Electron transfer conformational gating

DeVoe et al. have reported quantitative aspects of photosensitization of diphenyliodonium salt and bis(4-dimethylaminobenzylidene)acetone (DMBA) [101]. This ketone is a bis-vinylog of Michler s Ketone, which is a well-known sensitizer for onium salt initiated free radical polymerizations [102,103], The reaction with DMBA is an example of electron transfer sensitization gated by conformational relaxation of the sensitizer. The ratio of iodonium salt consumption to aminoketone consumption is two, the second iodonium salt equivalent is consumed by a second reducing equivalent from the aminoalkyl radical on the oxidized photosensitizer. 

The standard formalisms for describing ET processes assume that in reactions such as Eqs. (1) and (2) there is but a single stable conformational form for each of the precursor and successor electron-transfer states. However, for a system that displays two (or more) alternative stable conformations with different ET rates, dynamic conformational equilibrium can modulate the ET rates. Major protein conformational changes can occur at rates that are competitive with observed rates of ET [9], and such gating [10] may occur in non-rigid complexes such as that between zinc cytochrome c peroxidase (ZnCcP) and cytochrome c (see below) or even within cytochrome c [5]. 

The fact that ET and conformational reactions thus are sequential (Scheme III), and not concerted, is an important factor in efforts to disentangle eonforma-tional and electron-transfer influences, because standard detection methods monitor only the ET event, and not conformational changes within one electronic state. In many, if not most, instances the measured time course of a single gated ET reaction is likely to be indistinguishable from a reaction without gating. 

Davis WB, Ratner MA, Wasielewski MR (2001) Conformational gating of long distance electron transfer through wire-like bridges in donor-bridge-acceptor molecules. J Am Chem Soc 123 7877-7886... 

NADPH oxidation and NO synthesis by the enzyme, it supports a role for reduction of the heme iron in catalysis, and may explain why NOS functions only as an NADPH-dependent reductase in the absence of bound calmodulin (Klatt et ai, 1993). The mechanism of calmodulin gating is envisioned to involve a conformational change between the reductase and oxygenase domains of NOS, such that an electron transfer between the terminal flavin and heme iron becomes possible. Calmodulin may also have a distinct role within the NOS reductase domain, in that its binding dramatically increases reductase activity of the enzyme toward cytochrome c (Klatt et al., 1993 Heinzel et al., 1992). However, it is clear that several other NOS functions occur independent of calmodulin, including the binding of L-arginine and NADPH, and transfer of NADPH-derived electrons into the flavins (Abu-Soud and Stuehr, 1993). 

The mechanism of proton translocation in complexes I and IV is not yet understood. Here, the electron-transfer reactions may cause protein conformational changes that open gates for proton movement first on one side of the membrane and then on the other. 

Hoffman BM, Ratner MA. Gated electron transfer when are observed rates controlled by conformational interconversion J Am Chem Soc 1987 109 6237-43. 

Xu, Q., Baciou, L., Sebban, P., and Gunner, MR.. (2002) Exploring the Energy Landscape for QA- to QB Electron transfer in Bacterial photosynthetic reaction centers effect of substrate position and tail length on the conformational gating step, Biochemistry 41, 10021-10025. 

Graige, M. S., Feher, G., and Okamura, M. Y., 1998, Conformational gating of the electron transfer reaction Qa Qb QaQb bacterial reaction centers of Rhodobacter sphaeroides determined by a driving force assay. Proc. Natl. Acad. Sci. USA, 95 11679911684. 

Comprehensive laser flash photolysis stndies on chicken and human SO have probed the rates of intermolecular electron transfer (lET, see Intermolecular) between the Mo and heme centers. Data are consistent with a CEPT mechanism interconverting Fe V[Mo02] " and Fe /[MoO(OH)]+ centers. To account for the wide range of lET rates (20-1400 s ), it has been proposed that protein conformational changes effectively gate lET by changing the Fe- Mo distance. 

Hammes-Schiffer S. Theoretical perspectives on proton-coupled electron transfer reactions. Acc. Chem. Res. 2001 34 273-281. Khoshtariya DE, Wei J, Liu H, Yue H, Waldeck DH. Charge-transfer mechanism for cytochrome c adsorbed on nanometer thick films, distinguishing frictional control from conformational gating. J. Am. Chem. Soc. 2003 125 7704-7714. 

Temperature-jump experiments with two-electron reduced CPR were used to investigate internal electron transfer between the flavins." Two-electron reduced CPR is a 50/50 mixture of two species enzyme in which both flavins are in the semiquinone form and enzyme in which the FAD is oxidized while the FMN is fully reduced. Increasing the temperature shifts the equilibrium between these two states allowing electron transfer from FAD to FMN to be observed. Two reaction phases are observed during the experiment. The fast phase ( 2000s ) represents a conformational change around the FAD. The second phase ( 55 s ) represents electron transfer. This observed rate constant is much slower than the estimated intrinsic rate constant of electron transfer (10 ° s ) based on the distance between the two flavins." The rate of electron transfer is proposed to be conformationally gated." " ... 

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