Properties of M197, M160, M202, L131, and L166 mutants from QM/MM optimized structures of Rhodobacter Sphaeroides

Abstract

Site-directed mutagenesis has been employed by a number of groups to produce mutants of bacterial photosynthetic reaction centers, with the aim of tuning their operation by modifying hydrogen bond patterns in the close vicinity of the "special pair". Direct X-ray structural measurements of the consequences of mutation are rare. Attention has mostly focussed on effects on properties such as carbonyl stretching frequencies and mid-point potentials in order to infer indirectly the induced structural 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 determine (i) the orientation of the 2a-acetyl groups in the wild type, FY(M197), and FH(M197) series mutants of the neutral and oxidized reaction centre, (ii) the structure of the FY(M197) mutant and possible water penetration near the special pair, (iii) that significant protein chain distortions are required in order to assemble some M160 series mutants (LS(M160), LN(M160), LQ(M160) and LH(M160) are considered), (iv) that there is competition for hydrogen bonding between the 9-keto and 10a-ester groups for the introduced histidine in LH(L131) mutants, (v) that the observed mid-point potential of P for HL(M202) heterodimer mutants, including one involving also LH(M160), can be correlated with the change of electrostatic potential experienced at PL, (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of the special pair, and (vii) that competitive hydrogen bonding effects determine the change in properties of NL(L166) and NH(L166) mutants. A new technique is introduced for the determination of ionization energies at the Koopmans level from QM/MM calculations, and protein-induced Stark effects on vibrational frequencies are considered.