Proton-assisted Two-electron Transfer in Natural Variants of

The tetraheme cytochrome c3 isolated from Desulfo-microbium baculatum (DSM 1743) (Dsmb) was cloned, and the sequence analysis showed that this cytochrome differs in just three amino acid residues from the cyto-chrome c3 isolated from Desulfomicrobium norvegicum (Dsmn): (DsmnXXDsmb) Thr-373 Ser, Val-453 Ala, and Phe-88 3 Tyr. X-ray crystallography was used to deter-mine the structure of cytochrome c3 from Dsmb, show-ing that it is very similar to the published structure of cytochrome c3 from Dsmn. A detailed thermodynamic and kinetic characterization of these two tetraheme cy-tochromes c3 was performed by using NMR and visible spectroscopy. The results obtained show that the net-work of cooperativities between the redox and protonic centers is consistent with a synergetic process to stim