Crystallographic Analyses of Site-Directed Mutants of the Photosynthetic Reaction Center from Rhodobacter sphaeroides

Seven site-directed mutants of the bacterial photosynthetic reaction center (RC) from the 2.4.1 and WS 231 wild-type strains of Rhodobacter sphaeroides have been crystallized and their X-ray diffraction analyzed to resolutions between 3.0 and 4.0 Å. The mutations can be divided into four distinct categories: (1) mutations altering cofactor composition that affect electron transfer and quantum yield, His M202 → Leu (M202HL), His L173 → Leu (L173HL), and Leu M214 → His (M214LH); (2) a mutation in the proposed pathway of electron transfer altering electron-transfer kinetics, Tyr M210 → Phe (M210YF); (3) a mutation around the non-heme iron resulting in an iron-less reaction center, His M219 → Cys (M219HC); and (4) mutations around the secondary electron acceptor, a ubiquinone, affecting proton transfer and quinone turnover, Glu L212 → Gln (L212EQ) and Asp L213 → Asn (L213DN). Residues LI73 and M202 are within bonding distance of the respective magnesiums of the two bacteriochlorophylls of the BChl special pair, while M214 is close to the bacteriopheophytin on the active A branch of the RC. The L173HL and M202HL crystal structures show that the respective bacteriochlorophylls are replaced with bacteriopheophytins (i.e., loss of magnesium) without significant structural perturbations to the surrounding main-chain or side-chain atoms. In the M214LH mutant, the bacteriopheophytin has been replaced by a bacteriochlorophyll, and the side chain of His M214 is within ligand distance of the magnesium. The M210YF, L212EQ, and L213DN mutants show no significant tertiary structure changes near the mutation sites. The M219HC diffraction data indicate that the overall tertiary structure of the reaction center is maintained in the absence of the non-heme iron.