Homoserine kinase from Escherichia coli K 12: properties, inhibition by L threonine, and regulation of biosynthesis

Abstract

Homoserine kinase was partially purified 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 3x10-4 M. K+ (or NH4+) as well as Mg2+ were required for its activity. The sedimentation coefficient determined by ultracentrifugation in a sucrose density gradient was 5.0 ± 0.25S. 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.