The genus Shewanella produces a unique small tetraheme cytochrome c that is implicated in the iron oxide respiration pathway. It is similar in heme content and redox potential to the well known cytochromes c 3 but related in structure to the cytochrome c domain of soluble fumarate reductases from Shewanella sp. We report the crystal structure of the small tetraheme cyto-

The macroscopic and microscopic redox potentials of the four hemes of the small tetraheme cytochrome c from Shewanella oneidensis were determined. The microscopic redox potentials show that the order of reduction is from hemes in the C-terminal domain (hemes 3 and 4) to the N-terminal domain (heme 1), demonstrating the polarization of the tetraheme chain during reduction. This makes heme 4 the most e⁄cient electron delivery site. Furthermore, multi-step reduction of other redox centers through either heme 4 or heme 3 is shown to be possible. This has provided new insights into the two-electron reduction of the £avin in the homologous £avocytochrome c^fumarate reductase.

No abstract

Fumarate respiration is one of the most widespread types of anaerobic respiration. The soluble fumarate reductase of Shewanella putrefaciens MR-1 is a periplasmic tetraheme flavocytochrome c. The crystal structures of the enzyme were solved to 2.9 A for the uncomplexed form and to 2.8 A and 2.5 A for the fumarate and the succinate-bound protein, respectively. The structures reveal a flexible capping domain linked to the FAD-binding domain. A catalytic mechanism for fumarate reduction based on the structure of the complexed protein is proposed. The mechanism for the reverse reaction is a model for the homologous succinate dehydrogenase (complex II) of the respiratory chain. In flavocytochrome c fumarate reductase, all redox centers are in van der Waals contact with one another, thus providing an efficient conduit of electrons from the hemes via the FAD to fumarate.

No abstract