Chapter 5, Problem 5.9PS

Problem Set

QUANTITATIVE Succinate Oxidation. The oxidation of succinate to fumarate is an important cellular reaction because it is one of the steps in the citric acid cycle (see Figure 10-9). The two hydrogen atoms that are removed from succinate are accepted by a coenzyme molecule called flavin adenine dinucleotide (FAD), which is thereby reduced to FADH₂:

$\text{Succinate}+\text{FDA}\rightleftharpoons \text{Fumarate}+{\text{FADH}}_2$    (5-32)

ΔG°′ for Reaction 5-32 is 0 cal/mol.

(a)    If you start with a solution containing 0.01 M each of succinate and FAD and add an appropriate amount of the enzyme that catalyzes this reaction, will any fumarate be formed? If so, calculate the resulting equilibrium concentrations of all four species. If not, explain why not.

(b)    Answer part a assuming that 0.01 M FADH2 is also present initially.

(c)    If the steady-state conditions in a cell are such that the FADH₂/FAD ratio is 5 and the fumarate concentration is 2.5 μM, what steady-state concentration of succinate is needed to maintain ΔG′ for succinate oxidation at –1.5 kcal/mol?

Figure 10-9 The Citric Acid Cycle. The two carbon atoms of pyruvate that enter the cycle via acetyl CoA are shown in pink in citrate and subsequent molecules until they are randomized by the symmetry of the fumarate molecule. The carboxyl group of pyruvate that is lost as CO₂ is shaded, as are the two carboxyl groups of oxaloacetate that give rise to CO₂ in reactions CAC-3 and CAC-4. Five of the reactions are oxidations, with NAD⁺ as the electron acceptor in four reactions (PDH, CAC-3, CAC-4, and CAC-8) and FAD as the electron acceptor in one reaction (CAC-6). The reduced form of the coenzyme is shown in purple in each case. Note that when CO2 is released, no H⁺ is given off during NAD⁺ reduction, thereby maintaining the charge balance of these reactions. The generation of GTP shown in reaction CAC-5 is characteristic of animal mitochondria. In bacterial cells and plant mitochondria, ATP is formed directly.