Modeling the bacterial photosynthetic reaction center. 4. The structural, electrochemical, and hydrogen-bonding properties of 22 mutants of Rhodobacter sphaeroides

Site-directed mutagenesis has been employed by a number of groups to produc e mutants of bacterial photosynthetic reaction centers, with the aim of tun ing their operation by modifying hydrogen-bond patterns in the close vicini ty of the "special pair" of bacteriochlorophylls P drop PLPM. Direct X-ray structural measurements of the consequences of mutation are rare. Attention has mostly focused on effects on properties such as carbonyl stretching fr equencies and midpoint potentials to infer indirectly the induced structura l modifications. In this work, the structures of 22 mutants of Rhodobacter sphaeroides have been calculated using a mixed quantum-mechanical molecular -mechanical method by modifying the known structure of the wild type. We de termine (i) the orientation of the 2a-acetyl groups in the wild type, FY(M1 97), and FH(M197) series mutants of the neutral and oxidized reaction cente r, (ii) the structure of the FY(M197) mutant and possible water penetration near the special pair, (iii) that significant protein chain distortions ar e required to assemble some M160 series mutants (LS(M160), LN(M160), LQ(M16 0), and LH(M160) are considered), (iv) that there is competition for hydrog en-bonding between the 9-keto and 10a-ester groups for the introduced histi dine in LH(L131) mutants, (v) that the observed midpoint potential of P for HL(M202) heterodimer mutants, including one involving also LH(M160), can b e correlated with the change of electrostatic potential experienced at PL, (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of t he special pair, (vii) that the OH group of tyrosine M210 points away from PM, and (viii) that competitive hydrogen-bonding effects determine the chan ge in properties of NL(L166) and NH(L166) mutants. A new technique is intro duced for the determination of ionization energies at the Koopmans level fr om QM/MM calculations, and protein-induced Stark effects on vibrational fre quencies are considered.