We have determined the kinetic parameters of the individual steps of the threonine pathway from aspartate in Escherichia coli under a single set of experimental conditions chosen to be physiologically relevant. Our aim was to summarize the kinetic behaviour of each enzyme in a single tractable equation that takes into account the effect of the products as competitive inhibitors of the substrates in the forward reaction and also, when appropriate (e.g. near-equilibrium reactions), as substrates of the reverse reactions. Co-operative feedback inhibition by threonine and lysine was also included as necessary. We derived the simplest rate equations that describe the salient features of the enzymes in the physiological range of metabolite concentrations in order to incorporate them ultimately into a complete model of the threonine pathway, able to predict quantitatively the behaviour of the pathway under natural or engineered conditions.
Abbreviations used: AK, aspartate kinase; aspp, β-aspartyl phosphate; ASD, aspartate semialdehyde dehydrogenase; ASA, d,l-aspartic β-semialdehyde; HDH, homoserine dehydrogenase; HK, homoserine kinase; LDH, lactate dehydrogenase; PK, pyruvate kinase; TS, threonine synthase.
Present address: Diosynth France, BP 26, 60590 Eragny-sur-Epte, France.