Method Development for Determining the Stability of Heat Stable Proteins Combined with Biophysical Characterization of Human Calmodulin and the Disease Associated Variant D130G

Sammanfattning: Calmodulin is a highly conserved calcium ion binding protein expressed in all eukaryotic species. The 149 amino acid residues in the primary structure are organized in seven α helices with the highly flexible central α helix connecting the two non-cooperative domains of calmodulin. Each domain contains two EF-hand motifs to which calcium ions bind in a cooperative manner, hence the binding of four calcium ions saturate one calmodulin molecule. In the cardiovascular area calmodulin is involved in the activation of cardiac muscle contraction, and mutations that arise in the genetic sequence of the protein often have severe consequences. One such consequential mutation that can arise brings about the replacement of the highly conserved aspartic acid with glycine at position 130 in the amino acid sequence. In this research, the thermal and chemical stability within the C domain of the D130G variant of human calmodulin was investigated using a new method only requiring circular dichroism spectroscopic measurements. Affinity studies within the C domain of the D130G variant of human calmodulin were performed using fluorescence spectroscopy, and the ligands chosen for this purpose were trifluoperazine and p- HTMI. All analytical experiments were performed with the C domain of wild type human calmodulin as a reference. From the new method, it was concluded that the C domain of the D130G variant of human calmodulin has a slightly decreased stability in terms of Tm and Cm values compared to the C domain of wild type human calmodulin. The affinity analyses indicated that neither trifluoperazine nor p-HTMI discriminates between the C domain of the D130G variant of human calmodulin and the C domain of wild type human calmodulin in terms of dissociation constants. The pivotal outcome from this research is that the new method is applicable for determination of the stability parameters Tm and Cm of heat stable proteins.