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

Abstract

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.

Electron transfer from the tetraheme cytochrome to the special pair in isolated reaction centers of Rhodopseudomonas viridis

“…First, electron transfer to P+ is faster from heme c-552 than from heme c-559, as mentioned above: this difference does not agree with the proposed arrangement unless c-559 acts as an intermediate in the oxidation of c-552, a property that has not yet been evidenced. Second, according to the classical data of several laboratories obtained on a number of bacterial species, one low-potential heme remains fully photooxidizable at low temperatures (Vredenberg & Duysens, 1964;De Vault & Chance, 1966;De Vault et al, 1967; ; ; Dutton et al, 1970Dutton et al, , 1971; Dutton, 1971;Tiede et al, 1976; Nitschke & Rutherford, 1989) whereas á major part of the high-potential hemes is no longer capable of efficient electron donation to P+ (Vredenberg & Duysens, 1964;Dutton et al, 1970Dutton et al, , 1971Dutton, 1971;Rubin et al, 1989; Nitschke & Rutherford, 1989). This fact has long served as an argument for the concept of independent and parallel pathways for lowand high-potential heme oxidation by P+, a concept which could also account for room temperature results showing that a faster electron donation from low-potential hemes than from high-potential hemes takes place in many bacterial species Seibert & De Vault, 1970;Kononenko et al, 1974;Tiede et al, 1976; Dutton & Prince, 1978; Bixon & Jortner, 1986a,b, 1989.…”

The cytochrome subunit structure in the photosynthetic reaction center of Chromatium minutissimum

“…It is well known that both high‐potential and low‐potential cytochromes act as secondary electron donors to the photooxidized special pair of RC cytochromes and are intermediates of cyclic electron transport, whereas low‐potential hemes connect RC to a terminal electron donor (substrate of oxidation). We studied the kinetics of the dark reduction of P + under the redox conditions with only high‐potential hemes ( E h =200–300 bacteriochlorophylls (P + ) [1, 3, 4, 9, 11, 12]; the high‐potential mV), both high‐potential and mid‐potential hemes ( E h =50–150 mV), or the three pairs of hemes ( E h from −20 to −60 mV) poised in a chemically reduced state (therefore, capable of donating an electron to P + ). It was found that the rate of the photoinduced electron transfer from high‐potential hemes to P + , measured at E h =200–300 mV, coincides with the reaction rate measured earlier in chromatophores and whole cells of this species of photosynthetic bacteria [9].…”

“…We studied the kinetics of the dark reduction of P + under the redox conditions with only high‐potential hemes ( E h =200–300 bacteriochlorophylls (P + ) [1, 3, 4, 9, 11, 12]; the high‐potential mV), both high‐potential and mid‐potential hemes ( E h =50–150 mV), or the three pairs of hemes ( E h from −20 to −60 mV) poised in a chemically reduced state (therefore, capable of donating an electron to P + ). It was found that the rate of the photoinduced electron transfer from high‐potential hemes to P + , measured at E h =200–300 mV, coincides with the reaction rate measured earlier in chromatophores and whole cells of this species of photosynthetic bacteria [9]. Similar coincidence was observed under redox conditions when the three pairs of cytochrome hemes were chemically reduced ( E h from −20 to −60 mV).…”

“…A method of preparation of the isolated RC of C. minutissimum containing photoactive cytochromes c has been described in our previous work [8]. We also studied the temperature dependence of the rate constant of oxidation of cytochrome c and the molecular dynamics of the preparations obtained [9]. The goal of the present work was to elucidate the structure of the cytochrome subunit in the isolated preparations of RC C. minutissimum .…”