A study of the denaturation of human C-reactive protein in the presence of calcium ions and glycero-phosphorylcholine

Abstract

Differential scanning calorimetry, fourier transform infrared spectroscopy and surface plasmon resonance spectrometry have been used to study the thermal properties of C-reactive protein in its ligand-bound and ligand-free forms. Clear denaturation endotherms were detected during calorimeter heating scans. Upon binding either or both its ligands, Ca 2+ and glycero-phosphorylcholine, the denaturation temperature of the protein and the enthalpy change on denaturation were increased. Marked changes in the infrared spectrum of the protein were seen when it was heated. The main absorption peaks in the spectrum, at 1651 and 1643 cm −1 were replaced by a single peak at 1628 cm −1. There was an isosbestic point at 1630 cm −1. Comparison with the calorimetric data indicated that the changes in spectra were caused by protein denaturation. The spectra, like the calorimetric data, showed that the presence of glycero-phosphorylcholine raised the denaturation temperature of the protein. However, the presence of calcium ions had little effect on the spectra. This suggests that unlike glycero-phosphorylcholine, calcium binds to C-reactive protein in such a way that hydrogen bonding in the polypeptide backbone is not altered. Using surface plasmon resonance spectrometry the adsorption of C-reactive protein onto self-assembled phosphorylcholine monolayers was measured in the presence of the protein's ligands before and after its denaturation. Monolayers were formed by reacting a phosphorylcholine alkyl thiol with the surface of silvered microscope slides. It was possible to demonstrate a specific interaction between C-reactive protein and the phosphorylcholine surface and show that this interaction did not take place after the protein had been denatured.