| Summary: | The complement system is considered as the chief recognition and effector component of innate immunity; it is involved in inflammation and enhances the adaptive immune response. Factor I (fI) is a heterodimeric serine protease consisting of a heavy (HC) and a light-catalytic (LC) chain; it circulates in an active form regulating complement by selectively cleaving only C3b or C4b in the presence of a cofactor such as factor H (fH), CR1, MCP or C4bp. The cleavage of C3b occurs through a ternary complex formed between fI, C3b and a cofactor like fH and yields iC3b, a major opsonin. The structural and functional properties of fI were investigated. The interchain disulphide bond formed between C<sup>309</sup>-C<sup>435</sup> tnat links the HC and LC of fI as well as the composition of the TV-linked carbohydrates of fI were determined. By using two independent assays, the proteolytic and amidolytic assays, the catalytic properties of human fI were characterised in detail. The catalytic subunit, the SP domain, was shown to have a native conformation that accommodates substrate recognition and cleavage, fI has specificity similar to thrombin, but exhibits lower catalytic activity. fI amidolytic activity reaches optimum at pH 8.25 and is insensitive to ionic strength. This is in contrast to its proteolytic activity within the fI-C3b-fH reaction, in which the pH optimum for C3b cleavage is <5.5 and the reaction rate is highly dependent on ionic strength. The rate of cleavage of tripeptide AMC substrates by fI was unaffected by fH or C3(NH<sub>3</sub>) at optimum pH. fI and the isolated SP domain were found to have similar amidolytic activities, but strikingly different proteolytic activities on C3(NH 3 ). fl did not cleave C3(NH<sub>3</sub>) in the absence of fH, but cleaved it rapidly at two sites in its presence. The SP domain however, cleaved C3(NH<sub>3</sub>) slowly in the absence of fH, at more than two sites. Cleavage by the SP domain was inhibited, not stimulated, by fH. These results suggested that the HC domains and/or the cofactor must orient the natural substrates and restrict cleavage by fI to the two sites which yield iC3b. The implications of these findings are discussed.
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