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: rmgarav...@gmail.com >> <mailto:garav...@gmail.com>/ >> /****************************************************************/ >> >> >> >> 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: m-schm...@uwm.edu http://users.physik.tu-muenchen.de/marius/
