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I think this is somehow tortured, especially by a quick reading of Dale's explanation. All natural epimerases, isomerase and racemases use a mechanism based on L-amino acids to deal with a mirror-symmetric (quasi-, sometimes) reaction. In another word, these enzymes use a non-mirror symmetric structure to deal with a mirror-symmetric reaction, which itself causes the asymmetric kinetics for different direction, though the dG is 0. The Arrhenius Law k = A*exp(-dE/RT) should be understood like this: a mutation's effect to dE will be symmetric as Dale pointed out. However, the effects on A are asymmetric. A is related to intramolecular diffusion, substrate- and product-binding affinity, etc. That is why with mutation these enzymes changed their kinetics on two directions differently. Please check glutamate racemase, alanine racemase, aspartate racemase, DAPE epimerase, if you are interested. Never a 1000 to 1000 relation! Thus, mutation is possible to make one direction more favored---the point is you need the correct hit. Of course, such an experiment is never a Maxwell's demon. Lijun On May 19, 2010, at 8:51 AM, Maia Cherney wrote: > You absolutely right, I thought about it. > > Maia > > Marius Schmidt wrote: >> Interestingly, Maxwell's demon pops up here, whoooo... , >> don't do it. >> >> >> >> >>> If you change the reaction rate in one direction 1000 times slower >>> than >>> in the other direction, then the reaction becomes practically >>> irreversible. And the system might not be at equilibrium. >>> >>> Maia >>> >>> R. M. Garavito wrote: >>> >>>> 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] >>>> <mailto:[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 >>>>>> >>>>>> >>>>>> >>>>>> >> >> Dr.habil. Marius Schmidt >> Asst. Professor >> University of Wisconsin-Milwaukee >> Department of Physics Room 454 >> 1900 E. Kenwood Blvd. >> Milwaukee, WI 53211 >> >> phone: +1-414-229-4338 >> email: [log in to unmask] >> http://users.physik.tu-muenchen.de/marius/ >> >> >> Lijun Liu Cardiovascular Research Institute University of California, San Francisco 1700 4th Street, Box 2532 San Francisco, CA 94158 Phone: (415)514-2836