Conformation of the constant fragment of the immunoglobulin light chain: effect of cleavage of the polypeptide chain and the disulfide bond.

In order to understand the conformations and stabilities of the immunoglobulin domains, a derivative of the type-lambda constant fragment in which the peptide bond Arg190-Ser191 is cleaved and a derivative of the type-kappa constant fragment in which the peptide bond Arg142-Glu143 is cleaved were prepared by limited proteolysis with the proteinase from mouse submaxillary gland, endoproteinase Arg-C [EC 3.4.21.40]. The cleaved peptide bond of each derivative is located in the loop formed by the intrachain disulfide bond and the two peptides formed are linked by the disulfide bond. The two nicked CL fragments did not assume any ordered conformation. On the other hand, a derivative in which the intrachain disulfide bond is reduced had a conformation very similar to that of the intact CL fragment, although the stability was considerably low. Since the entropy of the nicked fragment should be nearly the same as that of the reduced CL fragment in the unfolded state, a destabilizing effect of cleavage of the polypeptide bond in the folded state in addition to the entropic effect in the unfolded state seems necessary to account for the conformations of the nicked CL fragments.