Cytochromes standard redox potentials

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

Boxes indicate electron-transport chain complexes, whereas ovals represent the electron transporters UQ, RQ and cytochrome c. The open boxes represent complexes involved in the classical aerobic respiratory chain, whereas grey boxes represent complexes involved in malate dismutation. The vertical bar represents a scale for the standard redox potentials in mV. Translocation of protons by the complexes is indicated by H+ +. Abbreviations Cl, Clll and CIV, complexes I, III and IV of the respiratory chain cyt c, cytochrome c FRD, fumarate reductase Fum, fumarate SDH, succinate dehydrogenase Succ, succinate RQ, rhodoquinone UQ, ubiquinone. 

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...

How can a simple cofactor, such as heme, give rise to a wide spectrum of protein functionalities While the Fe(III)/Fe(II) couple has a standard redox potential of 0.77 V, when complexed with a protoporphyrin to form free heme, it may decrease to —0.115 V [3-5]. When heme is introduced into a protein matrix, redox potential shows an impressive variation of around 1 V. The electrochemical data for structurally characterized heme proteins involved in electron transfer and redox catalysis has been compiled at the Heme Protein Database (HPD, http //heme.chem. columbia.edu/heme) [6]. The database comprises not only peroxidases but also catalases, oxidases, monooxygenases, and cytochromes. From b-type heme with histidine-tyrosine ligation (E° = 0.55 V) to c-type heme with histidine-methionine... 

The measurement of reduction potential is an important step in establishing the order of chain elements. Reduction potentials vary with relative concentrations of the two redox species, of course, with an order of magnitude increase in ratio of reduced to oxidized form leading to a 60-mV increase in reduction potential in one electron reactions. Differences of a few tens of millivolts between the standard reduction potentials of cytochrome / and plastocyanin in green plant photosynthesis are not enough to establish their relative order since relative concentrations could reverse the actual reduction potentials. Still, a cytochrome c with a standard reduction potential of -(-50 mV is unlikely to donate electrons to... 

As the free-energy change accompanying a redox reaction is given by Eqn. 25, where Eq is the standard reduction potential, n is the number of electrons transferred per mole and F is Faraday s constant, a reaction is thermodynamically feasible if the difference in reduction potentials is positive. Thus in the preceding list NADH will reduce flavins but cytochrome c cannot. 

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