Holms W H
Department of Biochemistry, University of Glasgow, Scotland, U. K.
Biochem Soc Symp. 1987;54:17-31.
The glyoxylate bypass and citric acid cycle operate concurrently in Escherichia coli when acetate is the sole source of carbon and energy to sustain aerobic growth. The overall carbon balance allows fluxes through the central metabolic pathways (CMPs) to be computed on the assumption that these metabolic pathways are known. Acetate is fluxed via the CMPs to the precursors required for synthesis of new biomass and also to generate the reducing power and ATP required to convert these precursors to biomass. Under these circumstances, a junction is created at isocitrate where isocitrate lyase (ICL) and isocitrate dehydrogenase (ICDH) compete for their common substrate. In general, flux through ICL generates the precursors used for biosynthesis while the larger part of the flux (95%) through ICDH is dedicated to the supply of reducing power and ATP. The system sustains a large intracellular pool of isocitrate to accommodate the rather low affinity of ICL for this substrate. Excessive flux of isocitrate through ICDH is prevented by regulation of ICDH activity: reversible inactivation of ICDH is achieved by a bifunctional kinase/phosphatase, as the phosphorylated form of ICDH has no activity. The kinase/phosphatase responds to two classes of effectors--intermediates of the CMPs generated by flux through ICL and the lower energy forms of ATP and NADPH (ADP, AMP and NADP+) generated when these intermediates are used for biosynthesis. The effect is to adjust flux through ICDH so that the rate of supply of NADPH and ATP is equal to the demands of biosynthesis. Biosynthetic fluxes are limited by the rate of supply of precursors which depends on flux through ICL. Growth rate is most likely limited by the primary flux of acetate to acetyl-CoA or flux through ICL. In the steady state, the flux through ICDH is regulated to be twice the throughput of ICL. The evolution of this complex pattern of control may have depended on alternatives to the citric acid for energy generation.
当乙酸盐是维持有氧生长的唯一碳源和能源时，乙醛酸循环支路和柠檬酸循环在大肠杆菌中同时运行。整体碳平衡使得在已知这些代谢途径的前提下，能够计算通过中心代谢途径（CMPs）的通量。乙酸盐通过CMPs流入合成新生物质所需的前体物质，同时也用于产生将这些前体物质转化为生物质所需的还原力和ATP。在这些情况下，在异柠檬酸处形成一个节点，异柠檬酸裂合酶（ICL）和异柠檬酸脱氢酶（ICDH）在此竞争它们的共同底物。一般来说，通过ICL的通量产生用于生物合成的前体物质，而通过ICDH的通量的较大部分（95%）则专门用于提供还原力和ATP。该系统维持着大量的细胞内异柠檬酸池，以适应ICL对该底物相当低的亲和力。通过调节ICDH活性可防止异柠檬酸过多地通过ICDH：一种双功能激酶/磷酸酶可实现ICDH的可逆失活，因为ICDH的磷酸化形式没有活性。该激酶/磷酸酶对两类效应物作出反应——通过ICL的通量产生的CMPs中间体以及这些中间体用于生物合成时产生的较低能量形式的ATP和NADPH（ADP、AMP和NADP+）。其作用是调整通过ICDH的通量，以使NADPH和ATP的供应速率等于生物合成的需求。生物合成通量受到前体物质供应速率的限制，而前体物质供应速率取决于通过ICL的通量。生长速率很可能受到乙酸盐向乙酰辅酶A的初级通量或通过ICL的通量的限制。在稳态下，通过ICDH的通量被调节为ICL通量的两倍。这种复杂控制模式的进化可能依赖于柠檬酸以外的能量产生途径。
