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The Thioester Bond of C3

1990; Springer Science+Business Media; Linguagem: Inglês

10.1007/978-3-642-74977-3_4

2196-9965

R. P. Levine, Alister W. Dodds,

Cellular transport and secretion

Thioester bonds are not uncommon in biochemical systems, being formed as intermediates in the reaction mechanisms of a number of enzymes (LIU 1976; DOUGLAS 1987). Thioester bonds are, however, important structural and functional features of certain proteins, including C3. C3 shares the thioester bond with several other proteins, principally the fourth complement protein, C4, and α2-macroglobulin. It is this bond that confers upon C3 the ability to form covalent bonds with cell-surface macromolecules, immune complexes, and a variety of small molecules in solution (LAW and LEVINE 1977; LAW etal. 1979, 1980, 1981). The bond in C3 lies in the α-polypeptide linking the—SH group of a cysteinyl residue to the y-carboxyl group of a glutaminyl residue. The same obtains in C4 and α-macroglobulin, and the three proteins share extensive amino acid homology throughout their structure, particularly in the vicinity of the bond.