Initiation of complement activation.

The complement system is an excellent one to look at as an example of a triggered enzyme system. It is found to use almost all the strategies of enzyme cascades and for initiation which were discussed in the introduction. The classical pathway is a fine example of an active zymogen form of activation and the alternative pathway is the canonical example of a tickover. Both may also be activated by exogenous enzymes from other systems. The strategies of enzymes waiting for substrates, of substrates waiting for enzymes, and of both waiting for modifying proteins are all seen in various stages of the reaction. There is an association of a linear cascade with the positive feedback amplification loop. This degree of evolutionary adaptation is not only aesthetically pleasing but must be taken to mean that the system is of considerable biological importance and has been over a long evolutionary time span.