In article <3366A28D.5953 at ix.netcom.com>, Bob Phair
<rphair at ix.netcom.com> writes
Work pressure prevented me from looking lately at the newsgroup but I
was pleasantly surprised to find today lots of interesting comment. I
realise a fair number of people half already commented on this but
here's another oar. I'm a bit rusty (not by choice I might add) but....
>>small-to-negligible flux control coefficients, but that Kacser always
>argued that one has no justification for assuming that quasi-equilibrium
>steps have very low flux-control coefficients.
>>I probably don't yet completely understand the terminology, but if a
>flux control coefficient is the same as Kacser's Sensitivity
>coefficient, Z, or even if it's the same as his response, R, (=Z*kappa),
Yes, Z is by today's terminology the flux control Coefficient, C^J_E.
>snip
>be rearranged in the form Z1:Z2:Z3 :: 1-p1:p1(1-p2):p1p2(1-p3), where
>the pi (rho-sub-i) are the mass-action ratios for each step divided by
>the equilibrium constant for that step.
>>This means, as Kacser is quick to point out, that for any step close to
>equilibrium, pi will be nearly 1 and the corresponding Z will be zero.
>Since R=kappa*Z, this is true of R as well.
What you say is correct. The only thing to point out is that this result
is limited to straight-chain pathways where the enzymes are working
below saturation so that the behaviour of the elasticities is dictated
by the degree of reaction equilibrium. It's only in this case you can
derive the nice equation you give above. The result is nevertheless
interesting and as Guy points out later on, when one thinks about it
operationally it's reasonably clear why it is the case. So without
enzyme saturation and in a linear chain the closer a step is to
equilibrium the smaller the flux control coefficient, no doubt about it.
The problem is that this result is not generally true and in pathways
where saturation occurs it can fall down. In fact it is relatively
straightforward to construct linear chains where the step closest to
equilibrium has the *greatest* flux control coefficient; but do these
situations occurs in real systems? This is a good example I think of
how theory can contribute to biochemical science, the theory suggests
some sort of behaviour to which the experimentalists can investigate,
isn't this how a lot of physics is done these days (or has always been
done like this)?
>>I've only been part of this newsgroup for a short while, but I already
>feel I should have been here years ago. Perhaps someone will educate me
>still further, but I found Kacser eye-opening and profound.
If only more could think like you.
>>>Regards,
>Bob
>--
Herbert M Sauro
email: HSauro at fssc.demon.co.uk
Telephone: 01974 282428
-----------------------------------------------------------------------
"He who cannot draw on 3000 years is living from hand to mouth" Goethe
"Ah, but a man's reach should exceed his grasp, or what's a heaven for?"
R Browning.
