Folding pathways of immunoglobulin domains. The folding kinetics of the Cgamma3 domain of human IgG1.

The in vitro folding kinetics of a fragment corresponding to an intact dimer of the Cgamma3 domain of human IgG1 (pFc') were monitored via the large changes in tryptophan fluorescence which accompany these processes. In going from the guanidine hydrochloride (Gdn.HCl) induced unfolded state (4.0 M Gdn.HCl) to the native state (0.5 M Gdn.HCl), three well-separated first-order processes were observed having time constants of 5, 50, and 350 s and roughly equal amplitudes. These values were concentration independent, a fact consistent with there being no fluorescence change accompanying dimerization. These time constants are one to two orders of magnitude slower than those observed for proteins of similar size such as ribonuclease or cytochrome c, most probably reflecting the complex processes involved in forming the correct beta-sheet arrangement of immunoglobulin domains. The corresponding unfolding transition is biphasic having time constant values of 50 and 500 s, the latter comprising 80% of the fluorescence change. These data indicate the presence of at least one species with intermediate fluorescence along the unfolding pathway. Gdn.HCl concentration jumps were also performed over various intervals within the transition zone. The results are not consistent with a fully reversible mechanism. In the absence of the intrachain disulfide bond, pFc' exists in an unfolded state even at 0.5 M Gdn.HCl. In a concomitant refolding and reoxidation experiment (at 0.5 M Gdn.HCl and using an optimal disulfide interchange catalytic system), the time constant for disulfide formation was in the range of 80--200 s and the fluorescence change revealed a lag phase analyzable in terms of rate-limiting reoxidation and refolding times consistent with those observed for the initially disulfide bonded species. Under similar conditions but a 4 M Gdn.HCl, reoxidation was more than two orders of magnitude slower, suggesting that reoxidation is directed by a refolding nucleation event.