PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Characterization of the thermodynamic properties of the natively unfolded manganese stabilizing protein of Photosystem II. AARON WYMAN^*,1, CHARLES YOCUM², ¹ DEPT OF MCD BIOLOGY, ANN ARBOR, MI, USA² DEPT OF MCD BIOLOGY AND CHEMISTRY, ANN ARBOR, MI, USA

ABSTRACT- The largest lumenal extrinsic protein of Photosystem II, Manganese Stabilizing Protein (MSP) is required for productive binding of the PSII Mn cluster and for high rates of oxygen evolution. The spinach protein possesses a number of unusual physical properties, including an elongated solution structure, a high ratio of charged to hydrophobic amino acid residues, a greater than expected molecular mass as determined by SDS-PAGE and gel filtration, and unusual thermostability. Although the disulfide bond is clearly important for maintaining MSP^,s solution structure, a mutant (C28A,C51A) which lacks the disulfide bridge in the protein retains the unusual thermostability. The thermodynamic properties of two MSP species (wild-type and C28A,C51A) were determined at pH 6.0 and 8.0 from thermal unfolding spectra generated by far-UV circular dichroism and by direct analysis using DSC, which permit calculations of the proteins^, T_m, S, H_m, and C_P values. C28A,C51A MSP has lower enthalpy and heat capacity values than the wild-type protein, indicating that the structure of MSP becomes less stable without the disulfide bond. Both MSP species had greater enthalpy and heat capacity values at pH 8.0 than at 6.0, indicating that both proteins have a more stable solution structure at the higher pH. This is consistent with the finding that MSP has a greater resistance to heat-induced protein aggregation at pH 8.0 than at 6.0. It is hypothesized that the increased negative charge of MSP at pH 8.0, along with the positioning of hydrophobic residues toward the protein^,s core at this pH, hinders interactions between MSP molecules, preventing their self-association in solution, while also producing a more stable solution structure.

KEY WORDS: OXYGEN EVOLUTION, THERMODYNAMIC PROPERTIES, PHOTOSYSTEM II, MANGANESE STABILIZING PROTEIN