title: Kinetic and thermodynamic stabilization of the betagamma-crystallin homolog spherulin 3a from Physarum polycephalum by calcium binding.
abstract: Globular proteins may be stabilized, either intrinsically, at the various levels of the structural hierarchy, or extrinsically, by ligand binding. In the case of the dormant all-beta protein spherulin 3a (S3a) from the slime mold Physarum polycephalum, binding of calcium ions causes extreme kinetic and thermodynamic stabilization. S3a is the only known single-domain member of the two Greek key superfamily of betagamma-crystallins sharing the extreme long-term stability of its homologs in vertebrate eye lens. Spectral analysis allows two Ca2+-binding sites with KD=9 microM and 200 microM to be distinguished. Unfolding in the absence and in the presence of Ca2+gives evidence for extreme kinetic stabilization of the protein: In the absence of Ca2+, the half-time of unfolding in 2. 5 M guanidinium chloride (GdmCl) equals 8.3 minutes, whereas in the presence of Ca2+, even in 7.5 M GdmCl, it exceeds nine hours. To reach the equilibrium of unfolding in the absence and in the presence of Ca2+takes one day and eight weeks, respectively. The corresponding Gibbs free energies (based on the two-state model) are 77 and 135 kJ/mol. Saturation of S3a with Ca2+leads to an upward shift of the temperature-induced equilibrium transition by ca 20 deg. C. The in situ Ca2+concentration in the spherules is sufficient for the complete complexation of S3a in vivo.
authors.affiliation: Institut für Biophysik und Physikalische Biochemie, Universität Regensburg, Regensburg, D-93040, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kretschmar
authors.forename: E M
authors.initials: EM
authors.lastname: Mayr
authors.forename: R
authors.initials: R
authors.lastname: Jaenicke
mesh_terms: D000818:Animals; D002118:Calcium / Q000737:chemistry*; D002942:Circular Dichroism; D003459:Crystallins / Q000737:chemistry*; D019791:Guanidine; D007700:Kinetics; D016839:Physarum polycephalum / Q000737:chemistry*; D011489:Protein Denaturation; D015800:Protozoan Proteins / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D003459:Crystallins; D015800:Protozoan Proteins; C108191:spherulin 3a protein, Physarum polycephalum; D019791:Guanidine; D002118:Calcium