Binding reaction between the third human complement protein and small molecules.

The covalent binding reaction of the third complement protein (C3) to receptive surfaces is thought to proceed by the following mechanism. An internal thioester [Tack, B. F., Harrison, R. A., Janatova, J., Thomas, M. L., & Prahl, J. W. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5764-5768; Law, S. K., Lichtenberg, N. A., & Levine, R. P. (1980b) Proc. Natl. Acad. Sci. U.S.A. 77, 7194-7198], which is usually hidden within the C3 molecule, is exposed upon proteolytic activation of C3 to C3b* (the hypothetical conformation of C3b which has the capacity to bind to receptive surfaces and small molecules). The exposed thioester is accessible to attack by hydroxyl groups on receptive surfaces. An acyl transfer reaction takes place, leading to the binding of C3b to the receptive surfaces via an ester linkage [Law, S. K., Lichtenberg, N. A., & Levine, R. P. (1979) J. Immunol. 123, 1388-1394]. We have used a fluid-phase system to demonstrate the specific binding of different small molecules to the labile binding site of C3. The small molecules include glycerol, different hexose monomers, sucrose, raffinose, and four amino acids. These molecules bind to C3b with different efficiencies, indicating that there is an order of preference of C3b* for these molecules. In certain cases, the small molecules bind to C3b via ester linkages (e.g., glucose); in others, the bond is an amide linkage (e.g., lysine). We have also studied the concentration dependence of the binding of small molecules to C3b. The binding is consistent with the following reaction scheme: (Formula: see text).