Bacterial cytochromes c′-β (cyts c′-β) are gas-binding heme proteins that contain a hydrophobic active site pocket within a largely beta sheet fold. Previous studies of cyt c′-β from Methylococcus capsulatus. Bath (McCP-β) have indicated that NO and CO gas can bind to the heme Fe(II) site, but no O2 complex has been reported. In this study of McCP-β, we used stopped-flow UV-visible absorbance to characterize the first O2 complex of any cyt c′-β. Under all conditions studied, a transient Fe(II)O2 complex forms rapidly within the stopped-flow mixing time (&lt; 1.5 ms), and subsequently undergoes autooxidation to the Fe(III) state within ~15 s. The dissociation constant of the Fe(II)O2 complex was determined by means of a saturation binding curve. Results are discussed in terms of the recent X-ray crystal structure of McCP-β, and compared to the gas binding properties of alpha helical cytochromes c′ (cyts c′-α).

Cytochrome c′-β binds O2 rapidly and transiently

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