The "working assumption" that the (physiological) Km's of the enzymes tend
to match the physiologial concentrations of substrates (S) is used in many
works on metabolic regulation. Yet, I never found a systematic survey
relating metabolite concentrations to the Km's of the enzymes catalysing
their consumption. Can anyone point me such a survey? (I mean something
similar to what Albe et al (1990) did for the ratios between enzyme and
metabolite concentrations.)
Cleland (1967) argues that S should approach the Km of the enzymes because
much lower S's waste the catalytic potential of the enzyme and much higher
S's would not ensure an adequate homeostasis.
These appear sound arguments. However it is not clear why, in many
circumstances, some catalytic potential should not be "wasted" to improve
homeostasis of metabolite concentrations. (The insensitivity of metabolite
concentrations to system parameters is an usual performance criteria in
BST's studies of metabolic design.)
So, the working assumption of S=Km should be (and probaly is) substantiated
by a systematic survey, even though such survey would meet the difficulties
found by Albe et al (1990).
Albe, K.R., Butler, M.H. and Wright, B.E. (1990), "Cellular
concentrations of enzymes and their substrates.", J.Theor.Biol. 143,
163-195
Cleland, (1967), "Enzyme kinetics", Ann. Rev. Biochem., ..., 77-112
Armindo Salvador
Grupo de Bioquimica e Biologia Teoricas - Instituto Rocha Cabral
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