Pulse Experiments as a Prerequisite for the Quantification of in Vivo Enzyme Kinetics in Aromatic Amino Acid Pathway of Escherichia coli

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Abstract
Glucose pulse experiments were performed to elucidate their effects on the carbon flux into the aromatic amino acid pathway in different Escherichia coli strains. Using a 3‐deoxy‐d‐ arabino‐heptulosonate 7‐phosphate (DAHP, aroB‐)‐producing strain, a fed‐batch fermentation strategy specialized for glucose pulse experiments was developed and further applied for 3‐dehydroshikimate (DHS, aroE‐)‐ and shikimate 3‐phosphate (S3P, aroA‐)‐producing E. coli strains. The strains overexpress a feedback‐resistant DAHP synthase and additional enzymes to prevent rate‐limiting steps in the aromatic amino acid pathway. Changes of carbon flux into the aromatic amino acid pathway were determined via extracellular metabolite accumulations using 1H NMR and HPLC measurements. As an important result, a close relationship between pulse intensity and aromatic metabolite formation rates was identified. The more downstream an aromatic pathway intermediate was located, the stronger the glucose pulse intensity had to be in order to detect significant changes in product formation. However, with the experimental conditions chosen, changes after pulse were detected even for shikimate 3‐phosphate, the most downstream accumulating metabolite of this experimental series. Hence glucose pulse experiments are assumed to be a promising tool even for the analysis of final pathway products such as, for example, l‐phenylalanine.