Chapter 12

Methanol is highly toxic, not because of its own biological activity but because it is converted metabolically to formaldehyde, through action of alcohol dehydrogenase. Part of the medical treatment for methanol poisoning involves administration of large doses of ethanol. Explain why this treatment is effective.

Suppose it were possible to label glucose with \({ }^{14} \mathrm{C}\) at any position or combination of positions. For yeast fermenting glucose to ethanol, which form or forms of labeled glucose would give the most radioactivity in \(\mathrm{CO}_{2}\) and the least in ethanol?

Because of the position of arsenic in the periodic table, arsenate \(\left(\mathrm{AsO}_{4}{\underline{\phantom{xx}}}^{3-}\right)\) is chemically similar to inorganic phosphate and is used by phosphaterequiring enzymes as an alternative substrate. Organic arsenates are quite unstable, however, and spontaneously hydrolyze. Arsenate is known to inhibit ATP production in glycolysis. Identify the target enzyme, and explain the mechanism of inhibition.

As early as the \(1930 \mathrm{~s}\), it was known that frog muscles could still contract when glycolysis was inhibited. Where did the ATP come from to drive these contractions?

Briefly discuss why each of the three common forms of galactosemia involves impaired utilization of galactose. Which metabolic process is blocked in each condition?

Enolase has a strict requirement for two \(\mathrm{Mg}^{2+}\) ions in its active site. Propose a role for these ions in the catalytic mechanism of the enzyme.

The muscle isozyme of lactate dehydrogenase is inhibited by lactate. Steady-state kinetic analysis yielded the following data, with lactate either absent or present at a fixed concentration. (a) Pyruvate is the substrate whose concentration is varied in one plot, NADH in the other. Identify each. Use an arrow and the appropriate letter (b, c, d, or e) to identify each of the following. (b) Reciprocal of \(V_{\max }\) for the uninhibited enzyme. (c) The line representing data obtained in the presence of lactate acting as a competitive inhibitor with respect to the variable substrate. (d) The line representing data obtained in the presence of lactate acting as a noncompetitive inhibitor with respect to the variable substrate. (e) Reciprocal of \(K_{\mathrm{M}}\) in the presence of lactate acting as a competitive inhibitor. (f) If \(K_{\mathrm{M}}\) for NADH is \(2 \times 10^{-5} \mathrm{M}\), then which of the following is the most appropriate NADH concentration to use when determining \(K_{\mathrm{M}}\) for pyruvate: \(10^{-7} \mathrm{M}, 10^{-6} \mathrm{M}, 10^{-5} \mathrm{M}, 10^{-4} \mathrm{M}\), or \(10^{-3} \mathrm{M}\) ?

How many high-energy phosphates are generated or consumed in converting (a) 1 mole of glucose to lactate? (b) 2 moles of lactate to glucose?

Avidin is a protein that binds extremely tightly to biotin, so avidin is a potent inhibitor of biotin-requiring enzyme reactions. Consider glucose biosynthesis from each of the following substrates and predict which of these pathways would be inhibited by avidin. (a) Lactate (b) Oxaloacetate (c) Malate (d) Fructose-6-phosphate (e) Phosphoenolpyruvate

\({ }^{14} \mathrm{CO}_{2}\) was bubbled through a suspension of liver cells that was undergoing gluconeogenesis from lactate to glucose. Which carbons in the glucose molecule would become radioactive?

Predict the effect of each of the following mutants on the rate of glycolysis in liver cells (increase, decrease, no change): (a) Loss of the allosteric site for ATP in PFK-1 (b) Loss of the binding site for citrate in \(\mathrm{PFK}-1\) (c) Loss of the phosphatase domain of PFK-2/FBPase-2 (d) Loss of the binding site for fructose-1,6-bisphosphate in pyruvate kinase (e) Loss of acetyl-CoA binding site in pyruvate carboxylase

Why does it make good metabolic sense for phosphoenolpyruvate carboxykinase, rather than pyruvate carboxylase, to be the primary target for regulating gluconeogenesis at the level of control of enzyme synthesis?

Write a one-sentence explanation for each of the following statements. (a) In liver, glucagon stimulates glycogen breakdown via cAMP. Although you might expect glucagon to stimulate catabolism of the glucose formed as well, glucagon inhibits glycolysis and stimulates gluconeogenesis in liver. (b) An individual with a glucose-6-phosphatase deficiency suffers from chronic hypoglycemia. (c) The action of phosphorylase kinase simultaneously activates glycogen breakdown and inhibits glycogen synthesis. (d) The presence in liver of glucose-6-phosphatase is essential to the function of the liver in synthesizing glucose for use by other tissues.

Pyruvate carboxylase is thought to activate \(\mathrm{CO}_{2}\) by ATP, through formation of carboxyphosphate as an intermediate. Propose a mechanism for the formation of this intermediate.