The reaction mechanism of the internal thioester in the human complement component C4

Abstract

A key step in the elimination of pathogens from the body is the covalent binding of complement proteins C3 and C4 to their surfaces. Proteolytic activation of these proteins results in a conformational change, and an internal thioester is exposed which reacts with amino or hydroxyl groups on the target surface to form amide or ester bonds, or is hydrolysed. We report here that the binding of the human C4A isotype involves a direct reaction between amino-nucleophiles and the thioester. A two-step mechanism is used by the C4B isotype. The histidine at position 1,106(aspartic acid in C4A) first attacks the thioester to form an acyl-imidazole intermediate. The released thiol then acts as a base to catalyse the transfer of the acyl group to amino- and hydroxyl-nucleophiles, including water.

Reticulocytes bearing C4d as biomarkers of disease activity for systemic lupus erythematosus

“…Both isoforms exhibit further polymorphism at the protein and DNA levels, with Ͼ40 alleles (including null alleles) identified to date (29)(30)(31)(32). Despite their striking sequence identity, C4A and C4B differ significantly in their functional features with regard to the expression of antigenic determinants, reactivity of the thioester site, hemolytic activity, and ability to bind/solubilize immune complexes (31)(32)(33)(34)(35). The activated thioester in C4A reacts predominantly with amino group-containing substrate and binds efficiently to immune complexes, whereas the activated thioester in C4B reacts preferentially with hydroxyl groups on carbohydrate substrates and binds effectively to erythrocytes (31)(32)(33)(34)(35).…”

“…Despite their striking sequence identity, C4A and C4B differ significantly in their functional features with regard to the expression of antigenic determinants, reactivity of the thioester site, hemolytic activity, and ability to bind/solubilize immune complexes (31)(32)(33)(34)(35). The activated thioester in C4A reacts predominantly with amino group-containing substrate and binds efficiently to immune complexes, whereas the activated thioester in C4B reacts preferentially with hydroxyl groups on carbohydrate substrates and binds effectively to erythrocytes (31)(32)(33)(34)(35). However, it should be noted that 2 erythrocyte blood group antigens, Rodgers and Chido, have been shown to correspond to the C4d region of C4A and C4B, respectively (16,28), suggesting that both C4A and C4B can bind to and persist on the surface membranes of normal erythrocytes.…”

“…The capacity of C4-derived complement activation products to attach covalently to cell surfaces via thioester bonds suggests that erythrocytes, once tagged with C4d, may carry the molecule during the remainder of their lifespan (40,41).…”

Fine-tuned characterization of RCCX copy number variants and their relationship with extended MHC haplotypes

“…3 C4A primarily takes part in removing immune complexes, whereas C4B takes part in the defense against intruders. 4 Lower levels of C4A confer higher susceptibility to autoimmune disorders, 5 and lower levels of C4B have been associated with increased prevalence of cardiovascular disease. 6 Integration of the HERV-K(C4) sequence presumably confers protection against exogenous retroviral attacks, but also decreases serum C4 concentrations.…”

Conformational Complexity of Complement Component C3

“…Residues Phe-1047 from the TED domain and Met-1378, Tyr-1425, and Tyr-1460 from the MG8 domain form a shield around the thioester, limiting access of small amino and hydroxyl nucleophiles. High reactivity toward hydroxyl nucleophiles requires a transformation of the thioester (Cys-988-Gln-991) to a free thiolate anion (Cys-988) and an acylimidazole (Gln-991-His-1104) [49][50][51] . Comparison of the native C3 and C3d 12 structures shows significant structural differences that can be correlated with this transformation from thioester to thiolate anion and acylimidazole intermediates.…”