PARENT SESSION

Symposium S5A Type II reaction centres: Structure
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 210A
Chair: Dave Britt
Co-Chair: Rick Debus

Bacterial reaction centers with a functional manganese binding site. Megan Theilges, Laszlo Kalman, Gregory Uyeda, Ana Camara-Artigas, Russell LoBrutto, JoAnn Williams, James Allen^*,1, ¹ Center for the Study of Early Events in Photosynthesis, Tempe, AZ, USA

ABSTRACT- A highly oxidizing bacteriochlorophyll dimer was generated by multiple amino acid substitutions in reaction centers from Rhodobacter sphaeroides (Kalman et al., 1999, Nature 401, 696-699). Carboxylate residues were then introduced into these modified reaction centers at a location that is analogous to the binding site of the manganese cluster in photosystem II. The residue Asp D1-170 is thought to play a critical role in the assembly of the manganese cluster in photosystem II. The corresponding residue, Glu M-173, was included as a ligand to manganese in four different designs for manganese binding sites in the reaction center. All four mutants were isolated and characterized by optical spectroscopy. At pH 9, the dissociation constants in the four mutants were found to range from no measurable change compared to wild type to having a tight binding site of approximately 1 micromolar. For the mutant with a tight binding site, the binding was independent of pH above 8.5 but strongly dependent upon pH below 8.5. Upon exposure to light, the bound manganese rapidly reduced the oxidized bacteriochlorophyll dimer with a rate that was independent of manganese concentration and an amplitude that increased in accordance with the measured dissociation constants. The three dimensional structure of the tight-binding mutant was determined by X-ray diffraction. The structure of the mutant showed the presence of manganese coordinated to the designed carboxylate groups, as well as a histidine residue that is present in wild type and a bound water molecule, with few overall changes compared to wild type reaction centers.

KEY WORDS: photosystem II, bacterial reaction center, manganese cluster