Photochemical initiation of electron transfer reactions

Phospholipid-linked quinones-mediated electron transfer on an electrode modified with lipid bilayers

Phospholipid-linked naphthoquinones separated by spacer methylene groups (C(n)), PE-C(n)-NQ (n=0, 5, 11), were synthesized to investigate the quinone-mediated electron transfers on a glassy carbon (GC) electrode covered with phospholipids membrane. The PE-C(n)-NQ could be incorporated in lipid bilayer composed of phosphatidylcholine and exhibited characteristic absorption spectral change corresponding to their redox state, quinone/hydroquinone. The cyclic voltammogram of PE-C(n)-NQ-containing lipid bilayer modified on a GC electrode indicated a set of waves corresponding to the consecutive two-electron and two-proton transfer reduction of the quinone moiety. The peak currents of PE-C(n)-NQ as a function of temperature showed a sharp break point in the current-temperature behavior, reflecting the gel-fluid phase transition. The shape of the cyclic voltammograms changed with the pH of the buffer solution. Below pH 6 the first step of the reduction of quinone was a monoprotonation of quinone, whereas above pH 10 the first step of the oxidation was a monodeprotonation of hydroquinone. This indicates that reaction sequences of quinone/hydroquinone were different with the change of the pH. These results showed that the PE-C(n)-NQ exhibited electron transfer associated with proton transfer in the lipid membranes, depending on the diffusivity of the redox species in the membrane and pH. Interestingly, less effect of the number of methylene of the spacer group on the peak currents was observed. Comparison of manganese porphyrin-mediated electron transfer that depends on the spacer methylene lengths [M. Nango, T. Hikita, T. Nakano, T. Yamada, M. Nagata, Y. Kurono, T. Ohtsuka, Langmuir 14 (1998) 407] is made.

Transient and time-resolved resonance Raman investigation of photoinitiated electron transfer in ruthenated cytochromes c

Ruthenation of exterior amino acid residues of heme proteins provides an effective means by which biological ET reactions can be studied within the context of highly complex protein environments. Resonance Raman spectroscopy can probe both ET kinetics and structural dynamics at the molecular level. Here we present the first comprehensive use of time-resolved and transient resonance Raman spectroscopies to examine photoinduced ET in cytochromes. Two ruthenated cytochromes c, Ru(lys72)-cyt.c and Ru(cyt102)cyt.c, were studied with TRRS using 10 ns laser pulses and with TRRRS on a 10 ns to 10 ms time scale. It was found that resonance Raman protocols can effectively trace ET kinetics and associated heme--protein structural dynamics. Care must be exercised, however, when drawing comparisons to measurements made by other methods (i.e., transient absorbance). The TRRS studies directly probe the heme and its local environment and reveal that the heme dynamics accompanying ET are very rapid relative to phenomenological ET kinetics. The heme and its local environment evolve to their equilibrium (ferrous) structure in less than 10 ns subsequent to ET, with no evidence for the existence of metastable heme pocket geometries analogous to those observed in the dynamic response of hemoglobins and oxidases. Finally, species-specific differences are observed in the photoinduced ET kinetics and heme structural dynamics. However, these differences are confined to nanosecond or faster time scales.

Structure-function studies of [2Fe-2S] ferredoxins

The ability to overexpress [2Fe-2S] ferredoxins in Escherichia coli has opened up exciting research opportunities. High-resolution x-ray structures have been determined for the wild-type ferredoxins produced by the vegatative and heterocyst forms of Anabaena strain 7120 (in their oxidized states), and these have been compared to structural information derived from multidimensional, multinuclear NMR spectroscopy. The electron delocalization in in these proteins in their oxidized and reduced states has been studied by 1H, 2H, 13C, and 15N NMR spectroscopy. Site-directed mutagenesis has been used to prepare variants of these ferredoxins. Mutants (over 50) of the vegetative ferredoxin have been designed to explore questions about cluster assembly and stabilization and to determine which residues are important for recognition and electron transfer to the redox partner Anabaena ferredoxin reductase. The results have shown that serine can replace cysteine at each of the four cluster attachment sites and still support cluster assembly. Electron transfer has been demonstrated with three of the four mutants. Although these mutants are less stable than the wild-type ferredoxin, it has been possible to determine the x-ray structure of one (C49S) and to characterize all four by EPR and NMR. Mutagenesis has identified residues 65 and 94 of the vegetative ferredoxin as crucial to interaction with the reductase. Three-dimensional models have been obtained by x-ray diffraction analysis for several additional mutants: T48S, A50V, E94K (four orders of magnitude less active than wild type in functional assays), and A43S/A45S/T48S/A50N (quadruple mutant).

Strategies for the study of cytochrome c structure and function by site-directed mutagenesis

The class I cytochromes c have been extensively studied by biochemical and biophysical methods; however, many questions remain concerning the roles of specific amino acids in electron transfer and stability properties. The method of site-directed mutagenesis, which substitutes specific amino acid residues by genetic methods, is ideal for addressing these questions of cytochrome c structure and function. Practical considerations of mutational effects on protein processing and stability will be addressed. The criteria for the selection of mutation sites will be discussed. Examples of site-directed mutagenesis studies, which were designed to elucidate the factors controlling biological electron transfer, protein processing, and protein stability, are given.

Use of laser flash photolysis time-resolved spectrophotometry to investigate interprotein and intraprotein electron transfer mechanisms

A description is given of the methodology developed in our laboratory for the application of laser flash photolysis to the elucidation of the kinetics and mechanism of electron transfer processes which occur intermolecularly between two protein molecules within a collisional complex, or intramolecularly between two redox centers within a single multisubunit or multidomain protein. This involves the use of flavin analogs, excited to their lowest triplet state by a laser flash, to initiate electron transfer, either by oxidation of a sacrificial donor followed by redox protein reduction via the flavin semiquinone, or by direct oxidation of a reduced redox protein by the flavin triplet. Time-resolved spectrophotometry is used to follow the course of the sequence of electron transfer events initiated by the laser flash. The application of this methodology to the following systems is described: cytochrome c/cytochrome c peroxidase; ferredoxin/ferredoxin NADP+ reductase; cytochrome c/plastocyanin; flavocytochrome b2; and sulfite oxidase.

Laser flash photolysis studies of electron transfer to the cytochrome b5-cytochrome c complex

Rate constants for electron transfer in the complex between recombinant rat mitochondrial outer membrane cytochrome b5 or the tryptic fragment of bovine liver cytochrome b5 and horse mitochondrial cytochrome c were measured by laser flash photolysis of 5-deazariboflavin-EDTA solutions. When an excess of cytochrome b5 was titrated with increasing amounts of cytochrome c at low ionic strength and electron transfer was initiated by a laser flash, both proteins were rapidly reduced by deazariboflavin semiquinone. The initial photoreduction was followed by a slower second-order reduction of b5 complexed oxidized cytochrome c by free reduced cytochrome b5. At an 8:1 ratio of cytochromes b5 to c, the pseudo-first-order rate constant for reduction of complexed cytochrome c increased 3-5-fold between ionic strengths of 5 and 40 mM, and then dropped precipitously at higher ionic strengths. The ionic strength dependent increase in rate constant is likely to be due to relief of steric hindrance via rearrangement of cytochrome c in the complex. The reaction rate showed no sign of saturation at any ionic strength, indicating a first-order rate constant greater than 10(4) s-1 within a transient ternary protein complex; i.e., interprotein electron transfer approaches the largest values previously reported for the stable binary protein complex (approximately 4 x 10(5) s-1). Our results emphasize the flexibility of electron-transfer protein complexes, which had previously been modeled in a single conformation with specific salt bridges. It appears that a variety of orientations can exist within such protein-protein complexes and that the population of conformations changes with ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)