Calcium-Dependent Conformational Rearrangements and Protein Stability in Chicken Annexin A5

Abstract

The conformational rearrangements that take place after calcium binding in chicken annexin A5 and a mutant lacking residues 3-10 were analyzed, in parallel with human annexin A5, by circular dichroism (CD), infrared spectroscopy (IR), and differential scanning calorimetry. Human and chicken annexins present a slightly different shape in the far-UV CD and IR spectra, but the main secondary-structure features are quite similar (70–80% α-helix). However, thermal stability of human annexin is significantly lower than its chicken counterpart (∼8°C) and equivalent to the chicken N-terminally truncated form. The N-terminal extension contributes greatly to stabilize the overall annexin A5 structure. Infrared spectroscopy reveals the presence of two populations of α-helical structures, the canonical α-helices (∼1650 cm −1) and another, at a lower wavenumber (∼1634 cm −1), probably arising from helix-helix interactions or solvated α-helices. Saturation with calcium induces: alterations in the environment of the unique tryptophan residue of the recombinant proteins, as detected by near-UV CD spectroscopy; more compact tertiary structures that could account for the higher thermal stabilities (8 to 12°C), this effect being higher for human annexin; and an increase in canonical α-helix percentage by a rearrangement of nonperiodical structure or 3 10 helices together with a variation in helix-helix interactions, as shown by amide I curve-fitting and 2D-IR.