12.3 The Citric Acid Cycle and Fatty Acid Oxidation

We now continue our detailed discussion of glucose oxidation and ATP generation, exploring what happens to the pyruvate generated during glycolysis (stage I, see Figures 12-1 and 12-3) after it is transported into the mitochondrial matrix. The last three of the four stages of glucose oxidation (Figure 12-14) are

• Stage II. Stage II can be subdivided into two distinct parts: (1) the conversion of pyruvate to acetyl CoA, followed by (2) oxidation of acetyl CoA to CO₂ in the citric acid cycle. These oxidations are coupled to reduction of NAD⁺ to NADH and of FAD to FADH₂. These two carriers can be considered the sources of high-energy electrons. (Fatty acid oxidation follows a similar route, with conversion of fatty acyl CoA to acetyl CoA.) Most of the reactions occur in or on the inner membrane facing the matrix.

• Stage III. Electron transfer from NADH and FADH₂ to O₂ via an electron-transport chain within the inner membrane converts the energy carried in those electrons into an electrochemical gradient across that membrane, called the proton-motive force.

• Stage IV. The energy of the proton-motive force is harnessed for ATP synthesis in the inner mitochondrial membrane. Stages III and IV are together called oxidative phosphorylation.

FIGURE 12-14 Summary of aerobic oxidation of glucose and fatty acids. Stage I: In the cytosol, glucose is converted to pyruvate (glycolysis) and fatty acid to fatty acyl CoA. Pyruvate and fatty acyl CoA then move into the mitochondrion. Mitochondrial porins make the outer membrane permeable to these metabolites, but specific transport proteins (colored ovals) in the inner membrane are required to import pyruvate (yellow) and fatty acids (blue) into the matrix. Fatty acyl groups are transferred from fatty acyl CoA to an intermediate carrier, transported across the inner membrane, and then reattached to CoA on the matrix side. Stage II: In the mitochondrial matrix, pyruvate and fatty acyl CoA are converted to acetyl CoA and then oxidized, releasing CO₂. Pyruvate is converted to acetyl CoA with the formation of NADH and CO₂; two carbons from fatty acyl CoA are converted to acetyl CoA with the formation of FADH₂ and NADH. Oxidation of acetyl CoA in the citric acid cycle generates NADH and FADH₂, GTP, and CO₂. Stage III: Electron transport reduces O₂ to H₂O and generates a proton-motive force. Electrons (blue) from reduced coenzymes are transferred via electron-transport complexes (blue boxes) to O₂ concomitant with transport of H⁺ ions (red) from the matrix to the intermembrane space, generating the proton-motive force. Electrons from NADH flow directly from complex I to complex III, bypassing complex II. Electrons from FADH₂ flow directly from complex II to complex III, bypassing complex I. Stage IV: ATP synthase, also called the F₀F₁ complex (orange), harnesses the proton-motive force to synthesize ATP in the matrix. Antiporter proteins (purple and green ovals) transport ADP and P_i into the matrix and export hydroxyl groups and ATP. NADH generated in the cytosol is not transported directly to the matrix because the inner membrane is impermeable to NAD⁺ and NADH; instead, a shuttle system (red) transports electrons from cytosolic NADH to NAD⁺ in the matrix. O₂ diffuses into the matrix, and CO₂ diffuses out.