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
CH3-C-CH2
o
C
S-CoA + HS-CoA ------
Acetoacetyl
CoA
Studies have shown that exercising less frequentlybut for a longer
duration is a good approachto
burning body fat. After 40 minutes
of exercising,the percentage of
energy supplied by fat is greater
than that supplied by carbohydrates.Still longer exerciseperiods
lead to an even higher percentage
of energy being supplied by body
fat.
o
CH3-C-S-CoA
o
+ CH3-C-S-CoA
In other words, the complete conversion of a fatty acyl CoA to two-carbon
fragments of acetyl CoA always produces one more molecule of aceryl CoA
than of NADH or FADH2.To summarize, the breakdornmof palmitic acid
giveseight molecules of acetyl CoA,but only sevenmolecules of NADH and
sevenmolecules of FADH2are produced.
PRACTICE
25.I
EXERCISE
Lauric acid is converted to acetyl CoA in beta oxidation. Determine the
yields of (a) acetyl CoA, (b) NADH, and (c) FADH2.
25.4ATPyield
AIM: To calculotethe numherof ATPmoleculesformed by the
oxidotion of o fotty ocid molecule.
Focus
The complete oxidation of I
molecule of palmitic acidyields
129 molecules ofAIP.
In Chapter 24 we saw that the carbons of the acetyl CoA produced by the
catabolism of glucose can be completely oxidized to carbon dioxide in the
citric acid cycle. Each molecule of acetyl CoA oxidized in this fashion yields
enough energy to make one molecule of AIB one molecule of FADH2,and
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 sameway. It was sho',,rm
that 38 AIP molecules
is the total useful energyyield of aerobic glucosecatabolism.
Molecules of acetyf CoA are the sam-e,regardlessof 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 calculate 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
25 Lipid Metabolism
CHAPTER
Table25.1ATPProduetion
from Complete
AerobicCatabolism
of OneMolecule
of PalmiticAcid
ATPyield
Pathway
fatty acid spiral (2 ATP invested to make palmitoyl
CoA to start spiral)
citric acid cycle (Bmolecules of acetyl CoA degraded)
oxidative pho sphorylation
TNADH from fatty acid spiral
7FADH2from fatty acid spiral
24NADH from citric acid cycle
BFADH2from citric acid cycle
The oxidation of I g of a fatty acid
yields about 0.5 molATP. The
oxidation of I g of glucoseyields
about 0.2 molAIP. This ratio,
0.5:0.2,is approximatelythe same
as the ratio of the energyyield for
the complete oxidation of fats and
carbohvdrates.9 kcal:4kcal.
-2
B
2l
I4
72
16
L29
phate bonds required to activate the fatty acid as two AIP molecules. We
can do this because hydrolysis of one molecule of AIP to AMP and 2P; is
equivalent to the hydrolysis of 2AIP to 2ADP andzPi. Table 25.1 shows that
for every molecule of palmitic acid completely oxidized to carbon dioxide
and water, 129moleculesof AIP are formed. No wonderfats are an important source of energy for cellular work.
Energy produciion 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, consists of 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.
PRACTICE
EXERCISE
25.2
Calculate how many molecules of ATP are produced by the total oxidation 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 aswell as fatty acids.Glycerol is converted by cells to dihydroxyacetone phosphate in two steps.
Glycerol produced by
hydrolysis of triglycerides
enters glycolysis as dihydroxyacetone phosphate.
H2C-OH
-t-t-l
H-C-OH
I
H2C-OH
H2C-OH
--Z---ATP
GlYcerol
H2C-OH
--Z--------_
H-C-SH
ADP
|
HrC-O-g)
^
NAD
,-o"it"".i',llr"
NADH-u
c:o
I
HrC-O-g)
t*:tjlY
phosphate
Dihydroxyacetone phosphate is one of the chemical intermediates of glycolysis. Thus the glycerol produced by hydrolysis of triglycerides con-