Athel wrote:
> Gustavo Parisi asked:
>> "We can find in several biochemistry books, the classical ATP pushed
> "reactions:
>> "A---B DeltaG >0
> "ATP----ADP + pi DeltaG <0
> _______________
> "A + ATP------B + ADP + pi DeltaG <0
>>> I think the usual representation is not quite as written above, but
> more
>> A + Pi = B + water DeltaG >0
> ATP + water = ADP + Pi DeltaG <0
> ______________________
>> A + Pi + ATP + water = ADP + Pi + B + water DeltaG <0
>> where the irrelevant species Pi and water cancel to give
>> A + ATP = ADP + B DeltaG <0
I had started to write a reply along these lines and then I realized
that there was a bit of a gap in my own understanding of these
processes. Certainly, most of the ATP driven reactions in living cells
fall into this category (phosphorylation by ATP rather than by free
phosphate), but what about active transport? As I understand it, it's
also often driven by ATP but the substance transported is not generally
phosphorylated. The transporter is, but the coupling seems somewhat
more indirect in these cases than, say, in the formation of
D-glucose-6-phosphate (in which case Athel's scheme is the correct one).
One furthermore supposes that the transporter is eventually
dephosphorylated so that it can undertake further transport cycles so
that the net reaction looks a lot like Gustavo's scheme if we identify A
as the species outside the cell and B as the same species inside, or
vice versa.
As a physical chemist who often uses biochemical examples in class,
I would be pleased if someone would tell me if I'm completely
out-to-lunch on this, and if so where my reasoning has gone off the
tracks.
Marc R. Roussel
(roussel at henri.chem.uleth.ca)
Department of Chemistry
University of Lethbridge
