Calcium-Dependent Conformational Change and Thermal Stability of the Isolated PsbO Protein Detected by FTIR Spectroscopy

Abstract

The structure and function of the photosystem II PsbO extrinsic protein is under intense research, being an essential part of the biomolecular engine that carries out water oxidation and oxygen production. This paper presents a structural analysis of the isolated PsbO protein by FTIR spectroscopy, reporting detailed secondary structure quantification and changes in the secondary structure content of the protein attributed to the effect of calcium (Ca(2+)). Measurements in H(2)O and D(2)O have allowed us to see the effect of calcium on the conformation of the protein. The results indicate that (i) the protein presents a major content of beta-structure (i.e., beta-sheet, beta-strands, beta-turns) as detected by the infrared bands at 1624-1625, 1678-1679, 1688-1689 cm(-1), which account for about 38% in water and 33% in heavy water, in the presence of calcium; and (ii) the amount of this beta-structure fraction increases 7-10% in the absence of calcium, with a concomitant decrease in loops and nonordered structure. The thermal denaturation profile of the protein in the presence of calcium showed low stability with T(m) approximately 56 degrees C. This profile also shows a second phase of denaturation above 60 degrees C and the appearance of aggregation signals above 70 degrees C. Our observations indicate that calcium is able to modify the conformation of the protein at least in solution and confirm that PsbO is mainly a beta-protein where beta-sheet is the major ordered secondary structure element of the protein core.

Structural and functional aspects of the MSP (PsbO) and study of its differences in thermophilic versus mesophilic organisms

“…Both S-MSP and T-MSP are thermostable, and have unfolding transitions which exhibit low cooperativity which is consistent with their assignment as molten globular proteins (Heredia and De Las Rivas 2003;Loll et al 2005a;Lydakis-Simantiris et al 1999;Sonoyama et al 1996). Both can refold after heating to 90°C and can restore activity to PSII upon cooling to 25°C (LydakisSimantiris et al 1999;Sonoyama et al 1996).…”

“…A multiple alignment of all known MSP sequences shows only 19 residues are completely conserved, and an additional 11 are functionally conserved, having side chains with similar properties. Most conserved residues are found in the highly conserved regions identified in references by Heredia and De Las Rivas (2003) and De Las Rivas and and shown in Fig. 3.…”

“…binding motifs have been predicted in MSP primary sequence of higher plants (Wales et al 1989). Calcium binding to isolated S-MSP induces conformational changes which increase the loop content of the protein and thermally destabilize it (Heredia and De Las Rivas 2003;Kruk et al 2003), although similar structural changes have not been observed in T. elongatus MSP (Loll et al 2005a). It has also been proposed that calcium is part of an activation process for converting the MSP to an 'open' conformation which allows controlled access of the substrate water to the active site.…”

Immobilization of Laccase on Magnetic Carriers and Its Use in Decolorization of Dyes

“…Stability of our free laccase and laccase immobilized on nanoparticles remained similar, but the stability of laccase immobilized on microparticles decreased. This could be caused by the changing conformation of laccase after immobilization on microparticles 35 .…”

Function of Extrinsic Proteins in Stabilization of the Photosynthetic Oxygen-Evolving Complex

“…It has been suggested that plant PsbO can bind Ca 2+ , which induces slight changes in secondary structure from a -sheet to a loop or nonordered structure, and facilitated the association of PsbO with the PSII core [Heredia & De Las Rivas, 2003, Kruk et al, 2003. However, an EPR study indicated that the functional Ca 2+ ion was not involved in the binding to PsbO [Seidler & Rutherford, 1996].…”