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

AUTOR(ES)

Theze, J.

RESUMO

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 Km values for l-homoserine and adenosine 5′-triphosphate were both 3 × 10−4 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.

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