The production of acetate by aerobically growing E. coli is examined. The problem is formulated in terms of a flow network that has as its objective maximal ATP synthesis. It is found that when loads are imposed and flux constraints exist either at the level of NADH turnover rate or the activity of a key Krebs cycle enzyme, switching to acetate overflow is predicted. Moreover, the result found for the latter constraint can be shown to be formally equivalent to a correlation experimentally determined for the specific rate of acetate production by E. coli K-12.
A computer model is described which is capable of predicting changes in cell composition, cell size, cell shape, and the timing of chromosome synthesis in response to changes in external glucose limitation. The model is constructed primarily from information on unrestricted growth in glucose minimal medium. The ability of the model to make reasonable quantitative predictions under glucose-limitation is a test of the plausibility of the basic biochemical mechanisms included in the model. Such a model should be of use in differentiating among competing hypotheses for biological mechanisms and in suggesting as yet unobserved phenomena. The last two points are illustrated with the testing of a mechanism for the control of the initiation of DNA synthesis and predictions on cellwidth variations during the division cycle.
We examine the effect of poly(ethylene glycol) (PEG) on pyrene solubilization behaviors in aqueous sodium dodecyl sulfate (SDS) solutions. These solutions display strong polymer-surfactant complexation. Following the definitions of Ikeda and Maruyama (J. Colloid Interface Sci. 1994, 166, 1) we distinguish between the macroscopic solubilizing power and the microscopic solubilization capacity. With pyrene as a model solubilizate, we use ultraviolet absorbance spectrophotometry to measure solubilizing powers. We use excimer fluorescence spectroscopy to identify polymer-surfactant binding transitions and to measure the aggregation numbers of free SDS micelles and of PEG-bound SDS aggregates that contain solubilized pyrene in order to calculate solubilization capacities. The solubilization capacity and solubilizing power of free SDS micelles both increase with increasing aggregation number, when the aggregation number is increased by increasing ionic strength. The solubilization capacity is approximately 3 times more sensitive than the solubilizing power to a change in aggregation number. For a given value of the ionic strength, the aggregation number of a PEG-bound SDS aggregate is approximately 50-60% smaller than that of a free micelle, while its solubilization capacity is within approximately 20% of that of a free micelle. As a result, PEG increases the macroscopic solubilizing power at all SDS concentrations above the critical aggregation concentration by virtue of the greater number of distinct surfactant aggregates formed for a given SDS concentration in the presence of PEG. Compared to free micelles, the solubilizing power of PEG-bound SDS aggregates is significantly more sensitive to ionic strength.
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