title: Amino acid sequence around the thiol and reactive acyl groups of human complement component C4.
abstract: Activation of the fourth component of complement (C4) by C1s results in the generation of a reactive acyl group, able to react with putrescine, and in the release of a free thiol group that cannot be detected in the native haemolytically active molecule. Both the reactive acyl group and the free thiol group have been shown to reside in C4d, a fragment of the alpha'-chain of C4b derived from digestion of the molecule with the control proteins C3b inactivator and C4-binding protein. Peptides derived from CNBr digestion of [1,4-14C]putrescine-labelled and iodo(2-14C]acetic acid-labelled C4d have been obtained and used to establish a continuous sequence of 88 residues from the N-terminus of the molecule. The thiol and reactive acyl groups are contained in an octapeptide that shows near identity with the equivalent sequences reported for alpha 2-macroglobulin and C3. Other adjacent short sections also show homology of sequence between the three proteins, and it is highly likely that they contribute to the overall structure that gives a unique reactivity to the thiol ester bond postulated to exist in the native forms of the three proteins.
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D003166:Complement Activating Enzymes / Q000378:metabolism; D003167:Complement Activation; D003173:Complement C1s; D003181:Complement C4 / Q000378:metabolism; D003488:Cyanogen Bromide / Q000378:metabolism; D006801:Humans; D010446:Peptide Fragments / Q000032:analysis; D011700:Putrescine / Q000378:metabolism; D013438:Sulfhydryl Compounds / Q000378:metabolism; D014357:Trypsin / Q000378:metabolism
chemical_list: D003181:Complement C4; D010446:Peptide Fragments; D013438:Sulfhydryl Compounds; D003166:Complement Activating Enzymes; D014357:Trypsin; D003173:Complement C1s; D003488:Cyanogen Bromide; D011700:Putrescine