Dear group,
Concerning Silvina's question, I share Athel's view that quasi-equilibrium
reactions have small-to-negligible flux control coefficients. Reactions with
high negative DeltaG values normally operate far from equilibrium. This seems
logical but is not so easy to prove. In fact they could be close to
equilibrium but with high product concentrations, which is certainly
prevented by appropriate means in the living cell (see David's comment).
Taken for granted they are far from equilibrium, these reactions have high
flux control coefficients. Since regulation in the cell should act somewhere,
it is sensible that it acts exactly on these enzymes because it would have no
effect if it acted on quasi-equilibrium reactions. Interestingly, the
existence of feedback loops acting on nearly irreversible reactions lowers
their control coefficients, which is not in contradiction of the above,
though.
A point Reinhart Heinrich often stresses is worth mentioning. Maybe
regulation acts on quasi-equilibrium reactions as well (as a relict from
evolution), but we don't "see" it because their control coefficients are so
small.
A useful reference regarding thermodynamic values is
M.L. Mavrovouniotis
Estimation of standard Gibbs energy changes of biotransformations
J. Biol. Chem. 266 (1991) 14440-14445.
Stefan Schuster (Berlin)
