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Dear Vinson, I would agree with you on choice B. There are probably many ways to look at it. Here are two that come to me at the moment. 1. If the reaction is reversible, then there's no opportunity to put energy into the system to reduce its overall entropy. So a reversible epimerase would be like a Maxwell's demon, violating the second law of thermodynamics. 2. Reversible reactions obey the principle of microscopic reversibility, i.e. the reaction mechanism and the transition states are the same in both directions. There's no way for an enzyme to selectively reduce the transition state barrier going in one direction but not the other. Regards, Randy Read On 18 May 2010, at 08:31, 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 > > ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: [log in to unmask] Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk