An improved fluorogenic substrate for the alternative complement pathway C3/5 converting enzyme CVFBb.

By lengthening the sequence of a previously available tripeptide to a pentapeptide, we were able to increase the specificity of the substrate for the complement enzyme CVFBb (EC 3.4.21.47) over Factor Xa (EC 3.4.21.6). This increase in specificity was achieved by both an increase in the kcat/Km for CVFBb for the longer substrate compared to the original substrate, and a decrease in the kcat/Km for Factor Xa. The new substrate, Boc-Nle-Gln-Leu-Gly-Arg-amino methyl coumarin (AMC) was synthesized by coupling Boc-Nle-Gln-Leu-Gly to Arg-AMC in solution. p-Toluenesulfonic acid was added to the coupling mixture to improve the solubility of the arginine derivative and avoid the need for covalent protection.