Lysine substitutions near photoactive cofactors in the bacterial photosynthetic reaction center have opposite effects on the rate of triplet energy transfer

Abstract

Rates of triplet energy transfer have been investigated by time-resolved pulsed electron paramagnetic resonance (EPR) spectroscopy in mutant photosynthetic reaction centers (RCs) from both Rhodobacter (R.) sphaeroides and R. capsulatus. Mutant RCs of both species harbor lysines in place of native residues of the L subunit at position 178 or 181 near monomeric bacteriochlorophyll BB. Analysis of kinetic measurements acquired throughout a range of temperatures demonstrated an increased rate of triplet transfer in the L178Ser ! Lys (‘‘L178K’’) mutant RCs and a decreased rate in the L181Phe ! Lys (‘‘L181K’’) mutant RCs from both species relative to their respective wild types. Activation energies extracted from Arrhenius plots suggest that triplet transfer in the L178K mutant RC is nearly isoenergetic with wild-type transfer while triplet transfer in the L181K mutant RC requires significantly less activation energy. This latter finding is counter to the observation of slower triplet transfer in the L181K mutant RC. Preliminary structural data offer evidence for direct ligation between the L181 lysine side chain and the central magnesium of BB, but the side chain of L178K is disordered. Our spectroscopic data suggest that these lysine substitutions elicit opposing changes in the relative geometric orientation of BB and that subtle changes in the protein environment can have dramatic and opposing effects on the rates of the photoprotective triplet energy transfer reaction. 2003 Elsevier B.V. All rights reserved.