Native human C3 is a naturally glycosylated (~2.7%) polypeptide containing two disulfide-linked chains. C3 is central to the activation of all three pathways of complement activation (Law, S.K.A. and Reid, K.B.M. (1995)). Initiation of each pathway generates proteolytic enzyme complexes (C3 convertases) which are bound to the target surface. These enzymes cleave a peptide bond in C3 releasing the anaphylatoxin C3a and activating C3b. For a brief time (~60 µs) this nascent C3b is capable of reacting with and covalently coupling to hydroxyl groups on the target surface. Carbohydrates are the favored target, but protein hydroxyls and amino groups also react. This process of tagging the target surface with C3b is called opsonization. The reactive site in nascent C3b is a thioester (Tack B.J., et al. (1980); Pangburn M.K. and MüllerEberhard H.J. (1980)) and C3b is linked to the target through a covalent ester bond (an amide bond is formed if C3b is attached to amino groups). Most of the C3 activated during complement activation never attaches to the surface because its thioester reacts with water forming fluid phase C3b which is rapidly inactivated by factors H and I forming iC3b. Surface-bound C3b is necessary in all three pathways for efficient activation of C5 and formation of C5b-9 complexes that lyse the target cell membrane. Surface-bound C3b and its breakdown products iC3b and C3d are recognized by numerous receptors on lymphoid and phagocytic cells which use the C3b ligand to stimulate antigen presentation to cells of the adaptive immune system. The end result is an expansion of target-specific B-cell and T-cell populations.

Sizes Available: 250 µg/vial

Concentration: 1.0 mg/mL (see Certificate of Analysis for actual concentration)

Form: Frozen liquid

Activity: >70% versus normal human serum standard (see Cert of Analysis).

Purity: >95% by SDS-PAGE

Buffer: 10 mM sodium phosphate, 145 mM NaCl, pH 7.2

Extinction Coeff. A_{280 nm} = 1.03 at 1.0 mg/mL

Molecular Weight: 185,000 Da (2 chains)

Preservative: None, 0.22 µm filtered

Source: Normal human serum (shown by certified tests to be negative for HBsAg, HTLV-I/II, STS, and for antibodies to HCV, HIV-1 and HIV-II).

Precautions: Use normal precautions for handling human blood products.

Molecular weight: 185,000 daltons composed of two disulfide linked chains. The alpha chain is 110,000 daltons (contains C3a and C3d domains) and the beta chain is 75,000 daltons. Alpha and beta chains are linked through a single disulfide bond. The pI of C3 is approx. 5.9 Upon cleavage of C3 by C3 convertases, C3a (77 amino acid fragment, 9083 Da) is released from the N-terminal of the alpha chain and C3b (176,000 Da) becomes attached covalently to the surface of the activator. The crystal-derived structures of both C3 and C3b have been described (Gros, P. (2008)) and these show that large conformational changes occur in the C3b portion of C3 following cleavage of the C3a-C3b peptide bond. Native C3 and C4 circulate in plasma with intramolecular thioester bonds linking a glycine and a glutamine residue in their C3d or C4d domains. These thioester bonds are susceptible to nuleophilic attack by amines such as ammonia, methylamine, hydroxylamine and hydrazine, all of which have been used to inactivate complement in serum.

CAS Number: 80295-41-6

MDL Number: MFCD00130836

The source of this protein is human serum, therefore precautions appropriate for handling any blood-derived product must be used even though the source was shown by certified tests to be negative for HBsAg, HTLV-I/II, STS, and for antibodies to HCV, HIV-1 and HIV-II.

-70℃ or below. Avoid freeze/thaw (>10 % activity loss per thaw).

C3 is essential for effective complement activation and subsequent presentation of antigens to the cells of the adaptive immune system (Lambris, J.D. (1988)). Following recognition of a target complement is activated by one of the three complement pathways and enzymes (C3 convertases) are formed on the target’s surface. These enzymes (C4b,C2a or C3b,Bb) cleave C3 after Arg 77 of the alpha chain releasing the anaphylatoxin C3a and depositing C3b on the target surface. Although there is a very weak C3 bypass system that operates through the classical and lectin pathways (C4b,C2a can activate C5 without C3b at about 1/2000 the rate of C3b,C4b,C2a), C3b is necessary for effective C5 activation (Rawal N. and Pangburn M.K. (2003)).