Charge Separation and L209 Mutation Give Rise to Similar Conformational Changes in the Reaction Center from Rhodobacter sphaeroides

Abstract

X-ray structures of Rhodobacter sphaeroides reaction centers (RCs) have been refined to 2.0 and 2.3 Å resolution for the neutral (dark) and charge-separated (illuminated) state, respectively. Similar to results of MHB Stowell et al. (Science 276, 812-816, 1997), the secondary quinone (QB) moves closer to the non-heme iron when charge separation occurs. Three further structures of RC variants, in which the native Pro L209 was changed to Tyr, Phe, or Glu, respectively, have been determined to 2.6-2.8 Å resolution in the neutral state. The Tyr and Phe mutant RCs do not interrupt a water chain as predicted by L Baciou and H Michel (Biochemistry 95, 7967-7972, 1995). Instead, both aromatic side chains point toward the QB molecule displacing the surrounding polar side chains Asp L213, Thr L226, and Glu H173 by up to 2.6 Å. The conformational changes induced by the bulky tyrosine ring of the Tyr L209 mutant lead, in the neutral state, to a displacement of the QB molecule similar to the one of the native RC observed, however, only in the charge-separated state. That means, charge separation and L209 mutation give rise to similar conformational changes. The rate of QA--QB electron transfer in the Tyr L209 mutant RC is similar to the rate observed in the native RC. It is therefore concluded that the movement of QB from the distal to the proximal position is not rate limiting for the QA--QB electron transfer.