Hi Nang,
Its me again...
I'm learning biochemistry by the time to do my exams in biology.
But biochemistry is certainly not my 'major'...
>QUESTION: Can we generalize from other available kinase data that the
>reaction shown above is most likely to be irreversible?
I think the reversibility of a given reaction is determined by the
difference in free energy, and the enzyme mechanism which is used.
Or, in terms of Michaelis-Menten: if nor Product-Enzyme-complex can
be formed, u have no backreaktion.
>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.
This is correct: it's not a physiological pathway.
>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?
If I understand u right, u need the Vmax, not the Keq. If u have an
[products]
equation like Keq= (shit)*-------------, and the reaction is irreversible,
[educts]
you have only products, and Keq is 1 (in a steady state).
If u r interested in product rates, u need the Vmax of the
slowest of the enzymes involved. Does Km_ATP or Km_NADP
has anything to do with this?
Sorry, but english is NOT my native language...perhaps I misunderstand u, Nang.
Hope this was at least correct...
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...
-------------------------------------------------
