Proton nuclear magnetic resonance study of the third component of complement: solution conformation of the carboxyl-terminal segment of C3a fragment

Abstract

A proton nuclear magnetic resonance (NMR) study is reported of des-Arg-C3a, which is a 76-residue fragment obtained from the N-terminal portion of the .alpha. chain of the third component of human complement. A method of carboxypeptidase digestion/difference spectroscopy [Endo, S., and Arata, Y. (1985) Biochemistry 24, 1561-1568] was used for the spectral assignments for Ala-76, Leu-75, Gly-74, His-72, His-67, and Ala-48. On the basis of the NMR results obtained for these residues, we conclude that in aqueous solution (1) the C-terminal segment Leu-73-Ala-76 is free from interactions with the rest of the C3a molecule and (2) the major part of the C-terminal segment takes an ordered conformation. We also suggest that the presence of a core, which is formed by segment Tyr-15-Tyr-59 [Huber, R., Scholze, H., Paques, E. P., and Deisenhofer, J. (1980) Hoppe-Seyler''s Z. Physiol. Chem. 361, 1389-1399] is essential for the C-terminal segment in maintaining the ordered structure in aqueous solution. 1H NMR spectral data were also obtained for the intact C3 from human and porcine sources. The resonances for the C2-H protons of His-67 and His-72, which exist in the C3a part of the human C3 molecule, were assigned. Comparisons of the results obtained with those for des-Arg-C3a demonstrate that (1) upon cleavage of C3a very little change, if any, is induced in microenvironments of His-67 and His-72 and (2) a piece of segment that contains His-72 is exposed to solvent and highly flexible. A possible role of the flexible segment joining C3a and C3b will be discussed briefly.