Autocatalytic activation of C1r subcomponent of the first component of human complement.
Autoactivation of the proenzyme form of a subunit of the first component (C1r) was performed in the presence and absence of diisopropyl fluorophosphate (DFP). The time-course of autoactivation of zymogen C1r followed a sigmoidal curve and was accelerated by addition of the enzyme C1r and by increasing the concentration of C1r, suggesting that autoactivation of C1r consists of two intermolecular reactions, i.e. zymogen(C1r)- and enzyme(C1r)-catalyzed reactions. In the presence of 10 mM DFP, the enzyme-catalyzed autoactivation of C1r was completely inhibited, while the zymogen-catalyzed autoactivation still proceeded depending upon C1r concentration. These results suggested that the zymogen-catalyzed autoactivation of C1r is a DFP-insensitive second-order reaction and is mediated by an active site generated in a single chain C1r through a conformational change (Kassahara et al. (1982) FEBS lett. 141, 128-131). Based on these results, a possible reaction process of autoactivation of C1r was proposed, as follows: (formula; see text) where C1r represents a conformational isomer which catalyzes the autoactivation of C1r, and the rate constants, k2 and k3, are of second-order. Utilizing a computer, we simulated the autoactivation of C1r and found the above scheme to be a reasonable model of C1r autoactivation. Evidence which supports the formation of a conformational isomer of C1r, C1r, as an intermediate in its autoactivation was also obtained by a surface radiolabeling method.
Kasahara
Y
Y
Odai
H
H
Takahashi
K
K
Nagasawa
S
S
Koyama
J
J
D003166:Complement Activating Enzymes / Q000378:metabolism*; D015923:Complement C1r; D003201:Computers; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D007531:Isoflurophate / Q000494:pharmacology; D007536:Isomerism; D007700:Kinetics; D011487:Protein Conformation
D007531:Isoflurophate; D003166:Complement Activating Enzymes; D015923:Complement C1r