Abstract
Mitochondrial cytochrome c oxidase (CcO) transfers electrons from cytochrome c (Cyt.c) to O2 to generate H2O, a process coupled to proton pumping. To elucidate the mechanism of electron transfer, we determined the structure of the mammalian Cyt.c–CcO complex at 2.0‐Å resolution and identified an electron transfer pathway from Cyt.c to CcO. The specific interaction between Cyt.c and CcO is stabilized by a few electrostatic interactions between side chains within a small contact surface area. Between the two proteins are three water layers with a long inter‐molecular span, one of which lies between the other two layers without significant direct interaction with either protein. Cyt.c undergoes large structural fluctuations, using the interacting regions with CcO as a fulcrum. These features of the protein–protein interaction at the docking interface represent the first known example of a new class of protein–protein interaction, which we term “soft and specific”. This interaction is likely to contribute to the rapid association/dissociation of the Cyt.c–CcO complex, which facilitates the sequential supply of four electrons for the O2 reduction reaction.
Highlights
Cytochrome c oxidase (CcO) is a typical aa3-type CcO, in which electrons are transferred to an active site consisting of heme a3 and CuB from CuA via heme a
The residues that interact with CcO have been investigated more extensively by NMR studies (Sakamoto et al, 2011), which revealed that hydrophobic residues on the surface of Cyt.c make major contributions to complex formation, whereas the charged residues near the hydrophobic core refine the orientation of Cyt.c to precisely control Electron transfer (ET)
We performed co-crystallization of Cyt.c and CcO at pH 8.0 under the same conditions used for 2D crystallization of the Cyt.c–CcO complex (Osuda et al, 2016)
Summary
Cytochrome c oxidase (CcO) is a typical aa3-type CcO, in which electrons are transferred to an active site consisting of heme a3 and CuB from CuA via heme a. Extensive steady-state kinetic analyses of oxidation of ferroCyt.c by CcO have revealed two Cyt.c-binding sites, both of which are actively involved in catalytic turnover (Ferguson-Miller et al, 1976). A significant amounts of data have accumulated regarding ET from Cyt.c to CcO (Speck et al, 1984; Sakamoto et al, 2011), and the X-ray structures of mammalian CcO (PDB 5B1A) and Cyt.c (Bushnell et al, 1990; De March et al, 2014) have been determined at high resolution, the underlying mechanism of ET remains incompletely understood. A novel mode of protein–protein interaction Satoru Shimada et al2012), which has a covalently tethered cytochrome c domain. It remains unclear whether this fused Cyt.c has functions analogous to those of the Cyt.c molecules that participate in catalytic turnover in the eukaryotic CcO system. The results revealed a novel mode of protein–protein interaction mediated by three water layers
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