Re: [HAPS-L] End-product inhibition

["John Cornell" <jccornell@xxxxxxxxx>]

To all,
The behavior of the output of the "end-product inhibition" system is certainly similar to what one would expect from a negative feedback system, but I agree with Margaret that it lacks elements that we usually consider when we discuss a control system.  I have pondered over this question before and I can see it several ways.  But,  if we consider "end-product inhibition" to be negative feedback then we must also consider chain reactions and cascades to be positive feedback -- something I am also a little uncomfortable with. One problem is that we risk increasing the complexity of the descriptions  and start losing our students on the fine points of the difference.  

Here is something that I tell my students: negative and positive feedback systems are very similar to each other in design--If we were to consider an electrical circuit, for example, we could turn a negative feedback circuit into a positive feedback circuit by simply reversing two wires.  The example I give is dual control electric blanket with the controls in normal and reversed positions.

John Cornell

On 2/12/07, Weck, Margaret <mweck@xxxxxxxxxx> wrote:

But isn't this at a lower level of organization than the systems we usually refer to as "control systems"?  I would say end-product inhibition is negative feedback, but it isn't an example of a "control system" because it is happening at the molecular rather than cellular level.

What do others think?

Margaret W.

 

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From: HAPS-L-owner@xxxxxxxxxxx [mailto: HAPS-L-owner@xxxxxxxxxxx] On Behalf Of James S. Miller
Sent: Monday, February 12, 2007 9:19 AM
To: HAPS-L@xxxxxxxxxxx
Subject: Re: [HAPS-L] End-product inhibition

 

Biochemistry is full of metabolic pathways where the end product, or even sometimes intermediates, inhibits the first or second enzyme in the pathway.  This was always considered to be negative feedback in every text I've ever seen.  So, if I understand you correctly, the answer is yes.  It's a negative feedback homeostatic mechanism.

Harbut, Chester wrote:

On a recent essay exam I asked my students to give me an example of negative feedback.  Most used
parameters such as glucose concentration, temperature, even calcium concentrtion.  Two students however
used "end-product inhibition" of enzymes (with specific examples).  I''m just curious, do most of you
consider this negative feedback in the classical sense?