Specific binding of CO to tetraheme cytochrome c₃

Carbon monoxide (CO) has been identified as another bioactive molecule like NO. Binding of CO to a tetraheme cytochrome c₃ (cyt c₃) was investigated using visible absorption spectroscopy, circular dichroism (CD), and NMR. CO was found to bind to the four hemes in different manners. CD spectra, however, indicated that only single-site CO binding can keep the protein intact. The K_d for the single-site binding was 8.0 μM, which is a typical value for a CO sensor protein. Furthermore, NMR spectra of uniformly ¹⁵N-labeled and specifically [¹⁵N]His-labeled proteins have provided evidence that CO specifically binds to the sixth coordination site of heme 2 via single-site binding. The CO-bound cyt c ₃ could conduct redox reactions. In light of triheme cytochrome c₇, the CO-bound cyt c₃ may work as an electron transporter. It was reported for sulfate-reducing bacteria that CO can be used as an energy source and CO cycling is operating like H₂ cycling. Therefore, the CO-bound cyt c₃ may play a role in maintaining electron transport pathways on accumulation of toxic CO for its utilization.