With reference to the question why close-to-equilibrium enzymes have
low control, and far from equilibrium steps may have high control over
pathway rates, I am suprised no one has given the obvious answer:
Close-to-equilibrium enzymes have high forward and backward
unidirectional rates compared to the net rate, and thus in general will
have high elasticities to both substrates and products. High elasticites
will in general cause a low control coefficient (depending on elasticities
of all other enzymes in the pathway) as indicated by the connectivity
theorm.
Far-from-equilibrium enzymes have a very low backward rate compared
to the net rate and thus may have zero elasticity to product (although they
may be designed to have non-zero elasticity to product by for example
competion between substrate and product or allosteric inhibition by product).
If the enzyme E has zero elasticity to product then no other enzyme
downstream of
that enzyme can have control, but of course there may be other far-from-
equilibrium enzymes upstream of the enzyme, which means that enzyme E
will have no control eventhough it is far-from-equilibrium.
Is this answer too obvious to bother with?
Dr Guy C. Brown
Department of Biochemistry,
University of Cambridge,
Tennis Court Road,
Cambridge CB2 1QW,
UK
Tel: +44 (0)1223 333342
Fax: +44 (0) 1223 333345
