NADPH and ATP must be provided in the cell. This can be accomplished in a series of reactions similar to case 3, if the fructose-6-P and glyceraldehyde-3-P produced in this way proceed through glycolysis to produce ATP and pyruvate, which itself can yield even more ATP by continuing on to the TCA cycle (Figure 23.40). The net reaction for this alternative is
3 Glucose-6-P + 5 NAD+ + 6 NADP+ + 8 ADP + 5 Pi->
5 pyruvate + 3 CO2 + 5 NADH + 6 NADPH + 8 ATP + 2 H2O + 8 H+
Note that, except for the three molecules of CO2, all the other carbon from glucose-6-P is recovered in pyruvate.
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