Solvent effects on the structure of rabbit Clq, a subcomponent of the first component of complement

The effect of solvent conditions on the conformation of rabbit Clq was studied by both spectroscopic and nonspectroscopic methods. The conformation of Clq in buffered saline solutions at pH 7.4 or 6.0 did not differ significantly from Clq at twice the saline concentration as determined with circular dichroism, difference spectroscopy, and tritium-hydrogen exchange techniques. Addition of calcium to the buffers had no structural effects in any of the conditions examined. Hydrogen exchange experiments performed at pH 7.4 were also unaffected by magnesium, manganese, or ethylenediaminetetraacetic acid. With all the methods used a pH effect was observable between 5.1 and 8.3. From solvent perturbation difference spectroscopy results it was calculated that the equivalent of 10 +/- 2 and 6 +/- 1 mol of tyrosine and tryptophan/mol of Clq, respectively, became exposed at the lower pH. A small positive CD band in the 231 to 235 nm region decreased in wavelength and increased in magnitude as a function of decreasing pH, indicating tyrosine exposure at the lower pH and possibly changes in the collagen-like structure of Clq. Hydrogen exchange experiments indicate a small, but significant, conformation transition occurring in the pH 5 region and a stabilization of conformation between pH 6 to 8. From these results the conformational pH dependence was interpreted as an acid expansion of Clq with a minor conformational transition occurring between pH 5 AND 6. These effects may in part be associated with decreased Clq-Ig interactions which have been observed at the lower pH.