Structural basis underlying the electron transfer features of a blue copper protein auracyanin from the photosynthetic bacterium Roseiflexus castenholzii
Auracyanin (Ac) is a blue copper protein that mediates the electron transfer between Alternative Complex III (ACIII) and downstream electron acceptors in both fort chains of filamentous anoxygenic phototrophs. Here, we extracted and purified the air-oxidized RfxAc from the photoheterotrophically grown Roseiflexus castenholzii, and we illustrated the structural basis underlying its electron transferring features. Spectroscopic and enzymatic analyses demonstrated the reduction of air-oxidized RfxAc by the ACIII upon oxidation of menaquinol-4 and menaquinol-7. Crystal structures of the air-oxidized and Na-dithionite-reduced RfxAc at 2.2 and 2.0 Å resolutions, respectively, showed that the copper ions are coordinated by His77, His146, Cys141, and Met151 in minor different geometries. The Cu1–Sδ bond length increase of Met151, and the electron density Fourier differences at Cu1 and His77 demonstrated their essential roles in the dithionite-induced reduction. Structural comparisons further revealed that the RfxAc contains a Chloroflexus aurantiacus Ac-A-like copper binding pocket and a hydrophobic patch surrounding the exposed edge of His146 imidazole, as well as an Ac-B-like Ser- and Thr-rich polar patch located at a different site on the surface. These spectroscopic and structural features allow RfxAc to mediate electron transfers between the ACIII and redox partners different from those of Ac-A and Ac-B. These results provide a structural basis for further investigating the electron transfer and energy transformation mechanism of bacterial photosynthesis, and the diversity and evolution of electron transport chains.
KeywordsAuracyanin Alternative complex III Roseiflexus castenholzii Crystal structure Electron transfer
Filamentous anoxygenic phototrophs
Alternative Complex III
Solvent accessible surface
We thank Professor Fei Sun at the Institute of Biophysics, Chinese Academy of Sciences for assisting in calculation of the Fourier difference map. We thank Jun Li at Shanghai Tech University for assistance in the data processing of the air-oxidized RfxAc. We thank the staff of the beamline BL19U at Shanghai Synchrotron Radiation Facility for assistance during data collection. This work was supported by the National Natural Science Foundation of China (Grant Numbers 31570738, 31870740 and 31400630).
CW and YYX performed purification, crystallization, and structure determination; CYZ performed the spectroscopic analyses; ZZM performed the enzymatic assays; JJQ assisted in bacterial culturing and protein extraction; XLX designed the experiments and wrote the manuscript. All authors have given approval to the final version of the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare no conflicts of interest.
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