But, if we consider "end-product inhibition" to be
negative feedback then we must also consider chain reactions and cascades to be
positive feedback
No we don't, because positive feedback means the end-result or end-product
feeds back and accelerates the change that initiated the process. That's not so
in cascades. For example, an activated kinase does not (to my knowledge) feed
back and accelerate the activation of adenylate cyclase in the cAMP
second-messenger system. Fibrin does not feed back and accelerate the
activation of factor XII in blood clotting. These cascades certainly have an
amplification effect (enzyme amplification), but they lack the
self-accelerating feedback aspect.
Ken
[Weck, Margaret] But doesn’t active thrombin also activate factors V, VIII and
conversion of more prothrombin to thrombin? So as long as it’s not
the fibrin that is doing the amplification, it isn’t positive feedback?