CHARACTERIZATION OF THE REACTION-CENTER BOUND TETRAHEME CYTOCHROME OFRHODOCYCLUS-TENUIS

Properties of the tetrahemic reaction center bound cytochrome have bee n investigated by different techniques. The mid-point potentials of th e four hemes were determined by redox titration. The best fit of the d ata was obtained with a (n = 1) Nernst curve by using the following va lues of the redox parameters: E-m = +420 mV for the two high-potential hemes and E-m = +110 and +60 mV for the two low-potential hemes. The mid-point potentials of the two high-potential hemes are the highest r eported so far. The spectral properties of the four hemes in the a-ban d have been determined by absorption spectroscopy and measurements of light-induced difference spectra in membranes of Rhodocyclus tenuis. T he two high potential hemes present very similar spectra centered at 5 57 nm. The absorption spectra of the two low-potential hemes are very similar, and their a-band centered around 551 nm. Spectral properties at 100 K and the linear dichroism of optical transitions allow the det ermination of the relative orientations of the hemes with respect to t he membrane plane. The orientation patterns thus obtained corresponds to none of the arrangements described so far for reaction center bound cytochromes.