Oxidation of Pyruvic Acid to Two-Carbon Segments.

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After entering mitochondria, pyruvic acid is shortened to a two-carbon segment in a cyclic series of reactions. These reac­tions remove two electrons, two hydrogens, and one carbon (as CO₂) from the pyruvic acid chain. The two-carbon segment produced by the oxidation is an acetyl (—CH₃CO~) group:

Fig.7-7 Coenzyme A, a car­rier of acetyl (— CH₃CO ~) groups.

Fig.7-8The overall reaction of pyruvic acid oxidation.

The overall reaction cycle in pyruvic acid oxidation (Fig. 7-8) thus yields as net products acetyl coenzyme A, reduced NAD, and CO₂:

Fig.7-10 An overview of the Krebs cycle.

Fig. 7-11The reactions and enzymes of the Krebs cycle.

Fig. 7-12NADP (nicotinamide-adenine dinucleotide phosphate), an alternate electron acceptor (for reaction 3 in Fig. 7-11) of the Krebs cycle.

Fig.7-13FAD (flavin adenine dinucleotide), an electron acceptor linked to succinic acid dehydrogenase in the cristae membranes of mitochondria.

Fig. 7-14 A summary of the Krebs cycle reactions.

Figure 7-15FMN (flavin mononucleotide). FMN and FAD are the prosthetic groups of the flavoprotein carriers of the electron transport system. The riboflavin group in reduced FMN and FAD binds two hy­drogens in the shaded positions to form FMNH₂ or FADH₂.

Figure 7-16The prosthetic group of cytochrome c, an iron porphyrin group. The iron in the center of the ring, by changing alternately from the ferrous (Fe²⁺) to the ferric (Fe³⁺) form, acts as an electron carrier. Similar porphyrin rings occur as central structures in hemoglobin, chlorophyll, and other molecules of biological importance (compare with Fig. 8-8).

Fig.7-17Coenzyme Q, the only carrier of the electron transport system not linked to a protein. The molecule may exist in the fully oxidized quinone or Q state (a), the partially reduced semiquinone or QH state (b), or the fully reduced hydroquinone or QH₂ state (c). The points where hydrogens attach in these reductions are shaded

Fig.7-18The absorption spectra for the oxidized and reduced forms of cytochrome c in solution at pH 7. The oxidized form has one major absorption peak; the reduced form has three (arrows).

Fig.7-19 A possible se­quence for the carriers of the elec­tron transport system of mitochondria). NAD, FMN, FAD, and coenzyme Q are hydrogen-electron carriers. The iron-sulfur proteins (Fe-S) and cytochromes carry only electrons.

Fig. 7-20The three sites of ATP synthesis in the electron transport chain.

Added to the total from Equation 7-13, the overall oxidation of glucose yields 38 molecules of ATP:

Figure 7-23Mitchell's version of the electron transport chain showing how electron transport sets up a gradient in H⁺ ions across the mitochondrial mem­branes.

inside (matrix) 2H⁺ 1/2O₂

Fig.7-24Mitchell's hypothesis for the mechanism synthesizing ATP in response to a gradient in H⁺ ions.

Fig. 7-27The major biochemical pathways oxidizing carbohy­drates, fats, and proteins. Coenzyme A, the acetyl carrier, forms a cen­tral channel funneling the products of many oxidative pathways into the Krebs cycle.

Fig.7-28The electron transport system of the bacterium E. coli. The electron carriers are embedded in the plasma membrane in bacteria.