ELECTRON-TRANSFER FROM THE TETRAHEME CYTOCHROME TO THE SPECIAL PAIR IN ISOLATED REACTION CENTERS OF RHODOPSEUDOMONAS-VIRIDIS

Kinetics of electron transfer from the bound tetraheme cytochrome c to the primary donor (P) have been measured in isolated reaction centers of the purple bacterium Rhodopseudomonas viridis by time-resolved flash absorption spectroscopy. The influence of two major parameters has been studied: temperature (7-305 K) and the redox state of the cytochrome. Most experiments were done with one heme (c-559), two hemes (c-559 and c-556), or three hemes (c-559, c-556, and c-552) poised in a reduced state before the flash. Measurements were done at 1283 nm in the absorption band of P+, and in the region of cytochrome alpha-bands. At room temperature, c-559 donates an electron to P+ with a half-time of 115, 190, or 230 ns (with three, two, or one heme reduced, respectively) and is then eventually rereduced by c-556 (t1/2 = 1.7 mus) or by c-552 (in less than 40 ns). The kinetics also include a minor microsecond phase of P+ reduction. At decreasing temperatures, the polyphasic character of P+ rereduction is accentuated. Fast phases (115 ns-10 mus) are slightly slowed down, following Arrhenius behavior with a weak activation energy (3.6-8.6 kJ.mol-1), until they become temperature-independent. Their extent decreases rather sharply, at temperatures which vary according to the redox poising: 250, 210, or 80 K when one, two, or three hemes are reduced, respectively. In the last case, P+ can still be reduced at low temperature, apparently directly by c-552 (t1/2 = 1.1 ms, nearly temperature-independent). The effects of temperature on electron transfer from c-556 to c-559 and on the triplet-state 3P have also been measured. The results are discussed with respect to the thermodynamic and spatial properties of cytochrome hemes. They indicate that the reaction center has several substates, the relative population of which is temperature-dependent. Within each substate, the kinetics have a weak temperature dependence in the high-temperature region, and they become temperature-independent at low temperature.