Location of the interchain disulfide bonds of the fourth component of human complement (C4): evidence based on the liberation of fragments secondary to thiol-disulfide interchange reactions.
Treatment of human C4 with chemical denaturants and heat produces rapid, autolytic peptide bond cleavage of the alpha-chain. These alpha-chain fragments are linked to the parent C4 molecule through disulfide bonds. On more prolonged heating, however, there is liberation of several peptides, including the beta-chain, the gamma-chain, and a C-terminal alpha-chain fragment. This reaction is inhibited by iodoacetamide. By using a fluorescent thiol reagent and 14C-iodoacetamide, the thiol group present on each peptide was analyzed. The results suggest that the thiol residue exposed by cleavage of the thioester bond induces thiol-disulfide interchange reactions to liberate the peptides. Based on the identification of fragments liberated, the kinetics of their appearance, their sulfhydryl content, and the reported primary structure of human C4, a model of the interchain disulfide bonds is proposed in which the amino terminal portion of the alpha-chain is disulfide-linked to both the beta- and gamma-chains, whereas the carboxyl terminal portion of the alpha-chain is disulfide-linked to only the gamma-chain.
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T
Seya
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S
Nagasawa
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JP
Atkinson
D000595:Amino Acid Sequence; D003181:Complement C4 / Q000032:analysis; D004220:Disulfides; D004228:Dithionitrobenzoic Acid; D004591:Electrophoresis, Polyacrylamide Gel; D006358:Hot Temperature; D006801:Humans; D007460:Iodoacetamide; D010446:Peptide Fragments / Q000302:isolation & purification*; D011487:Protein Conformation; D011489:Protein Denaturation; D013438:Sulfhydryl Compounds
D003181:Complement C4; D004220:Disulfides; D010446:Peptide Fragments; D013438:Sulfhydryl Compounds; D004228:Dithionitrobenzoic Acid; D007460:Iodoacetamide