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Vinson,

As Dale and Randy pointed out, you cannot change the ΔG of a reaction by mutation: enzyme, which is a catalyst, affects only the activation barrier (ΔE "double-dagger").  You can just make it a better (or worse) catalyst which would allow the reaction to flow faster (or slower) towards equilibrium.  Nature solves this problem very elegantly by taking a readily reversible enzyme, like an epimerase or isomerase, and coupling it to a much less reversible reaction which removes product quickly.  Hence, the mass action is only in one direction.  An example of such an arrangement is the triose phosphate isomerase (TIM)-glyceraldehyde 3-phosphate dehydrogenase (GAPDH) reaction pair.  TIM is readily reversible (DHA <=> G3P), but G3P is rapidly converted to 1,3-diphosphoglycerate by GAPDH.   The oxidation and phosphorylation reactions of GAPDH now make TIM "work" in one direction.

Since many epimerases are very optimized enzymes, why not consider making a fusion with a second enzyme (like a reductase) to make the system flow in one direction.  Of course, this depends on what you want to do with the product. 

Cheers,

Michael

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R. Michael Garavito, Ph.D.
Professor of Biochemistry & Molecular Biology
513 Biochemistry Bldg.   
Michigan State University      
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On May 18, 2010, at 11:54 AM, Dale Tronrud wrote:

Hi,

  I'm more of a Fourier coefficient kind of guy, but I thought that a
ΔG of zero simply corresponded to an equilibrium constant of one.  You
can certainly have reversible reactions with other equilibrium constants.
In fact I think "irreversible" reactions are simply ones where the
equilibrium constant is so far to one side that, in practice, the reaction
always goes all the way to product.

  As Randy pointed out the enzyme cannot change the ΔG (or the equilibrium
constant).  You could drive a reaction out of equilibrium by coupling it
to some other reaction which itself is way out of equilibrium (such as
ATP hydrolysis in the cell) but I don't think that's a simple mutation of
your enzyme.  ;-)

Dale Tronrud

On 05/18/10 00:31, Vinson LIANG wrote:
Dear all,

Sorry for this silly biochemistory question.  Thing is that I have a
reversible epimerase and I want to mutate it into an inreversible one.
However, I have been told that the ΔG of a reversible reaction is zero.
Which direction the reaction goes depends only on the concentration of
the substrate.  So the conclusion is,

A: I can mutate the epimerase into an inreversible one. But it has no
influence on the reaction direction, and hence it has little mean.

B: There is no way to change a reversible epimerase into an inversible one.

Could somebody please give me some comment on the two conclution?

Thank you all for your time.

Best,

Vinson