When fat stored in adipose tissue is going to be used as an energy source, lipase enzymes hydrolyze triglycerides into glyc-erol and free fatty acids in a process called lipolysis. These molecules (primarily the free fatty acids) serve as blood-borne energy carriers that can be used by the liver, skeletal muscles, and other organs for aerobic respiration.
A few organs can utilize glycerol for energy by virtue of an enzyme that converts glycerol to phosphoglyceraldehyde. Free fatty acids, however, serve as the major energy source derived from triglycerides. Most fatty acids consist of a long hydrocarbon chain with a carboxyl, or carboxylic acid group (COOH) at one end. In a process known as P-oxidation (P is the Greek letter beta), enzymes
\ / Cholesterol
, Citric acid (Krebs cycle)
■ Figure 5.12 Divergent metabolic pathways for acetyl coenzyme A. Acetyl CoA is a common substrate that can be used to produce a number of chemically related products.
remove two-carbon acetic acid molecules from the acid end of a fatty acid chain. This results in the formation of acetyl CoA, as the third carbon from the end becomes oxidized to produce a new carboxyl group. The fatty acid chain is thus decreased in length by two carbons. The process of oxidation continues until the entire fatty acid molecule is converted to acetyl CoA (fig. 5.13).
A sixteen-carbon-long fatty acid, for example, yields eight acetyl CoA molecules. Each of these can enter a Krebs cycle and produce ten ATP per turn of the cycle, so that eight times ten, or eighty, ATP are produced. In addition, each time an acetyl CoA molecule is formed and the end carbon of the fatty acid chain is oxidized, one NADH and one FADH2 are produced. Oxidative phosphorylation produces 2.5 ATP per NADH and 1.5 ATP per FADH2. For a sixteen-carbon-long fatty acid, these four ATP molecules would be formed seven times (producing four times seven, or twenty-eight, ATP). Not counting the single ATP used to start P-oxidation (fig. 5.13), this fatty acid could yield a grand total of 28 + 80, or 108 ATP molecules!
Remember that Brenda's coach advised her to exercise more gradually. Under these conditions, skeletal muscles utilize a higher proportion of fatty acids for energy. If her skeletal muscles used fatty acids more for energy, how would this help to alleviate her pain and fatigue?
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