title: 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 x 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 +/- 0.25S. 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.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000429:Alcohol Oxidoreductases / Q000378:metabolism; D001224:Aspartic Acid; D002474:Cell-Free System; D002499:Centrifugation, Density Gradient; D004794:Enzyme Repression; D004926:Escherichia coli / Q000201:enzymology*; D006714:Homoserine / Q000378:metabolism; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000378:metabolism; D010770:Phosphotransferases / Q000037:antagonists & inhibitors / Q000096:biosynthesis* / Q000378:metabolism; D013237:Stereoisomerism; D013912:Threonine / Q000494:pharmacology*
chemical_list: D013912:Threonine; D001224:Aspartic Acid; D006714:Homoserine; D000255:Adenosine Triphosphate; D000429:Alcohol Oxidoreductases; D010770:Phosphotransferases; D008274:Magnesium