Re: [HAPS-L] positive feedback

Re: [HAPS-L] positive feedback - coagulation

Subject: Re: [HAPS-L] positive feedback - coagulation

From: "John Cornell" <jccornell@xxxxxxxxx>

Date: Tue, 13 Feb 2007 18:45:46 -0600

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But, if we consider "end-product inhibition" to be negative feedback then we must also consider chain reactions and cascades to be positive feedback" statement, After more careful thought I believe that "end product inhibition" is a form of negative feedback where the control functions are performed by the enzyme at a molecular level.

As for chain reactions and cascade reactions, in my opinion these are not positive feedback although they may behave like one. I think I could argue that coagulation involves a cascade where thrombin feeds back positively to factor VIII.
John

On 2/13/07, Weck, Margaret <mweck@xxxxxxxxxx> 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?