Amino acid sequence around the proposed thiolester bond of human complement component C4 and comparison with the corresponding sequences from C3 and α2-macroglobulin

The amino acid sequence around the proposed thiolester bond in human complement component C4 has been determined. The sulphydryl and reactive acyl groups involved in the intrachain thiolester bond are contained in a short section of sequence which shows almost complete identity with the equivalent sequence from alpha 2-macroglobulin and C3. Other regions of homology between the three proteins near the proposed thiolester bond are also apparent.