Picosecond Infrared Study of Carbonmonoxy Cytochrome c Oxidase: Ligand Transfer Dynamics and Binding Orientations

Abstract

Cytochrome c oxidase (CcO), an enzyme which catalyzes the reduction of dioxygen to water in the terminal step of the respiratory chain, combines several fundamental chemical processes in performing its function; electron, proton and ligand transfers.[1] The coordination chemistry and ligation dynamics of the cytochrome a ₃-Cu_B site, where O2 and other small molecules such as CO, NO and isocyanates can bind, are essential to the function of the enzyme.[2] The sensitivity of the vibrational frequencies and bandwidths of small molecules to changes in coordination and environment makes infrared spectroscopy uniquely useful as a probe for those processes, particularly at CU_B ⁺, which generally is not observable by other spectroscopies. [1,2] Recent time-resolved infrared (TRIR) and visible absorption measurements have shown that coordination to Cu_B ⁺ is an obligatory mechanistic stop for CO entering the cytochrome a ₃ heme site and departing the protein after photodissociation. [2] The timescale (> 10⁻⁷ s) of the TRIR measurements, however, precluded observation of the ligation dynamics immediately following photodissociation. Here we report a picosecond timescale TRIR study of these events. The results reveal that the photoinitiated ligand transfer of CO from Fe_a3 ²⁺ to Cu_B ⁺, which are believed to lie 4–5 Å apart [1], occurs within 1 ps.