TOP-DOWN CONTROL ANALYSIS OF SYSTEMS WITH MORE THAN ONE COMMON INTERMEDIATE

The analysis of the control of complex metabolic systems can be greatl y simplified by application of the top-down approach of metabolic cont rol analysis, in which the reactions of the system are grouped togethe r into a small number of blocks connected by a common intermediate. Th e experimental application of the top-down approach has so far been li mited to systems that have only a single intermediate. In this study, we demonstrate that the connectivity and summation theorems of metabol ic control analysis hold with any number of intermediates between the metabolic blocks, and in doing so show that top-down analysis is valid for systems with multiple intermediates and so can be applied to most metabolic systems regardless of their complexity; an example of such an application is provided. Top-down control analysis has successfully described the control of mitochondrial respiration by dividing the sy stem into three blocks, the respiratory chain, phosphorylation system and proton leak, all linked by a single intermediate, proton motive fo rce. Here, we subdivide the respiratory chain into succinate consumers and cytochrome oxidase so that a second intermediate, cytochrome c re dox state, is generated. Despite the fact that the redox state of cyto chrome c is not measured, we solve the control over the system fluxes. In common with previous studies, we find that under conditions where there is no ATP turnover (state 4), respiration is largely controlled by proton leak, while at maximal ATP turnover (state 3) respiration is controlled by the respiratory chain and the phosphorylating system. I n state 4, 85% of the control by the respiratory chain resides with cy tochrome oxidase. As ATP turnover increases, the respiration rate incr eases, and the control by the respiratory chain shifts from cytochrome oxidase to the succinate consumers, so that in state 3 83% of the con trol by the respiratory chain lies in the reactions between succinate and cytochrome c and only 17% resides with cytochrome oxidase.