The molecular mechanisms of the interaction between complement proteases and substrates.
2017-03-22T01:44:13Z (GMT) by
MASP-2 and C1s are serine proteases of the lectin and classical complement pathways, respectively. As executioners of complement, their role in cleaving their substrates C4 and C2 is imperative to both innate and adaptive immunity, resulting in the formation of the key complement C3 convertase, C4bC2a. The dysregulated activation of these complement pathways has been shown to contribute to a number of diseases, thus there is the need to develop therapeutic molecules to control such proteases. Knowledge of the precise molecular mechanism whereby the proteases interact with their substrates will be key to the development of such therapeautic molecules. The proteases have similar domain structures and share 38% sequence identity, but the molecular interaction of the two proteases with their substrate C4 is quite unique. It has been shown in previous studies that MASP-2 cleaves C4 with high efficiency, while in comparison, C1s is significantly slower, with many studies indicating that the CCP domains of MASP-2 and C1s play a role in these differences. It has been suggested that the CCP domains are the location of an exosite, a site distant from the active site, which assists in correct conformational placement of the substrate C4 for efficient cleavage. This study has investigated the contribution of each domain of the catalytic region of the enzymes, CCP1, CCP2 and SP, to the binding and cleavage of C4. The work was carried out using a range of binding and cleavage assays, including measurement of affinity using an ELISA, catalytic efficiency by determining EC50 values, as well as kinetic assays and analytical affinity chromatography.