X-ray structure determination of three mutants of the bacterial photosynthetic reaction Centers from Rb. sphaeroides: Altered proton transfer pathways

摘要

In the photosynthetic reaction center (RC) from Rhodobacter sphaeroides, the reduction of a bound quinone molecule 013 is coupled with proton uptake. When Asp-L213 is replaced by Asn, proton transfer is inhibited. Proton transfer was restored by two second-site revertant mutations, Arg-M233-->Cys and Arg-H177-->His. Kinetic effects of Cd2+ on proton transfer showed that the entry point in revertant RCs to be the same as in the native RC. The structures of the parental and two revertant RCs were determined at resolutions of 2.10, 1.80, and 2.75 Angstrom. From the structures, we were able to delineate alternate proton transfer pathways in the revertants. The main changes occur near GluH173, which allow it to substitute for the missing AspL213. The electrostatic changes near GIu-H173 cause it to be a good proton donor and acceptor, and the structural changes create a cavity which accommodates water molecules that connect Glu-H173t o other proton transfer components.