An examination of how structural changes can affect the rate of electron transfer in a mutated bacterial photoreaction centre

A series of reaction centres bearing mutations at the (Phe) M197 position w ere constructed in the photosynthetic bacterium Rhodobacter sphaeroides. Th is residue is adjacent to the pair of bacteriochlorophyll molecules (P-L an d P-M) that is the primary donor of electrons (P) in photosynthetic light-e nergy transduction. All of the mutations affected the optical and electroch emical properties of the P bacteriochlorophylls. A mutant reaction centre w ith the change Phr M197 to Arg (FM 197R) was crystallized, and a structural model constructed at 2.3 Angstrom (1 Angstrom = 0.1 nm) resolution. The mu tation resulted in a change in the structure of the protein at the interfac e region between the P bacteriochlorophylls and the monomeric bacteriochlor ophyll that is the first electron acceptor (B-L). The new Arg residue at th e M197 position undergoes a significant reorientation, creating a cavity at the interface region between P and B-L. The acetyl carbonyl substituent gr oup of the P-M bacteriochlorophyll undergoes an out-of-plane rotation, whic h decreases the edge-to-edge distance between the macrocycles of P-M and B- L. In addition, two new buried water molecules partially filled the cavity that is created by the reorientation of the Arg residue. These waters are i n a suitable position to connect the macrocycles of P and B-L via three hyd rogen bonds. Transient absorption measurements show that, despite an inferr ed decrease in the driving force for primary electron transfer in the FM197 R reaction centre, there is little effect on the overall rate of the primar y reaction in the bulk of the reaction-centre population. Examination of th e X-ray crystal structure reveals a number of small changes in the structur e of the reaction centre in the interface region between the P and B-L bact eriochlorophylls that could account for this faster-than-predicted rate of primary electron transfer.