Problem 16
When the acetyl-CoA produced during \(\beta\) oxidation in the liver exceeds the capacity of the citric acid cycle, the excess acetyl-CoA forms ketone bodies - acetone, acetoacetate, and D- \(\beta\) hydroxybutyrate. This occurs in people with severe, uncontrolled diabetes; because their tissues cannot use glucose, they oxidize large amounts of fatty acids instead. Although acetyl-CoA is not toxic, the mitochondrion must divert the acetyl-CoA to ketone bodies. What problem would arise if acetyl-CoA were not converted to ketone bodies? How does the diversion to ketone bodies solve the problem?
Problem 17
Suppose you had to subsist on a diet of whale blubber and seal blubber, with little or no carbohydrate. a. What would be the effect of carbohydrate deprivation on the utilization of fats for energy? b. If your diet were totally devoid of carbohydrate, would it be better to consume odd- or even-number fatty acids? Explain.
Problem 18
In a laboratory experiment, investigators feed two groups of rats two different fatty acids as their sole source of carbon for a month. The first group gets heptanoic acid (7:0), and the second gets octanoic acid (8:0). After the experiment, those in the first group are healthy and have gained weight, whereas those in the second group are weak and have lost weight as a result of losing muscle mass. What is the biochemical basis for this difference?
Problem 20
Mutant Acetyl-CoA Carboxylase What would be the consequences for fat metabolism of a mutation in acetyl-CoA carboxylase that replaced the Ser residue normally phosphorylated by AMPK with an Ala residue? What might happen if the same Ser were replaced by Asp?
Problem 21
Acyl-CoA dehydrogenase uses enzyme-bound FAD as a prosthetic group to dehydrogenate the \(a\) and \(\beta\) carbons of fatty acyl-CoA. What is the advantage of using FAD as an electron acceptor rather than NAD \(^{+}\)? Explain in terms of the standard reduction potentials for the \(\mathrm{Enz}-\mathrm{FAD} / \mathrm{FADH}_{2}\left(E^{\prime \circ}=-0.219 \mathrm{~V}\right)\) and \(\mathrm{NAD}^{+} / \mathrm{NADH}\left(E^{\prime \circ}=-0.320 \mathrm{~V}\right)\) half- reactions.
Problem 22
Oxidation of Arachidic Acid How many turns of the fatty acid oxidation cycle are required for complete oxidation of arachidic acid \((20: 0)\) to acetyl-CoA?
Problem 23
Adding \(\left[3-{ }^{14} \mathrm{C}\right]\) propionate \(\left({ }^{14} \mathrm{C}\right.\) in the methyl group) to a liver homogenate leads to the rapid production of \({ }^{14} \mathrm{C}-l a b e l e d\) oxaloacetate. Draw a flowchart for the pathway by which propionate is transformed to oxaloacetate, and indicate the location of the \({ }^{14} \mathrm{C}\) in oxaloacetate.
Problem 25
The complete oxidation of palmitoylCoA to carbon dioxide and water is represented by the overall equation $$ \begin{gathered} \text { Palmitoyl-CoA }+23 \mathrm{O}_{2}+108 \mathrm{P}_{\mathrm{i}}+108 \mathrm{ADP} \rightarrow \\ \text { CoA }+16 \mathrm{CO}_{2}+108 \mathrm{ATP}+23 \mathrm{H}_{2} \mathrm{O} \end{gathered} $$ Water also forms in the reaction $$ \mathrm{ADP}+\mathrm{P}_{\mathrm{i}} \rightarrow \mathrm{ATP}+\mathrm{H}_{2} \mathrm{O} $$ but is not included as a product in the overall equation. Why?
Problem 26
Cattle, deer, sheep, and other ruminant animals produce large amounts of propionate in the rumen through the bacterial fermentation of ingested plant matter. Propionate is the principal source of glucose for these animals, via the route propionate \(\rightarrow\) oxaloacetate \(\rightarrow\) glucose. In some areas of the world, notably Australia, ruminant animals sometimes show symptoms of anemia with concomitant loss of appetite and retarded growth, resulting from an inability to transform propionate to oxaloacetate. This condition is due to a cobalt deficiency caused by very low cobalt levels in the soil and thus in plant matter. Explain.
Problem 27
Bears expend about \(25 \times 10^{6} \mathrm{~J} /\) day during periods of hibernation, which may last as long as seven months. The energy required to sustain life is obtained from fatty acid oxidation. How much weight (in kilograms) do bears lose after 7 months of hibernation? How could a bear's body minimize ketosis during hibernation? (Assume the oxidation of fat yields \(38 \mathrm{~kJ} / \mathrm{g}\).)
