title: Modeling the bacterial photosynthetic reaction center. 4. The structural, electrochemical, and hydrogen-bonding properties of 22 mutants 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" of bacteriochlorophylls P identical with P(L)P(M). Direct X-ray structural measurements of the consequences of mutation are rare. Attention has mostly focused on effects on properties such as carbonyl stretching frequencies and midpoint potentials 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 center, (ii) the structure of the FY(M197) mutant and possible water penetration near the special pair, (iii) that significant protein chain distortions are required 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 midpoint 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 P(L), (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of the special pair, (vii) that the OH group of tyrosine M210 points away from P(M), and (viii) 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.
authors.affiliation: Department of Biochemistry, School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: J M
authors.initials: JM
authors.lastname: Hughes
authors.forename: M C
authors.initials: MC
authors.lastname: Hutter
authors.forename: J R
authors.initials: JR
authors.lastname: Reimers
authors.forename: N S
authors.initials: NS
authors.lastname: Hush
mesh_terms: D006860:Hydrogen Bonding; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D011487:Protein Conformation; D011789:Quantum Theory; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D055672:Static Electricity; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D014867:Water