The metal reductase activity of some multiheme cytochromes c:: NMR structural characterization of the reduction of chromium(VI) to chromium(III) by cytochrome c7

The redox reaction between CrO42- and the fully reduced three-heme cytochrome c(7) from Desulfuromonas acetoxidans to give chromium(III) and the fully oxidized protein has been followed by NMR spectroscopy. The hyperfine coupling between the oxidized protein protons and chromium(III), which remains bound to the protein, gives rise to line-broadening effects on the NMR resonances that can be transformed into proton-metal distance restraints. Structure calculations based on these unconventional constraints allowed us to demonstrate that chromium(III) binds at a unique site and to locate it on the protein surface. The metal ion is located 7.9 +/- 0.4 Angstrom from the iron of heme IV, 16.3 +/- 0.7 Angstrom from the iron of heme III, and 22.5 +/- 0.5 Angstrom from the iron of heme 1. Shift changes caused by the presence of unreactive MoO42-, a CrO42- analogue, indicate the involvement of the same protein area in the anion binding. The titration of the oxidation of cytochrome c(7) shows a detailed mechanism of action. The presence of a specific binding site supports the hypothesis of the biological role of this cytochrome as a metal reductase.