ELECTRON-TRANSFER REACTIONS OF HIGH-POTENTIAL CYTOCHROMES IN THE REACTION-CENTER OF CHROMATIUM-TEPIDUM

In the thermophilic purple bacterium C. tepidum, the reaction centre ( RC) has a bound cytochrome, containing two high-potential hemes (E-m a bove +350 mV) and two low-potential hemes (E-m below +150 mV), which r e-reduces the photooxidized primary donor, P+. We have studied the eff ects of ambient redox potential and of temperature on the kinetics of that reaction by kinetic hash absorption spectroscopy in chromatophore s and isolated reaction centers. When both high-potential hemes are re duced prior to excitation by a short flash of light, the halftime incr eases slightly between 294 K (t(1/2) = 500 ns) and 217 K (t(1/2) = 104 0 ns) indicating an activation energy of 5.0 kJ mol(-1). The fraction of P+ which decays by this fast reaction decreases rather steeply arou nd 220 K from nearly 100% at 294 K to nearly 0% below 190 K where P+ d ecays slowly (t(1/2) approximate to 2.5 ms), probably by return of an electron from the quinone accepters. When the high-potential hemes are partially oxidized prior to the flash, an additional kinetic phase ha ving a halftime of 30 mu s at 294 K is observed. The fractions of RCs that give rise to the individual kinetic phases of pf reduction have b een monitored as a function of redox potential. The results can be int erpreted in terms of two high-potential hemes which have similar midpo int potentials of +380 +/- 10 mV and a weak electrostatic interaction.