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 were constructed in the photosynthetic bacterium Rhodobacter sphaeroides. This residue is adjacent to the pair of bacteriochlorophyll molecules (P_L and P_M) that is the primary donor of electrons (P) in photosynthetic light-energy transduction. All of the mutations affected the optical and electrochemical properties of the P bacteriochlorophylls. A mutant reaction centre with the change Phe M197 to Arg (FM197R) was crystallized, and a structural model constructed at 2.3 Å (1Å = 0.1nm) resolution. The mutation resulted in a change in the structure of the protein at the interface region between the P bacteriochlorophylls and the monomeric bacteriochlorophyll that is the first electron acceptor (B_L). The new Arg residue at the M197 position undergoes a significant reorientation, creating a cavity at the interface region between P and B_L. The acetyl carbonyl substituent group of the P_M bacteriochlorophyll undergoes an out-of-plane rotation, which 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 in a suitable position to connect the macrocycles of P and B_L via three hydrogen bonds. Transient absorption measurements show that, despite an inferred decrease in the driving force for primary electron transfer in the FM197R reaction centre, there is little effect on the overall rate of the primary reaction in the bulk of the reaction-centre population. Examination of the X-ray crystal structure reveals a number of small changes in the structure of the reaction centre in the interface region between the P and B_L bacteriochlorophylls that could account for this faster-than-predicted rate of primary electron transfer.