The road to the crystal structure of the cytochrome bc1 complex from the anoxigenic, photosynthetic bacterium Rhodobacter sphaeroides

The advantages of using bacterial systems to study the mechanism and function of cytochrome bc1 complexes do not extend readily to their structural investigations. High quality crystals of bacterial complexes have been difficult to obtain despite the enzymes’ smaller sizes and simpler subunit compositions compared to their mitochondrial counterparts. In the course of the structure determination of the bc1 complex from R. sphaeroides, we observed that the growth of only low quality crystals correlated with low activity and stability of the purified complex, which was mitigated in part by introducing a double mutations to the enzyme. The S287R(cyt b)/V135S(ISP) mutant shows 40% increase in electron transfer activity and displays a 4.3 °C increase in thermal stability over wild-type enzyme. The amino acid histidine was found important in maintaining structural integrity of the bacterial complex, while the respiratory inhibitors such as stigmatellin are required for immobilization of the iron-sulfur protein extrinsic domain. Crystal quality of the R. sphaeroides bc1 complex can be improved further by the presence of strontium ions yielding crystals that diffracted X-rays to better than 2.3 Å resolution. The improved crystal quality can be understood in terms of participation of strontium ions in molecular packing arrangement in crystal.