Human complex-forming glycoprotein, heterogeneous in charge: The primary structure around the cysteine residues and characterization of a disulfide bridge

Abstract

The amino acid sequence of the cysteine-containing regions of human complex-forming glycoprotein, heterogeneous in charge (protein HC) was determined by studies of the tryptic peptides of the completely reduced and radioalkylated protein. One of the cysteines was located in the amino-terminal part of the molecule at position 34: GlyProValPro ThrProProAspAsnIleGlnValGlnGluAsnPheAsn IleSerArgIleTyr GlyLysTrpTyrAsnLeuAlaIleGlySerThrCys ProLeuLys. Another was located in the carboxyl-terminal part of the molecule 14 residues from the carboxyl end: ThrMetAlaAspArgGlyGluCysValProGlyGlu GlnGluProGluProIleLeuIleProArg. The third was located somewhere in the middle of the molecule in the sequence: LysGlyValCysGluGluThrSerGlyAlaTyrGlu Lys. Diagonal map electrophoresis showed that the cysteine residue in the carboxy-terminal region was bridged to the cysteine containing sequence in the middle of the molecule. The function of the cysteine residue at position 34 remains elusive since the residue was not found on the diagonal maps. The release of cysteic acid and a small cysteic acid containing peptide after oxidation of the native protein HC molecule suggests that this cysteine residue may be involved in disulfide bridges with cysteine and small cysteine containing peptides.