Picosecond Infrared Study of Carbonmonoxy Cytochrome c Oxidase: Ligand Transfer Dynamics and Binding Orientations
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. The coordination chemistry and ligation dynamics of the cytochrome a 3-CuB site, where O2 and other small molecules such as CO, NO and isocyanates can bind, are essential to the function of the enzyme. 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 CUB +, which generally is not observable by other spectroscopies. [1,2] Recent time-resolved infrared (TRIR) and visible absorption measurements have shown that coordination to CuB + is an obligatory mechanistic stop for CO entering the cytochrome a 3 heme site and departing the protein after photodissociation.  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 Fea3 2+ to CuB +, which are believed to lie 4–5 Å apart , occurs within 1 ps.
KeywordsPump Pulse Alamos National Laboratory Linear Dichroism Polarization Ratio Ligand Transfer
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