Structural Research of Electron Transport Chain Complex IV

The electron transport chain (ETC) in mitochondria is made up of four complexes that transfer electrons from NADH and FADH2 to oxygen, which causes proton translocation across the inner mitochondrial membrane. Complex IV, also called cytochrome c oxidase (CcO), is the terminal enzyme in the ETC and is responsible for reducing dioxygen to water and coupling the released energy to proton translocation. Bovine CcO (bCcO) is a member of this family and has a molecular weight of about 400 kDa. It is composed of two identical parts, each made up of 13 subunits and four redox centers: a dinuclear copper center (CuA), a heme group (heme a), and a binuclear center composed of another heme (heme a3) and another copper (CuB).

The structural determination of CcO is challenging due to its large size, complex structure, and involvement of redox-active metal centers. Recent advancements in X-ray crystallography and cryo-electron microscopy have enabled high-resolution structural analysis of CcO, providing insights into its mechanism of action. A recent study describes the determination of the structure of bCcO in the CO-bound state at high resolution using serial femtosecond X-ray crystallography (SFX) with an X-ray free electron laser. Furthermore, the study presents an equivalent structure of bCcO at a resolution of 1.95 Å, obtained using a synchrotron light source. In the SFX structure, the CO is coordinated to the heme a3 iron atom, while in the synchrotron structure, the Fe-CO bond is cleaved, and CO relocates to a new site near CuB, resulting in CuB moving closer to the heme a3 iron by approximately 0.38 Å. The ligand binding to the heme a3 iron in the SFX structure triggers an allosteric structural transition, involving partial unwinding of the helix-X between heme a and a3, which establishes a communication linkage between the two heme groups, setting the stage for proton translocation during the ensuing redox chemistry.

Figure 1. Active site structure of bCcO. (Ishigami I, et al., 2019)

Table 1. Structural Research of Electron Transport Chain Complex IV.

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References

1. Ishigami I, et al. Crystal structure of CO-bound cytochrome c oxidase determined by serial femtosecond X-ray crystallography at room temperature. Proceedings of the National Academy of Sciences. 2017, 114(30): 8011-8016.
2. Ishigami I, et al. Snapshot of an oxygen intermediate in the catalytic reaction of cytochrome c oxidase. Proceedings of the National Academy of Sciences. 2019, 116(9): 3572-3577.