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
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/Professor of Biochemistry & Molecular Biology/
/513 Biochemistry Bldg. /
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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
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/
Lijun Liu
Cardiovascular Research Institute
University of California, San Francisco
1700 4th Street, Box 2532
San Francisco, CA 94158
Phone: (415)514-2836
