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 Kd 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.
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