Acerenza L
Sección Biofisica, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay.
J Theor Biol. 1996 Oct 7;182(3):277-83. doi: 10.1006/jtbi.1996.0165.
The response of metabolic variables to small rate changes is quantitatively described by the control coefficients (Kacser & Burns, 1973; Heinrich & Rapoport, 1974). Owing to the existence of structural and kinetic constraints in metabolism, the control coefficients are not all independent. Two quantities are defined to evaluate how constrained metabolic control is: the fraction of independent control coefficients (fw) and the number of independent control coeffcients per independent variable [N(C/V)]. It is shown that fw can be expressed in terms of the fraction of independent metabolite concentrations, the fraction of independent fluxes and the average fraction of independent metabolites affecting each rate. N(C/V) is equal to the average number of metabolite concentrations affecting each rate. The estimation of these quantities using experimental information available leads to the following conclusions concerning cellular metabolism: (i) only a small fraction of the control coefficients are independent; (ii) the number of rates (in average) that independently controls each independent variable is much smaller than its theoretical maximum; and (iii) the kinetic constraints are the main cause of the low value showed by fw and N(C/V). Finally, some arguments are given that could explain why living organisms do not evolve to less constrained metabolic responses.
代谢变量对小速率变化的响应由控制系数定量描述（Kacser和Burns，1973；Heinrich和Rapoport，1974）。由于代谢中存在结构和动力学限制，控制系数并非全部独立。定义了两个量来评估代谢控制的受限程度：独立控制系数的比例（fw）和每个独立变量的独立控制系数数量[N(C/V)]。结果表明，fw可以用独立代谢物浓度的比例、独立通量的比例以及影响每个速率的独立代谢物的平均比例来表示。N(C/V)等于影响每个速率的代谢物浓度的平均数量。利用现有实验信息对这些量进行估计，得出了关于细胞代谢的以下结论：（i）只有一小部分控制系数是独立的；（ii）平均而言，独立控制每个独立变量的速率数量远小于其理论最大值；（iii）动力学限制是fw和N(C/V)显示低值的主要原因。最后，给出了一些论据，这些论据可以解释为什么生物体没有进化出限制较少的代谢反应。
