Current topics in cellular regulation. Vol. 25 by Bernard L. Horecker, Earl R. Stadtman

By Bernard L. Horecker, Earl R. Stadtman

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2), \Sx) overall = (5x)a + (5X/b These two rules form the basis of much of the subsequent analysis and enable overall sensitivities of complex systems to be derived quite easily (see below). 2. INFLUENCE OF KINETIC RESPONSE OF ENZYMES The fundamental communication involved in almost all metabolic con­ trol is that between a small molecule and a protein, usually an enzyme. The small molecule may be the substrate, cofactor, product, or an allosteric activator or inhibitor of the enzyme. Let us consider the response of an enzyme to its substrate, S, in the following reaction, S—>P If this proceeds via simple equilibrium binding of S to enzyme, the rate of the reaction, v, is described by the Michaelis-Menten equation, v = Vm[S]/(Km + [S}) From Eq.

The effect of a near-equilibrium reaction in a substrate cycle can be included when calculating the sensitivity of each route by the product rule. Thus, if RF denotes the reversibility of F, s{ (s) , via interaction with F, = r ^ R F ( l + C/J) assuming that there is no catalystic component of rjs). , when 39 METABOLIC CONTROL C. Use of Intrinsic Sensitivities as "Building Blocks" for Calculating Net Sensitivities Intrinsic sensitivities, derived from the sensitivities of elementary communications, can be used to derive the overall (net) sensitivity of a system, reaction or flux to an effector in situ (12).

However, before this is done it is necessary to derive two important rules for combining metabolic sensitivities. 1. /A r el)(A r e l /X r e l ) = sls»As£ (8) Thus, for such a consecutive ("head-to-tail") mechanism the overall sen­ sitivity is the product of the sensitivities of the component steps. e. a Ί the overall communication is degenerate and the net sensitivity is the 32 B. CRABTREE AND E. A. NEWSHOLME TABLE I SENSITIVITIES O F ELEMENTARY C O M M U N I C A T I O N S 0 Elementary communication Effector, X, Effector, X,- Sensitivity , forward process r reverse process rx(=dv/dX-X/v) rx(=dV/dX-X/v) Forward process action net rate of re- R Reverse process tion net rate ofreac- -(R-l) Rate of forward reaction (]) of substrate cycle rnet flux Rate of reverse reaction of substrate cycle >net flux U + c/j) -c/j Comments** r c(x) = r (x) — rm(x); derived in this article.

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