EFFECTS OF CALCIUM, MAGNESIUM, AND PHOSPHORYLCHOLINE ON SECONDARY STRUCTURES OF HUMAN C-REACTIVE PROTEIN AND SERUM AMYLOID-P COMPONENT OBSERVED BY INFRARED-SPECTROSCOPY

The secondary structures of human C-reactive protein (CRP) and serum a myloid P component (SAP) in D2O-based solutions in the presence or abs ence of calcium, magnesium, and phosphorylcholine have been investigat ed using Fourier transform infrared spectroscopy. Quantitative analysi s provided estimations of about 50% beta-sheet, 12% alpha-helix, 24% b eta-turn, and 14% unordered structure for CRP and about 54% beta-sheet , 12% alpha-helix, 25% beta-turn, and 9% unordered structure for SAP. With both proteins significant calcium dependent changes were observed in conformation-sensitive amide I regions assigned to each type of st ructure. The CRP spectrum was also affected by magnesium, but the chan ges differed from those induced by calcium. The SAP spectrum was not a ffected by magnesium. Phosphorylcholine in the presence of calcium als o affected the spectrum of CRP but not the spectrum of SAP, Our presen t study provides the first direct comparison of the secondary structur es of the pentraxins human CRP and SAP and hamster female protein (Don g, A., Caughey, B., Caughey, W. S., Bhat, K. S., and Coe, J. E. (1992) Biochemistry 32, 9364-9370), These findings suggest that the three pe ntraxins have similar secondary structure compositions and calcium dep endent conformational changes, but differ significantly in their respo nses to phosphorylcholine and magnesium. Such properties are expected to be relevant to the incompletely understood roles of these highly co nserved proteins including binding to nuclear proteins, complement act ivation, and association with amyloids,