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Chapter 27: Problem 7

(Integrates with Chapters 23 and 24 .) Leptin not only induces synthesis of fatty acid oxidation enzymes and uncoupling protein 2 in adipocytes, but it also causes inhibition of acetyl-CoA carboxylase, esulting in a decline in fatty acid biosynthesis. This effect on acetyl CoA carboxylase, as an additional consequence, enhances fatty acid oxidation. Explain how leptin- induced inhibition of acetyl-CoA carboxylase might promote fatty acid oxidation.

Short Answer

Expert verified
Leptin-induced inhibition of acetyl-CoA carboxylase results in a decline in the production of malonyl-CoA. As malonyl-CoA is an inhibitor of carnitine palmitoyltransferase I, which controls the entry of long-chain fatty acyl CoAs into the mitochondria for oxidation, their reduction promotes the oxidation of fatty acids.

Step by step solution

01

Understand the role of leptin

Firstly, it is important to understand the role of leptin. Leptin is a hormone that reduces the synthesis of fatty acids in the body by inducing the synthesis of enzymes involved in their oxidation, such as uncoupling protein 2, and by inhibiting the enzyme acetyl-CoA carboxylase.
02

Understand the role of acetyl-CoA carboxylase

Next, acetyl-CoA carboxylase is a key enzyme involved in the biosynthesis of fatty acids. It catalyzes the conversion of acetyl-CoA to malonyl-CoA, the initial step in the fatty acid synthesis process. By inhibiting acetyl-CoA carboxylase, leptin causes a reduction in fatty acid biosynthesis.
03

Explain the role of uncoupling protein 2

Then, explain the role of uncoupling protein 2. This protein is an essential player in the oxidation process of fatty acids, enhancing the oxidation and energy expenditure while reducing ATP synthesis.
04

Relating inhibition of acetyl-CoA carboxylase to promotion of fatty acid oxidation

Now, connect the inhibition of acetyl-CoA carboxylase to the promotion of fatty acid oxidation. When acetyl-CoA carboxylase is inhibited by leptin, it leads to a decline in the production of malonyl-CoA. Malonyl-CoA is a potent inhibitor of carnitine palmitoyltransferase I, an enzyme that controls the entry of long-chain fatty acyl CoAs into the mitochondria for oxidation. Therefore, a decline in malonyl-CoA leads to an increase in this transport activity, promoting the oxidation of fatty acids.

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Most popular questions from this chapter

(Integrates with Chapters 19 and \(20 .\) ) Acetate produced in ethanol metabolism can be transformed into acetyl-CoA by the acetyl thiokinase reaction: $$\text { Acetate }+\mathrm{ATP}+\mathrm{CoASH} \longrightarrow \text { acetyl-CoA }+\mathrm{AMP}+\mathrm{PP}_{\mathrm{i}}$$ Acetyl-CoA then can enter the citric acid cycle and undergo oxidation to \(2 \mathrm{CO}_{2}\). How many ATP equivalents can be generated in a liver cell from the oxidation of one molecule of ethanol to \(2 \mathrm{CO}_{2}\) by this route, assuming oxidative phosphorylation is part of the process? (Assume all reactions prior to acetyl-CoA entering the citric acid cycle occur outside the mitochondrion.) Per carbon atom, which is a better metabolic fuel, ethanol or glucose? That is, how many ATP equivalents per carbon atom are generated by combustion of glucose versus ethanol to \(\mathrm{CO}_{2}\) ?

(Integrates with Chapters \(18 \text { and } 22 .)\) The reactions catalyzed by PFK and FBPase constitute another substrate cycle. PFK is AMP activated; FBPase is AMP inhibited. In muscle, the maximal activity of PFK (mmol of substrate transformed per minute) is ten times greater than FBPase activity. If the increase in [AMP] described in problem 5 raised PFK activity from \(10 \%\) to \(90 \%\) of its maximal value but lowered FBPase activity from \(90 \%\) to \(10 \%\) of its maximal value, by what factor is the flux of fructose- 6 - \(P\) through the glycolytic pathway changed? (Hint: Let PFK maximal activity = 10, FBPase maximal activity \(=1 ;\) calculate the relative activities of the two enzymes at low \([\mathrm{AMP}]\) and at high \([\mathrm{AMP}] ;\) let \(J,\) the flux of \(\mathrm{F}\) - 6 -P through the substrate cycle under any condition, equal the velocity of the PFK reaction minus the velocity of the FBPase reaction.)

The Human Biochemistry box, The Metabolic Effects of Alcohol Consumption, points out that ethanol is metabolized to acetate in the liver by alcohol dehydrogenase and aldehyde dehydrogenase: $$\begin{array}{r} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{NAD}^{+} \rightleftharpoons \mathrm{CH}_{3} \mathrm{CHO}+\mathrm{NADH}+\mathrm{H}^{+} \\\ \mathrm{CH}_{3} \mathrm{CHO}+\mathrm{NAD}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3} \mathrm{COO}^{-}+\mathrm{NADH}+2 \mathrm{H}^{+} \end{array}$$ These reactions alter the NAD \(^{+} /\) NADH ratio in liver cells. From your knowledge of glycolysis, gluconeogenesis, and fatty acid oxidation, what might be the effect of an altered \(\mathrm{NAD}^{+} / \mathrm{NADH}\) ratio on these pathways? What is the basis of this effect?

a. Leptin was discovered when a congenitally obese strain of mice \((o b / o b \text { mice })\) was found to lack both copies of a gene encoding a peptide hormone produced mainly by adipose tissue. The peptide hormone was named leptin. Leptin is an anorexic (appetitesuppressing agent; its absence leads to obesity. Propose an experiment to test these ideas. b. A second strain of obese mice \((d b / d b\) mice ) produces leptin in abundance but fails to respond to it. Assuming the \(d b\) mutation leads to loss of function in a protein, what protein is likely to be nonfunctional or absent? How might you test your idea?

Strenuous muscle exertion (as in the 100 -meter dash) rapidly depletes ATP levels. How long will 8 m \(M\) ATP last if 1 gram of muscle consumes \(300 \mu\) mol of ATP per minute? (Assume muscle is \(70 \%\) water.) Muscle contains phosphocreatine as a reserve of phosphorylation potential. Assuming [phosphocreatine] \(=40 \mathrm{m} M,[\text { creatine }]=4 \mathrm{m} M,\) and \(\left.\Delta G^{\circ \prime} \text { (phosphocreatine }+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \text { creatine }+\mathrm{P}_{\mathrm{i}}\right)=-43.3 \mathrm{kJ} / \mathrm{mol}\) how low must [ATP] become before it can be replenished by the reaction: phosphocreatine \(+\mathrm{ADP} \rightleftharpoons \mathrm{ATP}+\) creatine? [Remember \(\Delta G^{\circ \prime}\) (ATP hydrolysis) \(=-30.5 \mathrm{kJ} / \mathrm{mol} .\)]
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