Chapter 15

What is meant by intermediary metabolism?

Differentiate between anabolism and catabolism.

While walking to biochemistry class with a friend, you see the following graffiti spray painted on the wall of the science building: "When a system is in equilibrium, the Gibbs free energy is maximum." You are disgusted, not only at the vandalism, but at the ignorance of the vandal. Your friend asks you to explain. How do you respond?

What are the three primary uses for cellular energy?

Match the terms in the two columns. (a) Cellular energy currency _____ (b) Anabolic electron carrier _____ (c) Phototroph _____ (d) Catabolic electron carrier _____ (e) Oxidation-reduction reaction _____ (f) Activated carrier of two-carbon fragments _____ (g) Vitamin _____ (h) Anabolism _____ (i) Amphibolic reaction _____ (j) Catabolism _____ 1\. \(\mathrm{NAD}^{+}\) 2\. Coenzyme A 3\. Precursor to coenzymes 4\. Yields energy 5\. Requires energy 6\. ATP 7\. Transfers electrons 8\. \(\mathrm{NADP}^{+}\) 9\. Converts light energy into chemical energy 10\. Used in anabolism and catabolism

What factors account for the high phosphoryl-transfer potential of nucleoside triphosphates?

Why does it make good sense to have a single nucleotide, ATP, function as the cellular energy currency?

The standard free energy of hydrolysis for ATP is \(-30.5 \mathrm{kJ} \mathrm{mol}^{-1}\left(-7.3 \mathrm{kcal} \mathrm{mol}^{-1}\right):\) What conditions might be changed to alter the free energy of hydrolysis?

Metabolic pathways frequently contain reactions with positive standard free- energy values, yet the reactions still take place. How is it possible?

What is the structural feature common to ATP, FAD, \(\mathrm{NAD}^{+},\) and CoA?

What are the activated electron carriers for catabolism? For anabolism?

Thioesters, common in biochemistry, are more unstable (energy rich) than oxygen esters. Explain.

What are the three principal means of controlling metabolic reactions?

Glycolysis is a series of 10 linked reactions that convert one molecule of glucose into two molecules of pyruvate with the concomitant synthesis of two molecules of ATP (Chapter 16 ). The \(\Delta G^{\circ \prime}\) for this set of reactions is \(-35.6 \mathrm{kJ} \mathrm{mol}^{-1}\left(-8.5 \mathrm{kcal} \mathrm{mol}^{-1}\right),\) whereas the \(\Delta G^{\circ}\) is \(-90 \mathrm{kJ} \mathrm{mol}^{-1}\left(-22 \mathrm{kcal} \mathrm{mol}^{-1}\right) .\) Explain why the free-energy release is so much greater under intracellular conditions than under standard conditions.

Digestion is the first stage in the extraction of energy from food, but no useful energy is acquired in this stage. Why is digestion considered a stage in energy extraction?

The reaction of NADH with oxygen to produce \(\mathrm{NAD}^{+}\) and \(\mathrm{H}_{2} \mathrm{O}\) is very exergonic, yet the reaction of NADH and oxygen takes place very slowly. Why does a thermodynamically favorable reaction not take place rapidly?

The \(\mathrm{p} K_{\mathrm{a}}\) of an acid is a measure of its proton-group- transfer potential. (a) Derive a relation between \(\Delta G^{\circ \prime}\) and \(\mathrm{p} K_{\mathrm{a}}\) (b) What is the \(\Delta G^{\circ}\) ' for the ionization of acetic acid, which has a \(\mathrm{p} K_{\mathrm{a}}\) of \(4.8 ?\)

Fibrinogen, a precursor to the bloodclot protein fibrin, contains tyrosine-O- sulfate. Propose an activated form of sulfate that could react in vivo with the aromatic hydroxyl group of a tyrosine residue in a protein to form tyrosine-O-sulfate.

Creatine is a popular, but untested, dietary supplement. (a) What is the biochemical rationale for the use of creatine? (b) What type of exercise would most benefit from creatine supplementation?

The enzyme aldolase catalyzes the following reaction in the glycolytic pathway: Fructose \(1,6-\) bisphosphate \(\rightleftharpoons\) dihydroxyacetone phosphate \(+\) glyceraldehyde 3 -phosphate The \(\Delta G^{\circ \prime}\) for the reaction is \(+23.8 \mathrm{kJ} \mathrm{mol}^{-1}(+5.7 \mathrm{kcal}\) \(\left.\operatorname{mol}^{-1}\right),\) whereas the \(\Delta G^{\circ}\) in the cell is \(-1.3 \mathrm{kJ} \mathrm{mol}^{-1}\) \(\left(-0.3 \mathrm{kcal} \mathrm{mol}^{-1}\right) .\) Calculate the ratio of reactants to products under equilibrium and intracellular conditions. Using your results, explain how the reaction can be endergonic under standard conditions and exergonic under intracellular conditions.

On page 262 a reaction, \(\mathrm{A} \rightleftharpoons \mathrm{B},\) with a \(\Delta G^{\circ}=+16.7 \mathrm{kJ} \mathrm{mol}^{-1}\) \(\left(+4.0 \mathrm{kcal} \mathrm{mol}^{-1}\right)\) is shown to have a \(K_{e q}\) of \(1.15 \times 10^{-3}\). The \(K_{\text {eq }}\) is increased to \(2.67 \times 10^{2}\) if the reaction is coupled to ATP hydrolysis under standard conditions. The ATPgenerating system of cells maintains the \([\mathrm{ATP}] /[\mathrm{ADP}]\left[\mathrm{P}_{\mathrm{i}}\right]\) ratio at a high level, typically of the order of \(500 \mathrm{M}^{-1}\). Calculate the ratio of B/A under cellular conditions.