Conformational change of complement proteins C3 and C4 induced by methylamine: An X-ray scattering study

Analysis of X-ray scattering data from dilute solutions of the complement proteins C3, C4 and C5 yields the radii of gyration 4.5, 4.25 and 4.8 nm, and the maximum distances within the molecules (Dmax) of 18, 14.5 and 14 nm, respectively. The X-ray data of C4 are compatible with the scattering from one single triaxial body represented as an elliptic cylinder with the semiaxes 5.6 and 2.1 nm and a length of 11.0 nm. When C3 and c4 react with methylamine, at pH 8.0, the radii of gyration increase to 5.1 and 4.2 nm, and the Dmax)-values increase to 25 and 18 nm, respectively; for C5 there was no noticeable change in the X-ray scattering curve. The methylamine reactions are discussed in relation to conformational changes and possible dimerizations of C3 and C4.