Redox potential mitochondrial components

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

Figure 6-8. Components of the mitochondrial electron transport chain with midpoint redox potentials in parentheses. Also indicated are the four protein complexes (I-IV) involved. Spontaneous election flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

The respiratory chain catalyses transfer of reducing equivalents from NADH generated in the mitochondrial matrix or M space, to dioxygen (Fig. 2.1A). Fig. 2.1B shows a thermodynamic view, giving the operational redox potentials ( ,) for the main individual components (for details, see below). The total redox span is about 1.11 V for oxidation of NADH, and about 760 mV for oxidation of ubiquinol (or succinate). 

Most of the aerobic cell s free energy is captured by the mitochondrial electron transport system (Chapter 10). During this process, electrons are transferred from a redox pair with a more negative reduction potential (NADH/NAD+) to those with more positive reduction potentials. The last component in the system is the H20/ /2 02 pair ... 

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