Donors cytochrome

The cytochrome-c peroxidase test offers the advantages of stability of the peroxidase-hydrogen peroxide complex and high specificity for its hydrogen donor, cytochrome c. 

Under normal electron-transfer conditions, the lifetime of the reduced BO is quite short ( 200 ps) making it difficult to examine its spectral changes in detail. However, ifQ is pre-reduced, the separated charges in and recombine in 10 ni at ambient temperatures and, in principle, it should be possible to trap or accumulate B" if an efficient electron donor is available to reduce P. The physiological electron-donor, cytochrome c in its reduced form requires only a few microseconds to donate an electron to P but is still much slower than electron donation by BO and at first sight, it would appear to be rather difficult to accumulate the BO state. 

The various types of cytochrome oxidases contain different redox-active groups which are responsible for electron-transport from an external donor (cytochrome c or ubichinone) to oxygen [279] (Table 14). NO-reductase contains b- and c-type cytochromes but no CuA or CuB-sites. 

Interaction between cytochrome c peroxidase and its electron donor cytochrome C550... 

All these studies indicate that electron transfer within the flavocytochrome -cytochrome c complex is dependent upon a number of factors such as the distance between donor (cytochrome b2 core or TNS) and acceptor (cytochrome c). their relative orientation, their chemical nature and the structure of the protein medium involved in the electron transfer. 

NiR by its physiological electron donor cytochrome c551, and a smaller constant, 1.4 X 10 s for azurin which is its electron donor under stress. Nitrite... 

A theoretical analysis of charge distribution within supercomplexes (or clusters in which the movement of diffusible carriers is restricted) has been developed by Lavergne et al [4]. This theory predicts the evolution of the redox state of the carriers under continuous illumination or flash excitation for any cluster stoichiometry. The predictive power of this treatment is illustrated by the analysis of the light-induced oxidation of primary and secondary donors in isolated centers of Rhodopseudomonas viridis (Fig. 3). In this case, it is definitely established that the secondary donors (cytochromes) are irreversibly bound to the reaction center. In the absence of mediators, no electron exchange is expected to occur between photocenters. In the presence of 200yM ascorbate, only two of the four cytochromes (cyt 556 and cyt 559) are in their reduced state prior to the illumination. As expected, the apparent equilibrium constant between P and the cytochromes measured during the course of illumination is much lower than that computed from the value of the redox potentials (K = 50 for cyt 559 and K 1500 for cyt 556). The fit between the experimental data and the theoretical simulation (dashed lines) is excellent and clearly demonstrates that the measurement of electron transfer reactions under weak illumination is a powerful tool to characterize the degree of structuration of a photosynthetic electron transfer chain. 

Another important outcome of the structural analysis is the relative positioning of the metal sites and their distances in order to define plausible electron transfer pathways between electron donors and acceptors. A common pattern starts to emerge (the same applies to cytochrome oxidase (241, 242). Figure 11 gives a pictorial view of the electron transfer pathway ... 

Cytochromes. A cytochrome is a protein containing a heme with an iron cation bonded to four donor nitrogen atoms in a square planar array. Figure 20-29a shows the structure of cytochrome c, in which a histidine nitrogen atom and a cysteine sulfur atom occupy the fifth and sixth coordination sites of the octahedral iron center. 

The reductase in Geobacter sulfurreducens is located in the outer membrane and a soluble Fe(III) reductase has been characterized from cells grown anaerobically with acetate as electron donor and Fe(III) citrate or fumarate as electron acceptor (Kaufmann and Lovley 2001). The enzyme contained Fe, acid-labile S, and FAD. An extracellular c-type cytochrome is distributed in the membranes, the periplasm, and the medium, and functions as a reductase for electron transfer to insoluble iron hydroxides, sulfur, or manganese dioxide (Seeliger et al. 1998). 

Figure 18.2 Summary of respiratory energy flows. Foods ate converted into the reduced form of nicotinamide adenine dinucleotide (NADH), a strong reductant, which is the most reducing of the respiratory electron carriers (donors). Respiration can he based on a variety of terminal oxidants, such as O2, nitrate, or fumarate. Of those, O2 is the strongest, so that aerobic respiration extracts the largest amount of free energy from a given amount of food. In aerobic respiration, NADH is not oxidized directly by O2 rather, the reaction proceeds through intermediate electron carriers, such as the quinone/quinol couple and cytochrome c. The most efficient respiratory pathway is based on oxidation of ferrocytochrome c (Fe ) with O2 catalyzed by cytochrome c oxidase (CcO). Of the 550 mV difference between the standard potentials of c)Tochrome c and O2, CcO converts 450 mV into proton-motive force (see the text for further details).

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