Helix formation in reduced, S-carboxymethylated human choriogonadotropin beta subunit and tryptic peptides

The beta subunit of human choriogonadotropin (hCG beta) and its asialoderivative were digested with trypsin and then reduced and S-carboxymethylated. A series of peptides were purified which corresponded to residues 1-43, 44-95, 96-114, and 123-145 of the 145 amino acid residue glycoprotein. The two N-linked oligosaccharides were present on the amino terminal peptide, and three of the four O-linked oligosaccharides were present on the carboxy terminal peptide. Circular dichroic spectra between 190-240 nm were obtained on reduced, S-carboxymethylated (RCM) hCG beta and the above peptides, both in aqueous solution and in the helicogenic solvent 80% (vol/vol) trifluoroethanol (TFE). In aqueous solution there was evidence of only limited helicity in the peptides and RCM-hCG beta; however, in the presence of TFE, peptides 1-43 and 44-95 exhibited significant helicity, as did the full-length linear chain. The helicity developed in TFE by RCM-hCG beta appears much greater than that which occurs in the native, disulfide-intact form, thus suggesting that the disulfides prevent expression of helicity in regions with alpha-helix potential. Application of the Chou-Fasman secondary structure predictive algorithm to hCG beta suggested that several regions of helix potential, in particular regions 14-21, 59-69, and perhaps 80-88, may account for much of the helicity observed in peptides 1-43 and 44-95, respectively, in TFE. The region from 96-145 has no significant potential for helicity, consistent with the measured circular dichroic spectra of peptides 96-114 and 123-145. These results demonstrate that helicity can occur in the linear form of hCG beta, and this secondary structure can best be attributed to the amino terminal and the middle portion of the molecular. Several potential regions of beta-structure and beta-turns were also suggested.