Mechanistic insight into the heme-independent interplay between iron and carbon monoxide in CFTR and Slo1 BKCa channels.

Ion channels have been extensively reported as effectors of carbon monoxide (CO). However, the mechanisms of heme-independent CO action are still not known. Because most ion channels are heterologously expressed on human embryonic kidney cells that are cultured in Fe3+-containing media, CO may act as a small and strong iron chelator to disrupt a putative iron bridge in ion channels and thus to tune their activity. In this review CFTR and Slo1 BKCa channels are employed to discuss the possible heme-independent interplay between iron and CO. Our recent studies demonstrated a high-affinity Fe3+ site at the interface between the regulatory domain and intracellular loop 3 of CFTR. Because the binding of Fe3+ to CFTR prevents channel opening, the stimulatory effect of CO on the Cl- and HCO3- currents across the apical membrane of rat distal colon may be due to the release of inhibitive Fe3+ by CO. In contrast, CO repeatedly stimulates the human Slo1 BKCa channel opening, possibly by binding to an unknown iron site, because cyanide prohibits this heme-independent CO stimulation. Here, in silico research on recent structural data of the slo1 BKCa channels indicates two putative binuclear Fe2+-binding motifs in the gating ring in which CO may compete with protein residues to bind to either Fe2+ bowl to disrupt the Fe2+ bridge but not to release Fe2+ from the channel. Thus, these two new regulation models of CO, with iron releasing from and retaining in the ion channel, may have significant and extensive implications for other metalloproteins.