Existing, qualitative notions with respect to the way in which enzyme properties control metabolism are discussed in the light of the control analysis developed by H. Kacser and J. A. Burns ((1973) in: Rate Control of Biological Processes, Davies DD, ed., Cambridge University Press, pp. 63–104) and R. Heinrich and T. A. Rapoport ((1974) Eur. 3. Biochem. 42, 89–95), and recent experimental data. Points at which the existing notions should be adjusted are: (i) Metabolic control is shared by enzymes rather than confined to one rate-limiting enzyme per pathway. (if) Whether an enzyme exercises strong control on a flux cannot be deduced solely from its own properties, nor is it directly related to its distance from equilibrium. With respect to metabolic control, enzymes should be classified into four groups, rather than two (reversible versus irreversible). (iii) The distribution of control among the enzymes depends on the metabolic conditions. (iv) Control structures of metabolic pathways probably differ with the function of that pathway.

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