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The complement system proteins C3 and C4 and the plasma protease inhibitor alpha 2-macroglobulin, when activated by limited proteolysis, can bind covalently to other macromolecules. The three proteins also exhibit an unusual internal peptide-bond cleavage reaction when denatured. The covalent binding reaction is likely to occur by a transacylation mechanism involving an internal thiolester in the three proteins. However, the activated species of these proteins are much more reactive than simple thiolesters. Studies of molecular models of the thiolester region in C3 show that an intramolecular acid catalysis mechanism can both account for the exceptional reactivity of the activated form of these proteins and provide an explanation for the denaturation-induced peptide bond cleavage.

Original publication

DOI

10.1007/bf01121579

Type

Journal article

Journal

Biosci Rep

Publication Date

06/1981

Volume

1

Pages

461 - 468

Keywords

Amino Acid Sequence, Catalysis, Chemical Phenomena, Chemistry, Complement C3, Complement C4, Hydrogen Bonding, Models, Molecular, alpha-Macroglobulins