Electron transfer and protein dynamics

Role of hydrogen-bond networks on the donor side of photosynthetic reaction centers from purple bacteria

For the last decades, significant progress has been made in studying the biological functions of H-bond networks in membrane proteins, proton transporters, receptors, and photosynthetic reaction centers. Increasing availability of the X-ray crystal and cryo-electron microscopy structures of photosynthetic complexes resolved with high atomic resolution provides a platform for their comparative analysis. It allows identifying structural factors that are ensuring the high quantum yield of the photochemical reactions and are responsible for the stability of the membrane complexes. The H-bond networks are known to be responsible for proton transport associated with electron transfer from the primary to the secondary quinone as well as in the processes of water oxidation in photosystem II. Participation of such networks in reactions proceeding on the periplasmic side of bacterial photosynthetic reaction centers is less studied. This review summarizes the current understanding of the role of H-bond networks on the donor side of photosynthetic reaction centers from purple bacteria. It is discussed that the networks may be involved in providing close association with mobile electron carriers, in light-induced proton transport, in regulation of the redox properties of bacteriochlorophyll cofactors, and in stabilization of the membrane protein structure at the interface of membrane and soluble phases.

Dielectric asymmetry in the photosynthetic reaction center

Although the three-dimensional structure of the bacterial photosynthetic reaction center (RC) reveals a high level of structural symmetry, with two nearly equivalent potential electron transfer pathways, the RC is functionally asymmetric: Electron transfer occurs along only one of the two possible pathways. In order to determine the origins of this symmetry breaking, the internal electric field present in the RC when charge is separated onto structurally characterized sites was probed by using absorption band shifts of the chromophores within the RC. The sensitivity of each probe chromophore to an electric field was calibrated by measuring the Stark effect spectrum, the change in absorption due to an externally applied electric field. A quantitative comparison of the observed absorption band shifts and those predicted from vacuum electrostatics gives information on the effective dielectric constant of the protein complex. These results reveal a significant asymmetry in the effective dielectric strength of the protein complex along the two potential electron transfer pathways, with a substantially higher dielectric strength along the functional pathway. This dielectric asymmetry could be a dominant factor in determining the functional asymmetry of electron transfer in the RC.

Dynamics of electron transfer from high-potential cytochrome c to bacteriochlorophyll dimer in photosynthetic reaction centers as probed using laser-induced temperature jump

Laser-induced temperature jump experiments were used for testing the rates of thermoinduced conformational transitions of reaction center (RC) complexes in chromatophores of Chromatium minutissimum. The thermoinduced transition of the macromolecular RC complex to a state providing effective electron transport from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer within the temperature range 220-280 K accounts for tens of seconds with activation energy 0.166 eV/molecule. The rate of the thermoinduced transition in the cytochrome-RC complex was found to be three orders of magnitude slower than the rate of similar thermoinduced transition of the electron transfer reaction from the primary to secondary quinone acceptors studied in the preceding work (Chamorovsky et al. in Eur Biophys J 32:537-543, 2003). Parameters of thermoinduced activation of the electron transfer from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer are discussed in terms of cytochrome c docking onto the RC.

Low-temperature electron transfer from cytochrome to the special pair in Rhodopseudomonas viridis: role of the L162 residue

Electron transfer from the tetraheme cytochrome c to the special pair of bacteriochlorophylls (P) has been studied by flash absorption spectroscopy in reaction centers isolated from seven strains of the photosynthetic purple bacterium Rhodopseudomonas viridis, where the residue L162, located between the proximal heme c-559 and P, is Y (wild type), F, W, G, M, T, or L. Measurements were performed between 294 K and 8 K, under redox conditions in which the two high-potential hemes of the cytochrome were chemically reduced. At room temperature, the kinetics of P+ reduction include two phases in all of the strains: a dominant very fast phase (VF), and a minor fast phase (F). The VF phase has the following t(1/2): 90 ns (M), 130 ns (W), 135 ns (F), 189 ns (Y; wild type), 200 ns (G), 390 ns (L), and 430 ns (T). These data show that electron transfer is fast whatever the nature of the amino acid at position L162. The amplitudes of both phases decrease suddenly around 200 K in Y, F, and W. The effect of temperature on the extent of fast phases is different in mutants G, M, L, and T, in which electron transfer from c-559 to P+ takes place at cryogenic temperatures in a substantial fraction of the reaction centers (T, 48%; G, 38%; L, 23%, at 40 K; and M, 28%, at 60 K), producing a stable charge separated state. In these nonaromatic mutants the rate of VF electron transfer from cytochrome to P+ is nearly temperature-independent between 294 K and 8 K, remaining very fast at very low temperatures (123 ns at 60 K for M; 251 ns at 40 K for L; 190 ns at 8 K for G, and 458 ns at 8 K for T). In all cases, a decrease in amplitudes of the fast phases is paralleled by an increase in very slow reduction of P+, presumably by back-reaction with Q(A)-. The significance of these results is discussed in relation to electron transfer theories and to freezing at low temperatures of cytochrome structural reorganization.

Interaction between cytochrome c and the photosynthetic reaction center of purple bacteria: behaviour at low temperature

In purple photosynthetic bacteria the electron donor to the special pair, after its oxidation by a light-induced reaction, is a c-type cytochrome: either a soluble monoheme cytochrome which forms a transitory complex with the reaction center, or a tetraheme cytochrome which remains permanently bound to the reaction center. The effects of low temperatures on electron transfer in the complex are presented and discussed. They provide estimates for the reorganization energy. The most prominent effect of low temperature is that a dominant fast phase of electron transfer becomes impossible at a temperature of around 250 K (monoheme cytochrome) or located between 250 K and 80 K according to the redox state (tetraheme cytochrome). This inhibition is attributed to a freezing-like transition of pools of water molecules which blocks structural changes of the protein which are normally associated with the cytochrome oxidation.

Effect of temperature on the kinetics of electron transfer from the tetraheme cytochrome to the primary donor in Rhodopseudomonas viridis

The kinetics of electron transfer from the third highest potential heme (c-552, Em = +20 mV) to the primary donor (P-960) have been measured by flash absorption spectroscopy in isolated reaction centers of Rhodopseudomonas viridis between 300 K and 7 K. The data are analyzed on the basis of three exponential components with a very fast phase (t1/2 = 120 ns) dominating at high temperature and a very slow one (t1/2 = 1.2 ms) at low temperature. This multiphasic behavior is interpreted in terms of the existence of three states with a temperature-dependent population and a very limited effect of the temperature on the kinetics for each state.

Temperature dependence of cytochrome photooxidation and conformational dynamics of Chromatium reaction center complexes

A temperature dependence of multiheme cytochrome c oxidation induced by a laser pulse was studied in photosynthetic reaction center preparations from Chromatium minutissimum. Absorbance changes and kinetic characteristics of the reaction were measured under redox conditions where one or all of the hemes of the cytochrome subunit are chemically reduced (E h =+300 mV or E h =-20 to -60 mV respectively). In the first case photooxidation is inhibited at temperatures lower than 190-200 K with the rate constant of the photooxidation reaction being practically independent on temperature over the range of 300 to 190 K (k=2.2×10(5) s(-1)). Under reductive conditions (E h =-20 to -60 mV) lowering the temperature to 190-200 K causes the reaction to slow from k=8.3×10(5) s(-1) to 2.1×10(4) s(-1). Under further cooling down to the liquid nitrogen temperature, the reaction rate changes negligibly. The absorption amplitude decreases by 30-40% on lowering the temperature. A new physical mechanism of the observed critical effects of temperature on the rate and absorption amplitude of the multiheme cytochrome c oxidation reaction is proposed. The mechanism suggests a close interrelation between conformational mobility of the protein and elementary electron tunneling act. The effect of "freezing" conformational motion is described in terms of a local diffusion along a random rough potential.

Cytochrome oxidation in bacterial photosynthesis

In this paper we propose that the reduction of the bacteriochlorophyl dimer cation (P(+)) by cytochrome c in the photosynthetic bacteria Rps. viridis and Chromatium vinosum proceeds via two parallel electron transfer (ET) processes from two distinct cytochrome c molecules. The dominating ET process at high temperatures involves the activated oxidation of the high-potential cytochrome c at closest proximity to P, while the dominating low-temperature process involves activationless ET from a low-potential cytochrome c, which is further away from P. The available data for the effects of blocking the low-potential cytochrome c on ET dynamics are consistent with this model, which results in reasonable nuclear reorganization and electronic coupling parameters for the parallel cytochrome oxidation processes. The lack of universality in the cytochrome oxidation in reaction centres of various bacteria is emphasized.