From owner-metabolic-reg@net.bio.net Tue Jan 11 22:00:00 1994
Path: biosci!daresbury!not-for-mail
From: SCHLOSSER@ciit.org
Newsgroups: bionet.metabolic-reg
Subject: Re: sources on various Keq values?
Date: 12 Jan 1994 14:20:59 -0000
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Sender: daemon@mserv1.dl.ac.uk
Distribution: bionet
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Original-To: btk-mca@dl.ac.uk

In <9401120304.AA04594@net.bio.net>
mroussel@alchemy.chem.utoronto.ca (Marc Roussel) writes:

>This is only possible if there is a reactant flux.  A closed chemical
>system (with or without enzyme) will always relax to a state of
>thermodynamic equilibrium.  Enzymes cannot alter the equilibrium state.

Yes, but on what time scale?  A mixture of H2 and O2 is not at thermo.
equilibrium, but sans spark the relaxation to equilibrium is not of
practical significance.  I expect that the same would hold for the 
products of many enzymatic reactions.  For example I performed a series of
experiments with yeast in which the the yeast was harvested and placed in
an *anaerobic* medium containing glucose, but no nitrogen source.  The 
glucose was completely consumed (to the limit of detection) being converted
primarily to ethanol and glycerol.  I didn't do the calculations, but this
state is probably far from thermodynamic equilibrium.  Moreover, if I
were to take some of the "spent" broth (or any mixture of ethanol and
glycerol, keeping it sterile at 4C) it would probably take a very long time 
to reach equilibrium.

:-)

So, presumably, the "Keq" that one would want to model a reversible step
(one in which the enzymes for both the forward and reverse reactions are
present at signficant activities) in a metabolic pathway is not 
the true (thermodynamic) Keq, but the ratio of concentrations that exist 
at steady state.  Unfortunately, as Marc pointed out in a previous message, 
values for the steady state ratios are often not available.  If one could 
measure the metabolite levels at one steady state (ie., before changing the 
activities of *other* enzymes in the pathway) then the measured ratio could 
be assumed to be maintained after such a change.

Paul
schlosser@beta.ciit.org


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