Mitochondrial Oxidation of Fatty Acids Is Coupled to ATP Formation

Fatty acids are stored as triacylglycerols, primarily as droplets in adipose (fat-storing) cells. In response to hormones such as adrenaline, triacylglycerols are hydrolyzed in the cytosol to free fatty acids and glycerol:

Fatty acids released into the blood are taken up and oxidized by most other cells, constituting the major energy source for many tissues, particularly heart muscle. In humans, the oxidation of fats is quantitatively more important than the oxidation of glucose as a source of ATP. The oxidation of

► FIGURE 8-11 Oxidation of fatty acids in mitochondria and peroxisomes. In both mitochondrial oxidation (a) and peroxisomal oxidation (b), four identical enzyme-catalyzed reactions (shown down the center of the figure) convert a fatty acyl CoA molecule to acetyl CoA and a fatty acyl CoA shortened by two carbon atoms. Concomitantly (in reactions moving to the left of center for mitochondria and to the right of center for peroxisomes), one FAD molecule is reduced to FADH2, and one NAD+ molecule is reduced to NADH. The cycle is repeated on the shortened acyl CoA until fatty acids with an even number of carbon atoms are completely converted to acetyl CoA. In mitochondria, electrons from FADH2 and NADH enter the respiratory chain and ultimately are used to generate ATP; the acetyl CoA generated is oxidized in the citric acid cycle, resulting in synthesis of additional ATP Because peroxisomes lack the electron-transport complexes composing the respiratory chain and the enzymes of the citric acid cycle, oxidation of fatty acids in these organelles yields no ATP [Adapted from D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry, 3d ed., 2000, Worth Publishers.]

(a) MITOCHONDRIAL OXIDATION (b) PEROXISOMAL OXIDATION

(a) MITOCHONDRIAL OXIDATION (b) PEROXISOMAL OXIDATION

1 g of triacylglycerol to CO2 generates about six times as much ATP as does the oxidation of 1 g of hydrated glycogen, the polymeric storage form of glucose in muscle and liver. Triglycerides are more efficient for storage of energy because they are stored in anhydrous form and are much more reduced (have more hydrogens) than carbohydrates and therefore yield more energy when oxidized.

In the cytosol, free fatty acids are esterified to coenzyme A to form a fatty acyl CoA in an exergonic reaction coupled to the hydrolysis of ATP to AMP and PPi (inorganic pyrophosphate):

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment