FATTY ACID OXIDATION OCCURS IN THE MITOCHONDRIAL MATRIX

Unlike glucose, the oxidation of fatty acids for energy occurs completely in mitochondria and only in aerobic conditions. Before fatty acids can be transported into the matrix they must be activated by the enzymatic attachment of coenzyme A, a reaction that requires energy input equivalent to that of converting two molecules of ATP to ADP.

Once fatty acids have been transported into the mitochondrial matrix they are oxidized by a process called beta-oxidation. Similar to what occurred with the mitochondrial oxidation of pyruvate, beta-oxidation involves the attachment of coenzyme A and the cutting-off of two carbons at a time (as acetyl-CoA) from the fatty acid. However, with the release of each acetyl-CoA molecule from the fatty acid, two coenzymes (one NAD and one FAD) are reduced, instead of just one as occurred with pyruvate. Refer to the Cellular Respiration illustration.

For an 18-carbon fatty acid this produces nine acetyl-CoA molecules that will be oxidized by the citric acid cycle. Since one coenzyme A was attached to the fatty acid when it was activated, and the last reaction of beta-oxidation yields two acetyl-CoA molecules, this requires only eight rounds of beta-oxidation, producing 16 reduced coenzymes.

Tallying total ATP production by fatty acid oxidation

Recall that each turn of the citric acid cycle produces four reduced coenzymes and the equivalent of one ATP, so the oxidation of these nine acetyl-CoA molecules produced from an 18-carbon fatty acid will yield nine ATP and 36 reduced coenzymes. In addition, 16 coenzymes were reduced during beta-oxidation, so there is a total production of 52 reduced coenzymes. With the transfer of their high-energy electrons to the ETC the reduced coenzymes yield 113 ATP, for a total of 122 ATP produced. After subtracting the 2 ATP required for the initial activation of the fatty acid the final net yield of ATP produced from an 18-carbon fatty acid is 120 ATP.

Fat versus glucose—ATP production and oxygen consumption

The oxidation of fatty acids yields more ATP per carbon atom than is produced from glucose oxidation. Oxidation of an 18-carbon fatty acid produces a total of 120 ATP, while the oxidation of three molecules of glucose (with a total of 18 carbons) produces a total of 96 ATP (3 × 32 = 96).

Although the oxidation of fats produces more ATP per carbon than is obtained from carbohydrates, it also requires a greater amount of oxygen consumption. The complete oxidation of an 18-carbon fatty acid or three molecules of glucose both produce 18 CO2 molecules and 18 H2O molecules, containing a total of 54 oxygen atoms. Because the fatty acid initially contains only two oxygen atoms this requires an input of 52 oxygen atoms, or 26 molecules of O2, from the air we breathe. In contrast, the three molecules of glucose (with a total of 18 carbons) initially contain 18 oxygen atoms, so only 36 oxygen atoms, or 18 molecules of O2, from the air we breathe are needed.

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Aerobic Versus Anaerobic Glycolysis: In aerobic glycolysis reduced coenzymes are oxidized by the electron transport chain, while in anaerobic glycolysis reduced coenzymes are oxidized by converting pyruvate to lactate.

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Question

What are two ways that the coenzymes reduced during glycolysis can be returned to their oxidized form?

1). Reduced coenzymes are oxidized in the mitochondria as they deliver the hydrogen atom into mitochondria, with the electron entering the electron transport chain (ETC). 2). When oxygen is limited, reduced coenzymes are able to donate their hydrogen atom to pyruvate to form lactate; the electron enters the electron transport chain (ETC).

Question

When rates of glycolysis are very high, how are the vast majority of coenzymes returned to their oxidized form?

When rates of glycolysis are high, coenzymes are able to return to their oxidized form by transferring their hydrogen atom to pyruvate transforming it to lactate. This quickly regenerates oxidized coenzymes that can then participate in another round of glycolysis.

Question

How would the elimination of mitochondria from skeletal muscle affect both the availability of oxygen in muscle, and its reliance on anaerobic glycolysis for ATP production?

If muscle cells had no mitochondria, they would not be able to obtain energy through aerobic respiration (respiration with oxygen). They would have to use anaerobic glycolysis to produce ATP. This process converts glucose into pyruvate, but it is less efficient than aerobic respiration so only a small amount of the energy from glucose is released.

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Carbons ATP produced O2 consumed ATP/O2
Three glucose molecules 18 96 18 5.3
One 18-carbon fatty acid 18 120 26 4.6

An examination of the ratio of ATP produced to oxygen consumed during carbohydrate and fat oxidation reveals that carbohydrate oxidation produces more ATP per molecule of oxygen consumed.