This paper deals with the application of the metabolic control theory, especially the measurement of control coefficients, to the threonine pathway inE. coli. The control coefficient of a step on a metabolic flux quantitatively assesses the flux response to the step variations. This concept is particularly relevant both in pathological situations (decrease in the activity of an enzymatic step in the metabolism) and in biotechnologies, where, on the contrary steps are amplified.Measurement of the control coefficients of the steps of a metabolic network makes it possible to know those whose amplification should lead to a simultaneous increase in the fluxes.We have applied these concepts to threonine biosynthesis from aspartate inE. coli. The threonine pathway starting from aspartate involves five steps catalyzed by five enzyme activities: aspartokinase (AK), aspartate-semialdehyde-dehydrogenase (ASA-DH), homoserine dehydrogenase (HDH), homoserine kinase (HK) and hreonine synthetase activity (TS).Measurement of the control coefficient of the first step (AK, insensitive to threonine inhibition in the studied strain) has shown that it controls threonine production weakly. Our study has revealed a hitherto unknown inhibition of homoserine kinase activity by lysine.Mathematical modeling of this metabolic pathway has been undertaken to better understand our experimental results.相似文献
The influence of O2 concentrations on the Hill reactions in the presence of p-benzoquinone, ferricyanide, NADP+, NADP+ plus ferredoxin has been studied with isolated spinach chloroplasts.
Because of the partial reoxidation of the hydroquinone, which is depending upon the O2 concentration, it does not seem possible to localize a site of action for O2.
With ferricyanide the influence of O2 is weak. However, the rate of ferricyanide reduction is increased in the presence of O2. The observed stimulation is greater for 21% O2 than for 70% O2. Bicarbonate stimulates the ferricyanide reduction and decreases the stimulating effect of 21% O2.
O2 decreases the rate of NADP+ reduction. Ferredoxin as well as bicarbonate stimulate the NADP+ reduction and reduce the O2 inhibition.
These results seem to indicate that O2 may enter the electron transport chain at a site situated near Photosystem I and before the ferredoxin's site.
The inhibitory effect of O2 on the Hill reactions with p-benzoquinone and NADP+ is depending upon the plants' growth conditions. It is greater with plants grown under weak light. 相似文献