C3 deficiency

Gene Information


HUGO name: C3


  • Complement component 3
  • Complement C3 precursor


Reference sequences

Chromosomal location


Protein Information


Protein function:

C3 plays a central role in the activation of the complement system. Its processing by C3 convertase is the central reaction in both classical and alternative complement pathways. After activation C3b can bind covalently, via its reactive thiolester, to cell surface carbohydrates or immune aggregates.


C3 precursor is first processed by the removal of 4 Arg residues, forming two chains, beta and alpha, linked by a disulfide bond. C3 convertase activates C3 by cleaving the alpha chain, releasing C3a anaphylatoxin and generating C3b (beta chain + alpha' chain). During pregnancy, C3dg exists as a complex (probably a 2:2:2 heterohexamer) with agt and the proform of prg2.


C3b is rapidly split in two positions by Factor I and a cofactor to form IC3b (inactivated C3b) and C3f which is released.


There are two alleles: C3s (C3 slow), the most common allele in all races and C3f (C3 fast), relatively frequent in caucasoids, less common in black american, extremely rare in orientals.

Structures (PDB)

1C3D X-Ray Crystal Structure Of C3D: A C3 Fragment and Ligand For Complement Receptor 2


Anaphylatoxin-like domain: 693-728

    Properdin-binding domain: 1424-1456

      Other features

      Signal peptide : 1-22

        Complement C3: 23-1663

          Complement C3, beta chain: 23-667

            Complement C3, alpha chain: 672-1663

              Released active peptide C3a anaphylatoxin: 672-748

                C3b alpha' chain: 749-1663

                  Released active peptide C3c fragment: 749-954

                    Released active peptide C3dg fragment: 955-1303

                      Released active peptide C3g fragment: 955-1001

                        Released active peptide C3d fragment: 1002-1303

                          Released active peptide C3f fragment: 1304-1320

                            Disulfide bond interchain: 559-816

                              Disulfide bond : 627-662

                                Disulfide bond : 693-720

                                  Disulfide bond : 694-727

                                    Disulfide bond : 707-728

                                      Disulfide bond : 873-1513

                                        Disulfide bond : 1101-1158

                                          Disulfide bond : 1358-1489

                                            Disulfide bond : 1389-1458

                                              Disulfide bond : 1506-1511

                                                Disulfide bond : 1518-1590

                                                  Disulfide bond : 1537-1661

                                                    Disulfide bond : 1637-1646

                                                      N-linked (glcnac...) glycosylation site: 85

                                                        N-linked (glcnac...) glycosylation site: 939

                                                          N-linked (glcnac...) glycosylation site: 1617

                                                            Other related resourses:


                                                            Expression pattern for human

                                                            Tissue Expression (%) Clones
                                                            gall bladder 24.40 20:2445
                                                            adipose, white adipose 11.13 1:268
                                                            liver 6.58 81:36737
                                                            human spinal cord 6.01 1:496
                                                            nose, olfactory epithelium 5.33 2:1119
                                                            uterus, endometrium 4.98 3:1796
                                                            hepatocellular carcinoma 4.80 23:14298
                                                            corresponding non cancerous liver tissue 3.63 17:13955
                                                            testis, epididymus 3.16 1:945
                                                            adipose, white adipose tissue 3.01 1:990