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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 **************************************************************** R. Michael Garavito, Ph.D. Professor of Biochemistry & Molecular Biology 513 Biochemistry Bldg. Michigan State University East Lansing, MI 48824-1319 Office: (517) 355-9724 Lab: (517) 353-9125 FAX: (517) 353-9334 Email: [log in to unmask] **************************************************************** 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 >> >> >>