Chapter 26

  1. (a) C; (b) B and F; (c) G; (d) F; (e) E; (f) H; (g) I; (h) D; (i) A; (j) F; (k) B

  2. The oxidative phase generates NADPH and is irreversible. The nonoxidative phase allows for the interconversion of phosphorylated sugars.

  3. Glucose 6-phosphate dehydrogenase; NADP+ concentration

  4. When much NADPH is required. The oxidative phase of the pentose phosphate pathway is followed by the nonoxidative phase. The resulting fructose 6-phosphate and glyceraldehyde 3-phosphate are used to generate glucose 6-phosphate through gluconeogenesis, and the cycle is repeated until the equivalent of one glucose molecule is oxidized to CO2.

  5. Fava beans contain vicine, a purine glycoside that can lead to the generation of peroxides—reactive oxygen species that can damage membranes as well as other biomolecules. Glutathione is used to detoxify the ROS. The regeneration of glutathione depends on an adequate supply of NADPH, which is synthesized by the oxidative phase of the pentose phosphate pathway. People with low levels of the dehydrogenase activity are especially susceptible to vicine toxicity.

    C27

  6. The nonoxidative phase of the pentose phosphate pathway can be used to convert three molecules of ribose 5-phosphate into two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate. These molecules are components of the glycolytic pathway.

  7. The conversion of fructose 6-phosphate into fructose 1,6-bisphosphate by phosphofructokinase requires ATP.

  8. The label emerges at C-5 of ribulose 5-phosphate.

  9. Because red blood cells do not have mitochondria and the only means to obtain NADPH is through the pentose phosphate pathway. There are biochemical means to convert mitochondrial NADH into cytoplasmic NADPH.

  10. The Calvin cycle begins with the fixation of CO2 and proceeds to use NADPH in the synthesis of glucose. The pentose phosphate pathway begins with the oxidation of a glucose-derived carbon atom to CO2 and concomitantly generates NADPH. The regeneration phase of the Calvin cycle converts C6 and C3 molecules back into the starting material—the C5 molecule ribulose 1,5-bisphosphate. The pentose phosphate pathway converts a C5 molecule, ribose 5-phosphate, into C6 and C3 intermediates of the glycolytic pathway.

  11. The oxidative decarboxylation of isocitrate to α-ketoglutarate

  12. Lacking mitochondria, red blood cells metabolize glucose to lactate to obtain energy in the form of ATP. The CO2 results from extensive use of the pentose phosphate pathway coupled with gluconeogenesis. This coupling allows the generation of much NADPH with the complete oxidation of glucose by the oxidative branch of the pentose phosphate pathway.

  13. C-1 and C-3 of fructose 6-phosphate are labeled, whereas erythrose 4-phosphate is not labeled.

  15. ΔE′0 for the reduction of glutathione by NADPH is +0.09 V. Hence ΔG°′ is −17.4 kJ mol−1 (−4.2 kcal mol−1), which corresponds to an equilibrium constant of 1126. The required [NADPH]/[NADP+] ratio is 8.9 × 10−5.

  16. Incubate an aliquot of a tissue homogenate with glucose labeled with 14C at C-1, and incubate another with glucose labeled with 14C at C-6. Compare the radioactivity of the CO2 produced by the two samples. The rationale of this experiment is that only C-1 is decarboxylated by the pentose phosphate pathway, whereas C-1 and C-6 are decarboxylated equally when glucose is metabolized by the glycolytic pathway, the pyruvate dehydrogenase complex, and the citric acid cycle. The reason for the equivalence of C-1 and C-6 in the latter set of reactions is that glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are rapidly interconverted by triose phosphate isomerase.