Covalent binding of C3b to C4b within the classical complement pathway C5 convertase. Determination of amino acid residues involved in ester linkage formation

J Biol Chem. 1992 Feb 25;267(6):4171-6.


C5 convertase of the classical complement pathway is a protein complex consisting of C4b, C2a, and C3b. Within this complex C3b binds to C4b via an ester linkage. We now present evidence that the covalent C3b-binding site on human C4b is Ser at position 1217 of C4. We also show that formation of the covalently linked C4b.C3b complex occurs in the mouse complement system and that the C3b-binding site on mouse C4b is Ser at position 1213 which is homologous to Ser-1217 of human C4. Therefore, covalent binding of C3b to a single specific site on C4b within the classical pathway C5 convertase is likely a common phenomenon in the mammalian complement system. Specific noncovalent association of metastable C3b with C4b would occur first, leading to reaction of the thioester with a specific hydroxy group. This is supported by two lines of experimental evidence, one which shows that a mutant C4 that does not make a covalent linkage with C3b is still capable of forming C5 convertase and a second in which the C4b.C3b complex has been demonstrated by cross-linking erythrocytes bearing this C5 convertase.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Amino Acids / metabolism*
  • Animals
  • Autoradiography
  • Base Sequence
  • Binding Sites
  • Cell Line
  • Complement C3-C5 Convertases / metabolism*
  • Complement C3b / metabolism*
  • Complement C4 / pharmacology
  • Complement C4b / metabolism*
  • Complement Pathway, Classical*
  • Electrophoresis, Polyacrylamide Gel
  • Esters / metabolism
  • Hemolysis / drug effects
  • Humans
  • Mice
  • Molecular Sequence Data
  • Plasmids
  • Precipitin Tests
  • Recombinant Proteins / metabolism
  • Sequence Homology, Nucleic Acid


  • Amino Acids
  • Complement C4
  • Esters
  • Recombinant Proteins
  • Complement C3b
  • Complement C4b
  • Complement C3-C5 Convertases