From owner-metabolic-reg@net.bio.net Tue Jan 11 22:00:00 1994
Path: biosci!daresbury!not-for-mail
From: prm@aber.ac.uk (Pedro Mendes)
Newsgroups: bionet.metabolic-reg
Subject: Re: sources on various Keq values?
Date: 12 Jan 1994 10:36:48 -0000
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Sender: daemon@mserv1.dl.ac.uk
Distribution: bionet
Message-ID: <2h0js0$j2v@mserv1.dl.ac.uk>
Original-To: btk-mca@dl.ac.uk

>Paul (schlosser@beta.ciit.org) writes:

>A single enzyme is capable of taking a reaction essentially to completion,
>far from thermodynamic equilibrium.  If the enzymes for both a forward and
>reverse reaction are operating simultaneously, then one can expect a 
>steady state, or "dynamic equilibrium" to be achieved, but is this necessarily
>the same as the thermodynamic equilibrium?

 Not necessarily. In fact, the probabiblity of that steady state being the 
thermodynamic equilibrium state is very small (and depends *only* on the 
amount of the fixed substrates and products, the presence of the enzyme only 
affects how *fast* the system relaxes to the steady state).

 Remember that,according to the established chemistry theory, enzymes are 
only able to accelerate reactions (and in both directions!), not alter 
equilibrium constants. (However chemists forget the possibility that sometimes 
reactants are the enzyme-substrate complexes, rather than the substrates 
alone, and in this case the Keq is different because the reaction is different)

>  Won't it be changed by changing
>the relative amounts of the forward and reverse enzyme?  (At the limit when
>one of the two is completely removed, one should return to the essentially
>irreversible case.)

 That is quite right, the only thing you must note is that the state of your 
system when all the concentrations stopped changing is *not* the thermodynamic 
equilibrium, but simply a steady state.

>  And if this is true, how applicable is the thermodynamic equil. constant to 
>computing the relative levels of the compunds *in vivo*?

 It has no applicability at all! Equilibrium constants alone cannot serve for 
those purposes.

Pedro Mendes
prm@aber.ac.uk


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