Electron self-exchange in high-potential iron-sulfur proteins. Characterization of protein I from Ectothiorhodospira vacuolata.

During previous research on oxidized and reduced high-potential iron-sulfur proteins (HiPIP hereafter), qualitative different electron self-exchange rates were noticed. We have now investigated this phenomenon in detail for HiPIP I and II from Ectothiorhodospira vacuolata, which differ significantly in total charge and in which the sequence homology is the largest among all known HiPIPs. We have also characterized the electronic structure of HiPIP I through 1H NMR and EPR spectroscopies to parallel the existing characterization of HiPIP II and other HiPIPs. This investigation has allowed us to propose a model, according to which the productive collisions for electron transfer occur through a hydrophobic patch near the cluster. The effects of total charge and redox potential are considered. The possible formation of dimers through the hydrophobic patch at liquid helium temperature is discussed in light of the EPR spectra.