Sounds like a good explanation. Thank you.
Maia
Dale Tronrud wrote:
If you change the reaction rate in one direction 1000 times slower then
the reaction rate in the other direction will also be 1000 times slower
and the equilibrium will be in exactly the same place. You can't make
the transition state less stable when approached "from the left" without
making it less stable when approached "from the right".
Dale Tronrud
On 05/18/10 12:34, Maia Cherney wrote:
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
