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sources on various Keq values?

Hartmut Schütze schuetze at mailer.uni-marburg.de
Tue Feb 1 14:35:34 EST 1994

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 

equation like Keq= (shit)*-------------, and the reaction is irreversible, 

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 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...

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