25.a ATPYield 771

beta-oxidation cycle is two carbons shorter than in the previous round. For this reason, the pathway for the degradation of fatty acids to acetyl CoA is ofien called thefatty acid spiral. Every round of the spiral produces one molecule each of acetyl CoA, NADH, and FADH2 until the fatty acyl CoA molecule is only four carbons Iong. At this point, the first three steps of the flnal round of beta oxidation produce the compound acetoacetyl CoA. The fourth step, the reaction of acetoacetyl CoA with CoA, produces an extra molecule of acetyl CoA from the tail end of the fattv acid without formation of NADH and FADHz. o o CH3-C-CH2 C S-CoA + HS-CoA ------AcetoacetylCoA Studies have shown that exercis- o o ing less frequentlybut for a longer CH3-C-S-CoA + CH3-C-S-CoA duration is a good approachto burning body fat. After 40 minutes In other words, the complete conversion of a fatty acyl CoA to two-carbon of exercising,the percentage of fragments of acetyl CoA always produces one more molecule of aceryl CoA energy supplied by fat is greater than of NADH or FADH2.To summarize, the breakdornmof palmitic acid than that supplied by carbohy- giveseight molecules of acetyl CoA,but only sevenmolecules of NADH and drates.Still longer exerciseperiods lead to an even higher percentage sevenmolecules of FADH2are produced. of energy being supplied by body fat. PRACTICEEXERCISE 25.I Lauric acid is converted to acetyl CoA in beta oxidation. Determine the yields of (a) acetyl CoA, (b) NADH, and (c) FADH2.

25.4ATP yield AIM: To calculotethe numherof ATPmolecules formed by the oxidotion of o fotty ocid molecule.

In Chapter 24 we saw that the carbons of the acetyl CoA produced by the Focus catabolism of glucose can be completely oxidized to carbon dioxide in the The complete oxidation of I citric acid cycle. Each molecule of acetyl CoA oxidized in this fashion yields molecule of palmitic acidyields enough energy to make one molecule of AIB one molecule of FADH2,and 129 molecules ofAIP. three molecules of NADH. The reducing power of each molecule of NADH can make three molecules of AIP by cellular respiration; FADH2produces two molecules of AIP in the same way. It was sho',,rmthat 38 AIP molecules is the total useful energyyield of aerobicglucose catabolism. Molecules of acetyf CoA are the sam-e,regardless of their source. Like acetyl CoA molecules produced from glucose,the acetyl CoA molecules formed in the fatty acid spiral can be oxidized in the citric acid cycle. Since we can find the yield of NADH, FADH2, and ATP from the beta-oxidation reactions of the fatty acid spiral and from the citric acid cycle, we can cal- culate how many molecules of AIP are produced by the total oxidation of one molecule of any fatty acid to carbon dioxide and water. Table 25.1 shows a calculation of this kind for palmitic acid. In calculating the total AIP yield obtained from the complete oxidation of the fatty acid, we can count the investment of two high-energy phos- 772 CHAPTER25 Lipid Metabolism

Table25.1 ATP Produetion from CompleteAerobic Catabolism of OneMolecule of PalmiticAcid

Pathway ATPyield

fatty acid spiral (2 ATP invested to make palmitoyl CoA to start spiral) -2 citric acid cycle (Bmolecules of acetyl CoA degraded) B oxidative pho sphorylation TNADH from fatty acid spiral 2l 7FADH2from fatty acid spiral I4 24NADH from citric acid cycle 72 BFADH2from citric acid cycle 16 L29

The oxidation of I g of a fatty acid phate bonds required to activate the fatty acid as two AIP molecules. We yields about 0.5 molATP. The can do this because hydrolysis of one molecule of AIP to AMP and 2P; is oxidation of I g of glucoseyields equivalent to the hydrolysis of 2AIP to 2ADP andzPi. Table 25.1 shows that about 0.2 molAIP. This ratio, for every molecule of palmitic acid completely oxidized to carbon dioxide 0.5:0.2,is approximatelythe same and water, 129molecules of AIP are formed. No wonderfats are an impor- as the ratio of the energyyield for tant source of energy for cellular work. the complete oxidation of fats and produciion carbohvdrates.9 kcal:4kcal. Energy is not the only useful function of beta oxidation. Cells using NADH and FADH2to reduce oxygen also produce a good deal of water as a by-product of cellular respiration. Certain animals have become physiologically adapted to take advantage of this fact. A camel's hump, for example, consistsof fat that has been stored in times of plenty. Aerobic catabolism of this fat supplies enough energy and water to permit camels to surviveduringlongperiods of famine and drought.

PRACTICEEXERCISE 25.2 Calculate how many molecules of ATP are produced by the total oxida- tion of lauric acid to carbon dioxide and water.

25.5 Glycerolmetobolism AIM: To describehow glycerolis usedos on energys.ource.

The hydrolysis of triglycerides produces glycerol as well as fatty acids. Glyc- erol is converted by cells to dihydroxyacetone phosphate in two steps. Glycerol produced by H2C-OH H2C-OH H2C-OH hydrolysis of triglycerides -t-t-l enters glycolysis as dihydroxy- H-C-OH --Z---- H-C-SH --Z------_ c:o acetone phosphate. I ATP ADP | ^ NAD NADH-u I H2C-OH HrC-O-g) HrC-O-g) GlYcerol t*:tjlY ,-o"it"".i',llr" phosphate

Dihydroxyacetone phosphate is one of the chemical intermediates of gly- colysis. Thus the glycerol produced by hydrolysis of triglycerides con-