- Complement component 3
Complement component 3
PDB rendering based on 1c3d.
Available structures PDB , , , , , , , , , , , , , , , , , , , Identifiers Symbols External IDs GeneCards: EC number Gene Ontology Molecular function •
Cellular component •
Biological process •
Sources: Amigo / QuickGO Orthologs Species Human Mouse Entrez Ensembl UniProt RefSeq (mRNA) RefSeq (protein) Location (UCSC) PubMed search
Complement component 3, often simply called C3, is a protein of the immune system. It plays a central role in the complement system and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.
C3 plays a central role in the activation of complement system. Its activation is required for both classical and alternative complement activation pathways. People with C3 deficiency are susceptible to bacterial infection.
One form of C3-convertase, also known as C4b2a, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of C3 into C3a and C3b, generated during activation through the classical pathway as well as the mannan-binding lectin pathway. C3a is an anaphylotoxin and the precusor of some cytokines such as ASP, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by C3bBb, another form of C3-convertase composed of activated forms of C3 (C3b) and factor B (Bb). Once C3 is activated to C3b, it exposes a reactive thioester that allows the peptide to covalently attach to any surface that can provide a nucleophile such as a primary amine or a hydroxyl group. Activated C3 can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex, C3bBb, is called the alternative pathway (AP) C3 convertase.
C3bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by complement regulatory proteins having decay accelerating factor (DAF) activity. Next, C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d. Factor I is the protease that performs these cuts but it requires the help of another protein to supply what is termed cofactor activity.
Complement tests C4 (C) FB (A) C3 CH50 Conditions · ↓ ↓ ↓ PSG, C3 NeF AA ↓ · ↓ · HA, C4D · · · ↓ TCPD ↓ · /↓ ↓ ↓ SLE ↑ ↑ ↑ ↑ inflammation
Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with some types of kidney disease such as post-infectious glomerulonephritis and shunt nephritis.
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- GeneReviews/NCBI/NIH/UW entry on Atypical Hemolytic-Uremic Syndrome
- OMIM entries on Atypical Hemolytic-Uremic Syndrome
- GeneReviews/NCBI/NIH/UW entry on Dense Deposit Disease/Membranoproliferative Glomerulonephritis Type II
- MeSH Complement+C3
Proteins: complement system (C, L, A) Activators/enzymesEarlyMiddleLate Inhibitors Complement receptors Amyloid Other positive Negative
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