Electron transfer in the Cytochrome

Figure 1. Simplified scheme for the electron transfer in the Cytochrome P-450 mediated monooxygenase activity. In the liver, the fiavoprotein is Cystochrome c reductase. R is the compound being metabolized. NAD and Cytochrome bs have...

Honeychurch MJ, Hill AO, Wong LL (1999) The thermodynamics and kinetics of electron transfer in the cytochrome P450cam enzyme system. FEES Lett 451 351-353... 

Incubation of the cell-free extracts from S. carbophihts with pyridine nucleotides and ML-236B Na indicated that the cytochrome P-450sca monooxygenase system required NADH as an electron donw. We used NADH as electron donor and DCIP as an artificial electron acceptor for assay of electron transfer in the cytochrome P-450sca monooxygenase system. In general, actinomycetous cytochrome P-450 systems require NADH. This is in contrast with mammalian cytochrome P-450 systems, in which NADH is not required (13). 

The pioneering works of Hill and Eddows have opened the way to realize fast and efficient electron transfer of enzymes at the electrode surface. They modified a gold electrode with 4,4 -bipyrydyl, an electron promoter, not a mediator since it does not take part in electron transfer in the potential region of interest, to accomplish rapid electron transfer of cytochrome [1], Their work has triggered intensive investigation of electron transfer of enzymes using modified electrodes [2]. 

Eventually, the electrons in PQBH2 pass through the cytochrome b6f complex (Fig. 19-49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production. 

Yu, C. A., Nagaoka, S., Yu, L., and King, T. E., 1980, Evidence of ubisemiquinone radicals in electron transfer at the cytochromes b and c, region of the cardiac respiratory chain. Arch. Biochem. Biophys. 204 59n70. 

The initial electron acceptor can be made to accumulate in the reduced state (I ) if reaction centers which have bound (or added) cytochromes are illuminated continuously after the reduction of [56,67-69]. Each time the radical-pair state P I is formed, has a brief opportunity to oxidize the cytochrome instead of recovering an electron from I. The probability of electron transfer from the cytochrome is low, because the back reactions between P and I are much faster than the cytochrome oxidation. After many turnovers, however, essentially all of the reaction centers may be left with I reduced, particularly if the return of electrons... 

Electron-transfer reactions between cytochrome c and cytochrome c peroxidase have been studied extensively because of the well-characterized structures and biophysical properties of the reactants [146-150]. It is well known that the resting ferric form of cytochrome c peroxidase is oxidized by hydrogen peroxide to compound I, which contains an oxyferryl heme moiety in which the iron atom has a formal oxidation state of 4-1- [146-150]. The other is a porphyrin n radical cation or organic radical (R +) localized on an amino acid residue of Trp-191 [151-154] this is formed by transfer of the oxidized equivalent to the amino acid side chain [150]. The site of electron transfer in the reduction of compound I has been controversial and two forms of compound II have been identified, (P)Fe =0 containing the oxyferryl heme Fe(IV) [155-158] and [(P)Fe ] + containing Fe(III) and the porphyrin % radical cation which oxidizes the amino acid side-chain to produce an organic radical [(P)Fe +, R" ] [159 165] as shown in Scheme 10. 

Figure 5. The robustness of long-distance electron transfer through the cytochrome c heme chain in Rhodopseudomonas viridis is apparent from the relative insensitivity to extensive changes in the redox midpoint potentials of the individual hemes along the chain. Using the equations described in the text, most cytochrome C2 to BChl2 rates remain physiologically rapid as the zigzag potential profile is changed.

The kinetics of the elementary processes of electron transfer in the photochemical hydrogen evolution with hydrogenase using cytochrome C3 and methylviologen as carriers have been reported." The results indicate that cytochrome C3 is reduced by reduced methylviologen (MV ), and that this is followed by rapid electron transfer from C3 to hydrogenase. Molecular hydrogen has been obtained... 

We now discuss kinetic evidence that supports the notion that a reduced cytochrome is the direct electron donor to the photooxidized P870. . In subsequent sections we discuss properties and reactions of the RC-associated cytochromes, i.e., those cytochromes that are firmly associated with the reaction centers. The topics to be discussed include the temperature-insensitive electron transfer from the cytochrome to the reaction center and the spatial arrangement of the hemes in the tetraheme cytochrome subunit. 

II. Cytochrome Electron Transfers in the Photosynthetic Bacteria can be Temperature Insensitive... 

Experiments like the one depicted In Figure 8-19 have shown that four protons are translocated across the membrane per electron pair transported from C0QH2 through the CoQH2-cytochrome c reductase complex. Thus this complex transports two protons per electron transferred, whereas the cytochrome c oxidase complex transports only one proton per electron transferred. An evolutlonarlly conserved mechanism, called the Q cycle, accounts for the two-for-one transport of protons and electrons by the CoQH2-cytochrome c reductase complex. 

The acceptor for electrons transferred through the cytochrome bci complex is a soluble cytochrome, a one-electron carrier, in the periplasmic space, which is reduced from the Fe to the Fe state. The reduced cytochrome (analogous to cytochrome c in mitochondria) then diffuses to a reaction center, where it releases its electron to a positively charged chlorophyll, returning the chlorophyll to the ground state and the cytochrome to the Fe state. This cyci/c electron flow generates no oxygen and no reduced coenzymes. 

There are totally 11 chromophores in the bacterial photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) virids. Since the excitation process of the reaction center is the primary event of the photo-induced electron transfer in the reaction center, the detailed analysis of the absorption spectrum is one of the key steps for the understanding of photochemistry of the system. The chromophores included in the PSRC are bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll in L- and M-branches (Bl and B ), bacteriopheophytin in L- and M-branches (Hl and Hm), menaquinone (MQ), ubiquinone (UQ) and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit. 

---