Hello again, everyone:
In article <schuetze.61.2D3C3BBF at mailer.uni-marburg.de>,
schuetze at mailer.uni-marburg.de (Hartmut Schtze) wrote:
> Hi Ing-Nang,
> >1) gluconate + ATP -----> 6-phosphogluconate(6PG) + ADP phosphorylation
> this one starts the Glycolysis and is totally irreversible. If u want to
> undo it, u need another enzyme: gluconate-6-phosphatase.
> So the equilibrium is somewhere very near to 6PG...
QUESTION: Can we generalize from other available kinase data that the
reaction shown above is most likely to be irreversible?
> >2) 6PG -----> 2-keto-3-deoxy-phosphogluconate + H20 dehydration
> don't know this one, but i'll have a look..
This reaction is basically irrversible. For the mechanism involved with
the formation and ketonization of an enol.
see:
Kovachevich, R., and Wood, W. A. (1955) JBC 213: 745-756
Meloche, H. P., and Wood, W. A. (1964) JBC 239: 3505-3510
> >3) 6PG + NADP ------> ribulose 5-phosphate + CO2 + NADPH
> again, u picked an irreversible reaction. I do not remember the very special
> step which is irreversible, but it's the same as 1), i think.
As far as I know, this reaction is not as irreversible as we think. I
remember there is one experiment being done on this enzyme. The author(s)
forced the reverse reaction by jacking up high concentrations of ribulose
5-P + NADPH + CO2. I can't find the reference right now. But I know it
has been done. I was kind of surprised at the time. However, the
concentration may be too high to be any significance in vivo.
<<.....>>
> Hartmut
> -------------------------------------------------
> Hartmut Schuetze always
> e-mail: schuetze at mailer.uni-marburg.de look
> snail : H.Schuetze Chemnitzer Str.47 on the
> 35039 Marburg / Germany bright
> tube : germany 06421/43365 :-) side...
> -------------------------------------------------
The reason I send out the first query about the sources on various Keq
values is because I need those values to plug into enzyme kinetic rate
equations (OR DO I NEED TO ?). At least 2 out of 3 reactions mentioned
above are presumably irreversible, possibly all 3 of them. Also at the
same time, I know from personal experience that both Km_ATP for the kinase
and Km_NADP for the dehydrogenase are very small. That is, inside an E.
coli cell both enzymes are likely to be saturated by these two cofactors (
assuming we adopt the published in vivo ATP and NADP concentration for E.
coli). Since this is a branched pathway we are talking about, can we just
simplify the potentially ugly looking explicit steady state rate equation
into a simpler one? That is, using Michaelis-Menten equation to
approximate each branch. The assumptions are :1) the reactiona are
irrversible, and 2) the cofactors are saturating in vivo. To add a new
twist on this. By adopting a simplified version of the steady state
equation for the branched pathway I just mentioned, we are implying that
the kinase step will determined the steady state flux, which, again from
personal experience, I know is not true. So in a sense, I'm stuck. Any
help or suggestion on this?
--
Ing-Nang Wang
ingnang at sbbiovm.sunysb.edu
