Redox potential midpoint

Fig. 6. Schematic representation of the midpoint redox potentials and electron and protron balances relating the various active site states as detected by FTIR (65).

The midpoint redox potentials were estimated to be +230 mV (pH = 8.6) or +281 mV (pH = 7.0) for the Rd-like centers, and +339 and +246 mV (pH = 7.0) for the diiron-oxo center 38, 43). This is a surprising observation, since the normal redox potential of Rd centers is about 0 mV. All spectroscopic evidence points to the fact that the monomeric iron centers present in Rr are virtually identical to the ones found in Rd. Hence, it is reasonable to assume that the first coordination sphere of these centers cannot be held responsible for the 250 mV difference in the midpoint redox potentials. 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

Redox titrations monitored by visible and EPR spectroscopies show that the clusters have very different midpoint redox potentials approximately 0 mV for center I, and < - 300 mV for center II (139). 

As described above, the combination of EPR and Mossbauer spectroscopies, when applied to carefully prepared parallel samples, enables a detailed characterization of all the redox states of the clusters present in the enzyme. Once the characteristic spectroscopic properties of each cluster are identified, the determination of their midpoint redox potentials is an easy task. Plots of relative amounts of each species (or some characteristic intensive property) as a function of the potential can be fitted to Nernst equations. In the case of the D. gigas hydrogenase it was determined that those midpoint redox potentials (at pFi 7.0) were —70 mV for the [3Fe-4S] [3Fe-4S]° and —290 and —340mV for each of the [4Fe-4S]> [4Fe-4S] transitions. 

Not all cytochromes from sulfate-reducing bacteria reduce Fe(III) or other metals. D. vulgaris produces a cyt C553, which has a molecular mass of 9 kDa, midpoint redox potential of OmV, and a single heme and the iron atom is coordinated by histidine methionine. It is unclear at this time if the inability of this cyt C553 to reduce metals is due to lack of a bishistidinyl iron coordination or to some other factor, such as steric hinderance owing to orientation of heme in the protein. 

The midpoint redox potential of a His/His coordinated heme is likely to be less than -l-lOO mV. Given that the d heme is catalyzing a reaction with a midpoint potential of approximately -1-350 mV, it can be expected to have a potential considerably more positive than -l-lOO mV. This would account for the stoichiometric transfer of electrons from the c heme to the di heme under the conditions of the pulse radiolysis experiment. 

Potentiometric reductive titration, using both fresh thylakoids and PS II particles previously oxidized with ferricyanide, has revealed that the LP couple exhibits a constant midpoint redox potential ( o- +° 12 v ) above pH 7.6, but becomes pH-dependent below this pH, with a slope of about -60 mV pH pH unit, whereas the HP couple is pH-independent in the pH range between 6.5 an 8.5 ( o-+0 36 V) in general, cytochrome b-559 exhibits potential values about 40 mV lower in thylakoids than in PS II particles. After mild heating of the fresh preparations or treatment with the detergent Triton X-100, the HP couple is converted into the LP couple, which preserves its characteristic pH-dependence. In contrast, in the presence of the uncoupler CCCP, the HP couple is also converted into the LP couple, but the pH-dependence proper to the latter is now lost. 

Name Molecular mass (kDa) Approximate number per 600 chlorophylls Numbers of electrons accepted or donated per molecule Approximate midpoint redox potential (V) Comment... 

The hydrogen half-cell is not very convenient for routine laboratory usage—indeed, 1 m H+ (corresponding to a pH of 0 ) and 1 atm H2 (explosive) are dangerous. Hence, secondary standards are used, e.g., mercury/mercurous (calomel) or silver/silver chloride electrodes, which have midpoint redox potentials of 0.244 V and 0.222 V, respectively. 

In Chapter 5 (Section 5.5B), we introduced the various molecules involved with electron transfer in chloroplasts, together with a consideration of the sequence of electron flow between components (Table 5-3). Now that the concept of redox potential has been presented, we will resume our discussion of electron transfer in chloroplasts. We will compare the midpoint redox potentials of the various redox couples not only to help understand the direction of spontaneous electron flow but also to see the important role of light absorption in changing the redox properties of trap chi. Also, we will consider how ATP formation is coupled to electron flow. 

Although the ratio of reduced to oxidized forms of species j affects its redox potential [Ej = EfH - (RT/qF)ln(reduced))(oxidized)) Eq. 6.9], the actual activities of the two forms are usually not known in vivo. Moreover, the value of the local pH (which can affect h7) is also usually not known. Consequently, midpoint (standard) redox potentials determined at pH 7 are usually compared to predict the direction for spontaneous electron flow in the lamellar membranes of chloroplasts. We will assume that free energy is required to transfer electrons to a compound with a more negative midpoint redox potential, whereas electrons spontaneously flow toward higher midpoint redox potentials. 

Figure 6-4. Energy aspects of photosynthetic electron flow. The lengths of the arrows emanating from the trap chi s of Photosystems I and II represent the increases in chemical potential of the electrons that occur upon absorption of red light near the Xmax s of the trap Chip s. The diagram shows the various midpoint redox potentials of the couples involved (data from Table 5-3) and the three types of election flow mediated by ferredoxin. Spontaneous electron flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

From ferredoxin to the next component in the noncyclic electron flow sequence, NADP+, electrons go from —0.42 to —0.32 V (midpoint redox potentials of the couples Fig. 6-4). Again, moving toward higher redox potentials is energetically downhill for electrons, so this step leading to the reduction of the pyridine nucleotide follows spontaneously from the reduced ferredoxin —a step catalyzed by the enzyme ferredoxin-NADP+ ox-idoreductase (Table 5-3 and Fig. 6-4). 

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.

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