RE: [HAPS-L] positive feedback - coagulation

[Ken Saladin <ksaladin@xxxxxxxxxx>]

At 03:05 PM 2/13/2007, you wrote:
> 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?
>  


Yes, there is a small positive feedback loop within the overall
process of coagulation, but coagulation on the whole is not a positive
feedback process. Moreover, we can't call it a positive feedback loop
merely because it's a reaction cascade -- which was the point I was
originally responding to. The thrombin-prothrombin loop is really a side
issue, which is why I didn't bring that up.

Ken