23
Fatty Chapter Acid Catabolism
Problems and Solutions
2. Calculate the approximate number of ATP molecules that can be obtained from the oxidation of cis-11-heptadecenoic acid to CO2 and water. Answer: cis-11-Heptadecenoic acid is a 17-carbon fatty acid with a double bond between carbons 11 and 12. -oxidation, ignoring for the moment the presence of the double bond, will produce 7 acetyl-CoA units and one propionyl-CoA. The 7 acetyl-CoA units are metabolized in the citric acid cycle with the following stoichiometry: 7 Acetyl-CoA + 14 O2 + 70 ADP + 70 Pi > 7 CoA + 77 H2O + 14 CO2 + 70 ATP In producing the 7 acetyl-CoAs, 7 -carbons had to be oxidized, and only 6 of these by the And, if complete -oxidation pathway. Thus, 6 FAD 6 FADH2, and 6 NAD+ 6 NADH. electrons from FADH2 produce 1.5 ATP, whereas electrons from NADH produce 2.5 ATPs, we have 9 ATPs from FADH2 and 15 ATPs from NADH. Now we will deal with the double bond. The presence of a double bond means that a carbon is already partially reduced, so, the FADdependent reduction step is bypassed and only NADH is generated. Thus, one acetyl-CoA unit is generated along with only 2.5 ATPs. Finally, we consider the propionyl-CoA unit that is metabolized into succinyl CoA. In this pathway, propionyl-CoA is converted to methylmalonylCoA, a process driven in part by ATP hydrolysis. Succinyl-CoA is a citric acid cycle intermediate that is metabolized to oxaloacetate. This process yields 1 GTP, one FAD-dependent oxidation, and one NAD+-dependent reaction. Thus a net of (1 + 1.5 + 2.5 - 1) 4 ATPs are produced. Succinyl-CoA cannot be consumed by the citric acid cycle and we have only converted it to oxaloacetate. One way of oxidizing succinate completely is to remove the carbons from the mitochondria as malate, and convert them to CO2 and pyruvate using malic enzyme. This reaction is an oxidative-decarboxylation; NADP+ is reduced in the cytosol. There is a slight reduction in ATP production potential for electrons on NADPH in the cytosol but we will ignore this and assume that the reduction is equivalent energetically to reduction of malate to oxaloacetate (which we have already taken into account). Pyruvate is metabolized to acetyl-CoA with production of NADH, which supports 2.5 ATPs synthesized. The acetyl-CoA unit results in 10 ATP. To summarize: Oxidation of 7 -carbons produces 24 ATPs; oxidation of an additional carbon (the one involved in a double bond) produces 2.5 ATPs; oxidation of 7 acetyl-CoAs contributes 70 ATPs; oxidation of an additional acetyl-CoA (derived from propionyl-CoA) yields 16.5 ATPs. The total is 113. 3. Phytanic acid, the product of chlorophyll that causes problems for individuals with Refsum's disease, is 3,7,11,15-tetramethylhexadecanoic acid. Suggest a route for its oxidation that is consistent with what you have learned in this chapter. (Hint: The methyl group at C-3 effectively blocks hydroxylation and normal -oxidation. You may wish to initiate breakdown in some other way.) Answer: The structure of phytanic acid is: CH3 CH3 CH3 CH CH2 CH2 CH2 CH3 CH3
CH2 CH CH2 CH COOCH2 CH2 CH2 CH2 CH2
Normally it is metabolized by -oxidation. The enzyme phytanic acid -oxidase hydroxylates the -carbon to produce phytanic acid, which is decarboxylated by phytanate -oxidase to produce pristanic acid. Pristanic acid can form a coenzyme A ester which is metabolized by -oxidation to yield 3 propionyl-CoAs, 3 acetyl-CoAs and 2-methyl-propionyl-CoA. The reaction sequence is shown below.
Chapter 23 . Fatty Acid Catabolism
CH3 H3 C CH C H2 CH 3 C H2 CH CH3 CH C H2 CH3 C H2 CH C H2 COO-
H2 C
C C H2 H2 phytanic acid
H2 C
H2 C
CH3 H3 C CH C H2
H2 C
phytanic -hydrox ylase CH 3 CH3 CH3 H2 H2 CH CH CH COOC C C C CH C C C H2 H2 H2 H2 H2 OH phytanic acid H2 O CO2 phytanic -oxidase
CH 3 H3C CH C H2
H2 C
CH3
CH CH C C C C H2 H2 H2 H2 pristanic acid
H2 C
CH3
H2 C
CH3 C H2 CH COO-
CoA-SH
Actyl-CoA synthetase
CH3 C H2 CH C H2 CH 3 C H2 CH COO-
CH3 H3C CH C H2
H2 C
CH3 C H2 CH C H2
H2 C
H2 C
O H3 C O CH3 H3 C O H3 C O H3 C H3 C O H3 C C S-CoA O C CH2 C S-CoA O CH2 C S-CoA S-CoA
H3 C CH C S-CoA 2-methyl-propionyl-CoA
CH2 C S-CoA propionyl-CoA
C S-CoA acetyl-CoA
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Chapter 23 . Fatty Acid Catabolism
4. Even though acetate units, such as those obtained from fatty acid oxidation, cannot be used for net synthesis of carbohydrate in animals, labeled carbon from 14C-labeled acetate can be found in newly synthesized glucose (for example in liver glycogen) in animal tracer studies. Explain how this can be. Which carbons of glucose would you expect to be the first to be labeled by 14C-labeled acetate?
Answer: Acetate, as acetyl-CoA, enters the citric acid cycle where its two carbons show up as carbons 1 and 2 or carbons 3 and 4 of oxaloacetate after one turn of the cycle. C-1 and C-4 label derive from acetate labeled at the carboxyl carbon, whereas C-2 and C-3 label derive from label at the methyl carbon of acetate. Oxaloacetate is converted to PEP with carbons 1, 2, and 3 becoming carbons 1, 2, and 3 of PEP, and so label is expected at carbon 1 if carboxy-labeled acetate is used and at carbons 2 or 3 if methyl-labeled acetate is used. Conversion of PEP to glyceraldehyde-3-phosphate results in label either at carbon 1 or at carbon 2 or 3. Isomerization to dihydroxyacetone phosphate labels the same carbons. Carbons 1 of DHAP and glyceraldehyde-3-phosphate become carbons 3 and 4 of fructose-1,6-bisphosphate so aldolase will label fructose-1,6-bisphosphate at carbons 3 and 4 from carboxy-labeled acetate and carbons 1, 2, 5, and 6 from methyl-labeled acetate.
8. Write a properly balanced chemical equation for the oxidation to CO2 and water of (a) myristic acid, (b) stearic acid, (c) -linolenic acid, and (d) arachidonic acid.
Answer: a. acid Myristic is a saturated 14:0 fatty acid, CH3(CH2)12COOH. To metabolize myristic acid, it is first activated to a coenzyme A derivative in the following reaction: CH3(CH2)12COOH + ATP + CoA-SH > CH3(CH2)12CO-S-CoA + AMP + PPi Six cycles of -oxidation convert myristoyl-CoA to 7 acetyl-CoA units. The balanced equation for -oxidation is: CH3(CH2)12CO-S-CoA + 6 CoA-SH + 6 H2O + 6 NAD+ + 6 FAD > 7 CH3CO-S-CoA + 6 NADH + 6 H+ + 6 FADH2 The citric acid cycle metabolizes acetyl-CoA units as follows: 7 CH3CO-S-CoA + 21 H2O + 21 NAD+ + 7 FAD > 14 CO2 + 7 CoA-SH + 21 NADH + 21 H+ + 7 FADH2 This is accompanied by substrate-level GDP phosphorylation, which is equivalent to: 7 ADP + 7 Pi > 7 ATP + 7 H2O Electron transport recycles NAD+ as follows: 27 NADH + 27 H+ + 13.5 O2 > 27 NAD+ + 27 H2O that supports the production of 2.5 27 (ADP + Pi > ATP +H2O) or 67.5 ADP + 67.5 Pi > 67.5 ATP + 67.5 H2O FAD is recycled by: 13 FADH2 + 6.5 O2 > 13 FAD + 13 H2O which supports the production of 1.5 13 (ADP + Pi > ATP +H2O) or 19.5 ADP + 19.5 Pi > 19.5 ATP + 19.5 H2O The AMP and PPi produced in the very first reaction can be metabolized by hydrolysis of PPi and phosphorylation of AMP to ADP using ATP. Thus, PPi + H2O > 2 Pi and AMP + ATP > 2 ADP: AMP + ATP + PPi + H2O > 2 ADP + 2 Pi If we sum these equations we find: CH3(CH2)12COOH + 92 ADP + 92 Pi + 20 O2 > 92 ATP + 14 CO2 + 106 H2O b. Stearic acid or octadecanoic acid is 18:0 and its oxidation is given by: CH3(CH2)16COOH + 12O ADP + 120 Pi + 26 O2 > 12O ATP + 18 CO2 + 138 H2O c. -Linolenic acid is a polyunsaturated 18-carbon fatty acid with double bonds at carbons 9, 12, and 15. In two out of a total of 8 rounds of -oxidation, reduction by FAD is bypassed. Therefore 2 fewer moles of FADH2 are reduced in the electron transport chain than for a fully saturated fatty acid and 1.0 fewer moles of O2 are consumed. The amount of ATP is reduced by 1.5 2 = 3.0. In addition, a NADH was actually consumed after the fifth cycle of -oxidation to resolve a conjugated double bond. This accounts for 2.5 few ATP and 0.5 fewer O2 that in b. The balanced equation is: CH3CH2(CH=CHCH2)3(CH2)6COOH + 114.5 ADP + 114.5 Pi + 24.5 O2 > 114.5 ATP + 18 CO2 + 129.5 H2O
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Chapter 23 . Fatty Acid Catabolism
d. Arachidonic acid is 5,8,11,14-eicosatetraenoic acid, a 20-carbon fatty acid with four double bonds. It undergoes a total of 9 cycles of oxidation, with 2 cycles not producing FADH2 and consumption of two NADH to resolve two cases of conjugated double bonds. The balanced equation is: CH3(CH2)4(CH=CHCH2)4(CH2)2COOH + 126 ADP + 126 Pi + 27 O2 > 126 ADP + 20 CO2 + 142 H2O
14. Write a reasonable mechanism for the HMG-CoA synthase reaction shown in Figure 23.28.
Answer:
CoA S
O C
H C H B E H CoA S
O C
H CH
CH3 C O HB E
CH2 C O
SCoA
O H HC C H
CH3 CoASH H2 0 C OH CoA S
O C
H C H
CH3 C OH B E
CH2 C O
CH2 C O
SCoA
SCoA
15. Discuss the changes of the oxidation state of cobalt in the course of the methylmalonyl-CoA mutase reaction. Why do they occur as shown in Figure 23.21?
Answer: The mechanism shown in Figure 23.21 involves free-radical formation on methylmalonyl-CoA, which results in rearrangement to succinyl CoA. Free-radical formation is initiated by cleavage of the Co-C bond in cobalamin by hemolytic cleavage. Homolytic cleavage is decomposing of a compound into two uncharged atoms or radicals. The electron pair in the Co-C bond is in effect ...