Chapter 19

Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at \(\mathrm{C}_{\gamma}\) with \(^{14} \mathrm{C}\)

Describe the effect on the TCA cycle of (a) increasing the concentration of \(\mathrm{NAD}^{+},\) (b) reducing the concentration of \(\mathrm{ATP}\), and (c) increasing the concentration of isocitrate.

(Integrates with Chapter \(15 .\) ) The serine residue of isocitrate dehydrogenase that is phosphorylated by protein kinase lies within the active site of the enzyme. This situation contrasts with most other examples of covalent modification by protein phosphorylation, where the phosphorylation occurs at a site remote from the active site. What direct effect do you think such active-site phosphorylation might have on the catalytic activity of isocitrate dehydrogenase? (See Barford, D., 1991. Molecular mechanisms for the control of enzymic activity by protein phosphorylation. Biochimica et Biophysica Acta \(1133: 55-62 .\)

The first step of the \(\alpha\) -ketoglutarate dehydrogenase reaction involves decarboxylation of the substrate and leaves a covalent TPP intermediate. Write a reasonable mechanism for this reaction.

In a tissue where the TCA cycle has been inhibited by fluoroacetate, what difference in the concentration of each TCA cycle metabolite would you expect, compared with a normal, uninhibited tissue?

In addition to fluoroacetate, consider whether other analogs of TCA cycle metabolites or intermediates might be introduced to inhibit other, specific reactions of the cycle. Explain your reasoning.

Based on the action of thiamine pyrophosphate in catalysis of the pyruvate dehydrogenase reaction, suggest a suitable chemical mechanism for the pyruvate decarboxylase reaction in yeast: Pyruvate \(\longrightarrow\) acetaldehyde \(+\mathrm{CO}_{2}\)

(Integrates with Chapter 3 .) Aconitase catalyzes the citric acid cycle reaction: Citrate \(\rightleftharpoons\) isocitrate The standard free energy change, \(\Delta G^{\circ \prime}\), for this reaction is +6.7 \(\mathrm{kJ} / \mathrm{mol}\). However, the observed free energy change \((\Delta G)\) for this reaction in pig heart mitochondria is \(+0.8 \mathrm{kJ} / \mathrm{mol}\). What is the ratio of [isocitrate]/[citrate] in these mitochondria? If [isocitrate] = \(0.03 \mathrm{m} M,\) what is [citrate]?

Describe the labeling pattern that would result if \(^{14} \mathrm{CO}_{2}\) were incorporated into the TCA cycle via the pyruvate carboxylase reaction.

Describe the labeling pattern that would result in the glyoxylate cycle if a plant were fed acetyl-CoA labeled at the \(-\mathrm{CH}_{3}\) carbon.