CRYSTALLOGRAPHIC ANALYSES OF SITE-DIRECTED MUTANTS OF THE PHOTOSYNTHETIC REACTION-CENTER FROM RHODOBACTER-SPHAEROIDES

Seven site-directed mutants of the bacterial photosynthetic reaction c enter (RC) from the 2.4.1 and WS 231 wild-type strains of Rhodobacter sphaeroides have been crystallized and their X-ray diffraction analyze d to resolutions between 3.0 and 4.0 Angstrom. The mutations can be di vided into four distinct categories: (1) mutations altering cofactor c omposition that affect electron transfer and quantum yield, His M202 - -> Leu (M202HL), His L173 --> Leu (L173HL), and Leu M214 --> His (M214 LH); (2) a mutation in the proposed pathway of electron transfer alter ing electron-transfer kinetics, Tyr M210 --> Phe (M210YF); (3) a mutat ion around the non-heme iron resulting in an iron-less reaction center , His M219 --> Cys (M219HC); and (4) mutations around the secondary el ectron acceptor, a ubiquinone, affecting proton transfer and quinone t urnover, Glu L212 --> Gin (L212EQ) and Asp L213 --> Asn (L213DN). Resi dues L173 and M202 are within bonding distance of the respective magne siums 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 sur rounding main-chain or side-chain atoms. In the M214LH mutant, the bac teriopheophytin has been replaced by a bacteriochlorophyll, and the si de chain of His M214 is within ligand distance of the magnesium. The M 210YF, L212EQ, and L213DN mutants show no significant tertiary structu re changes near the mutation sites. The M219HC diffraction data indica te that the overall tertiary structure of the reaction center is maint ained in the absence of the non-heme iron.