Département de chimie moléculaire

CnrX, the dimeric metal sensor of the three-protein transmembrane signal transduction complex CnrYXH of Cupriavidus metallidurans CH34, contains one metal-binding site per monomer. Both Ni and Co elicit a biol. response and bind the protein in a 3N2O1S coordination sphere with a nearly identical octahedral geometry as shown by the X-ray structure of CnrXs, the sol. domain of CnrX. However, in soln. CnrXs is titrated by 4 Co-equiv and exhibits an unexpected intense band at 384 nm that was detected neither by single-crystal spectroscopy nor under anaerobiosis. The data from a combination of spectroscopic techniques (spectrophotometry, ESR, X-ray absorption spectroscopy) showed that two sites correspond to those identified by crystallog. The two extra binding sites accommodate Co(II) in an octahedral geometry in the absence of oxygen and are occupied in air by a mixt. of low-spin Co(II) as well as EPR-silent Co(III). These extra sites, located at the N-terminus of the protein, are believed to participate to the formation of peroxo-bridged dimers. Accordingly, we hypothesize that the intense band at 384 nm relies on the formation of a binuclear $μ$-peroxo Co(III) complex. These metal binding sites are not physiol. relevant since they are not detected in full-length NccX, the closest homolog of CnrX. X-ray absorption spectroscopy demonstrates that NccX stabilizes Co(II) in two-binding sites similar to those characterized by crystallog. in its sol. counterpart. Nevertheless, the original spectroscopic properties of the extra Co-binding sites are of interest because they are susceptible to be detected in other Co-bound proteins. [on SciFinder(R)]