About Gibbs energies values of biochemicals: I know Prof. Robert A. Alberty has done a lot of
work on that. He has a recent paper in Arch. Biochem. Biophysics (1998) 353, 116-130.
"Calculation of standard transformed Gibbs energies and standard transformed enthalpies of
biochemical reactants." Although trp is not on the list there, I suspect some of the other papers
cited in it will have relevant data.
I agree that many experiments done to determine thermodynamic properties of biomolecules are
not at the exact T and P (or concentrations) of cellular processes. However all is not lost - there are
accurate equation of states (e.g. HKF, Plyasunov's...) available that can account of wide range of
temperatures and pressures. And knowing the activity coefficients, free energy values deviated
from standard state concentrations can also be accounted for.
It's true that kinetics is important in biochemical reactions. But the reaction must first have the
thermodynamic drive to proceed. If the Gibbs free energy is positive (i.e. unfavorable going from
left to right), there is no amount of rate constant that can make the reaction to go from left to right.
Gavin
Petr Kuzmic wrote:
>dbk at aber.ac.uk ("Douglas Kell") wrote in message news:<3BB4AAA7.8790.DF1D392A at localhost>...
> > We are trying to get hold of delta-G (or K-eq) values for common
> > biochemical reactions to see which are <ahem> 'irreversible'. There
> > is a special interest in trp biosynthesis right now. Does anyone know
> > where these can easily be got?
> >
> > Many thanks,
> > Douglas.
>> Hi,
>> Unfortunately I do not have a ready list of equilibrium constants. I
> do wonder, though, how justified it would be to take {\Delta}G or
> K_{eq} values obtained in the test tube and use them to understand
> cellular processes. It seems that, in certain documented cases,
> thermodynamic equilibria for component enzyme reactions are not useful
> at all. The cellular processes seem to be under kinetic control, not
> under thermodynamic control.
>> For example, consider the equilibrium constants for the binding of
> zinc to various proteins. Zinc dissociation constants for zinc enzymes
> typically range from only 10-6 to 10-12 M. Based on the total amount
> of zinc in cells, these values of would suggest that there is a large
> pool of free (unbound) zinc in the cytoplasm.
>> However, it is now known that the cells contain relatively large
> amounts of zinc transport proteins with binding constant in the
> femtomolar range (10-15 M). To quote from the article attached
> below:
>> "[The] results contradict the long-held assumption that pools of free
> zinc--and of other transition metals, such as copper--exist in cells.
> [...] [Zinc]--and other transition metals--are always held tightly by
> biopolymers and [...] their trafficking within cells must be
> kinetically controlled by metalloregulatory proteins and perhaps by
> other factors, such as metal chaperones. These are proteins that can
> mobilize bound metal and conduct it to sites where it is required.
> [...] It appears that zinc does not necessarily end up in the tightest
> binding sites, as would be the case with simple thermodynamic
> control."
>> Response, anyone?
>> - Petr
>> ---------------------------------------------------
> P e t r K u z m i c, Ph. D. * BioKin Ltd.
> 1652 S. Grand Ave. * Suite 337 * Pullman, WA 99163
> 509.334.4131 * 509.332.3493 fax * http://www.biokin.com>>
