Chapter 20

Variation on a theme. Sedoheptulose 1,7 -bisphosphate is an intermediate in the Calvin cycle but not in the pentose phosphate pathway. What is the enzymatic basis of this difference?

Total eclipse. An illuminated suspension of Chlorella is actively carrying out photosynthesis. Suppose that the light is suddenly switched off. How would the levels of 3 -phosphoglycerate and ribulose 1,5 -bisphosphate change in the next minute?

\(\mathrm{CO}_{2}\) deprivation. An illuminated suspension of Chlorella is actively carrying out photosynthesis in the presence of \(1 \% \mathrm{CO}_{2}\) The concentration of \(\mathrm{CO}_{2}\) is abruptly reduced to \(0.003 \%\). What effect would this reduction have on the levels of 3 -phosphoglycerate and ribulose 1,5 -bisphosphate in the next minute?

When one equals two. In the Ca pathway, one ATP molecule is used in combining the \(\mathrm{CO}_{2}\) with phosphoenolpyruvate to form oxaloacetate (Figure 20.17 ), but, in the computation of energetics bookkeeping, two ATP molecules are said to be consumed. Explain.

Global warming. \(\mathrm{C}_{3}\) plants are most common in higher latitudes and become less common at latitudes near the equator. The reverse is true of \(\mathrm{C}_{4}\) plants. How might global warming affect this distribution?

Tracing glucose, Glucose labeled with \(^{14} \mathrm{C}\) at \(\mathrm{C}\) - 6 is added to a solution containing the enzymes and cofactors of the oxidative phase of the pentose phosphate pathway. What is the fate of the radioactive label?

Recurring decarboxylations. Which reaction in the citric acid cycle is most analogous to the oxidative decarboxylation of 6 phosphogluconate to ribulose 5 -phosphate? What kind of enzyme-bound intermediate is formed in both reactions?

Carbon shuffling. Ribose 5 -phosphate labeled with \(^{14} \mathrm{C}\) at \(\mathrm{C}-1\) is added to a solution containing transketolase, transaldolase, phosphopentose epimerase, phosphopentose iso. merase, and glyceraldehyde 3 -phosphate. What is the distribution of the radioactive label in the erythrose 4 -phosphate and fructose 6 -phosphate that are formed in this reaction mixture?

An alternative approach. The mechanisms of some aldolases do not include Schiff-base intermediates. Instead, these enzymes require bound metal ions. Propose such a mechanism for the conversion of dihydroxyacetone phosphate and glyceraldehyde 3 -phosphate into fructose 1,6 -bisphosphate.

Photosynthetic efficiency. Use the following information to estimate the efficiency of photosynthesis. The \(\Delta G^{\circ}\) for the reduction of \(\mathrm{CO}_{2}\) to the level of hexose is \(+477 \mathrm{kJ} \mathrm{mol}^{-1}\left(+114 \mathrm{kcal} \mathrm{mol}^{-1}\right)\) A mole of 600 -nm photons has an energy content of \(199 \mathrm{kJ}\) \((47.6 \mathrm{kcal})\) Assume that the proton gradient generated in producing the required NADPH is sufficient to drive the synthesis of the required ATP.