Site-Directed Mutations Near the L-Subunit D-Helix of the Purple Bacterial Reaction Center: A Partial Model for the Primary Donor of Photosystem II

Abstract

We have engineered a photosynthetically competent mutant of the purple non-sulfur bacterium Rhodobacter capsulatus which seeks to mimic the behavior of the primary electron donor (P) of the plant photosystem II (PS II) reaction center (RC). To construct this mutant (denoted D1-ILMH), four residues in the bacterial L subunit were mutagenized, such that an 11-residue segment was made identical to the analogous segment from the D1 subunit of PS II. The electronic properties of the bacteriochlorophyll (Bchl) dimer which constitutes the primary donor are substantially altered by these modifications, to the degree that the dimer becomes functionally much more "monomeric". The changes include (1) an increase in the values of the zero-field splitting (ZFS) parameters, as measured by electron paramagnetic resonance (EPR), for the spin-polarized triplet state, 3P, from /D/ = 185 x 10(-4) cm(-1) and /E/ = 31 x 10(-4) cm(-1) in wild-type (WT) chromatophore membranes to /D/ = 200 x 10(-4) cm(-1) and /E/ = 44 x 10(-4) cm(-1) in the mutant and (2) an increase in the EPR line width of the oxidized state, P+, from 0.97 mT in WT to 1.09 mT in D1-ILMH RCs. However, unlike the PS II primary donor (P680), the orientation of 3P in the D1-ILMH mutant is the same as in WT bacteria and does not display the unusual orientation found for PS II. And whereas the redox couple P/P+ has a very high midpoint potential in PS II, P/P+ in the D1-ILMH mutant has a lower midpoint (90 mV more negative) than in WT Rb. capsulatus. In addition, Raman measurements indicate that the hydrogen bond between HisL168 and the C2 acetyl carbonyl oxygen of the Bchl on the active electron transfer pathway (P(A)) is absent in the mutant, due to the fact that HisL168 in the WT sequence has been replaced by a leucine in D1-ILMH. However, the Raman data also reveal the presence of a new hydrogen bond in the D1-ILMH RCs, between the C9 keto carbonyl oxygen of P(A) and an unknown hydrogen-bond donor. Thus, although the protein environment around one of the Bchls of the special pair is significantly changed in D1-ILMH, the chimeric RC does not, as a result of these changes, have a primary donor that is oriented like the one in PS II.