This continues the discussion with Denise Dagnino, e-mailing as Jan Schripsema.
Denise:
A reply to your BTK message of 6/6/96.
The elasticities I mention in my first paragraph refer to overall or group
elasticities, i.e. the combined elasticity of all the enzymes that use your
intermediate, or that produce it, when it is not fixed. [The overall
elasticity is the sum of the individual elasticities of all the enzymes to
loganin, weighted by the flux control coefficients of the enzymes over your
end-product production rate].
Intermediates become parameters if you draw the system around them to
exclude them, but remain as variables if you draw the system to include
them. The critical test is: who has control over their concentration? If
you do, they are parameters, if the system does, they are system variables.
The best review that discusses double modulation is the one in Biochem. J.
by David Fell a couple of years ago.
"I did not really understand why I can't conclude that steps in a pathway
have low flux control when addition of their substrate leads to increases
in flux if I know the enzyme amounts don't change".
Well, imagine a pathway of one single enzyme that converts loganin to end
product, in which the enzyme therefore has a control coefficient of 1.0.
Now alter the concentration of loganin. The rate increases, maybe even
proportionally (or maybe it doesn't change at all), but this certainly does
not tell you that the single enzyme has a flux control coefficient of zero,
or even a low flux control coefficient, even if the concentration of the
enzyme is not allowed to change. Does this help?
Martin Brand
Martin D. Brand
Department of Biochemistry, University of Cambridge
Tennis Court Road, Cambridge CB2 1QW UK
phone: +44 1223 333648, fax: +44 1223 333345
Internet: http://www.bio.cam.ac.uk/dept/biochem/brand/
