Homoserine Kinase from Escherichia coli K-12: Properties, Inhibition by l -Threonine, and Regulation of Biosynthesis

Abstract

We have partially purified homoserine kinase from a genetically derepressed strain of Escherichia coli K-12. The optimum pH of the enzyme-substrate reaction was 7.8 and the K m values for l -homoserine and adenosine 5′-triphosphate were both 3 × 10 −4 M. K + (or NH 4 + ) as well as Mg 2+ were required for its activity. The sedimentation coefficient determined by ultracentrifugation in a sucrose density gradient was 5.0 ± 0.25 S . l -Homoserine was an excellent protector against heat inactivation of homoserine kinase. l -Threonine was a competitive inhibitor of homoserine kinase, suggesting that end-product inhibition of this enzyme plays a role in vivo in the overall regulation of threonine biosynthesis. The specific activity of aspartokinase I-homoserine dehydrogenase I and of homoserine kinase showed a strong positive correlation in extracts from strains under widely varying conditions of genetic or physiological derepression; it was concluded that these two enzymes are coordinately regulated in E. coli K-12.