Homoserine Kinase from Escherichia coli K12

Homoserine kinase was purified to apparent homogeneity from a derepressed strain of Escherichia coli K12, using standard fractionation techniques. It is a dimer (Mr= 60000) composed of apparently identical polypeptide chains (Mr= 29 000). Its amino acid composition and N‐terminal sequence have been determined. l‐Threonine is a competitive inhibitor of the substrate l‐homoserine; this inhibition is straightforward and shows no sign of co‐operativity. Evidence is presented that homoserine and threotine bind to the same site of this non‐allosteric enzyme. The binding of homoserine and threonine can also be studied by difference spectroscopy; the latter studies reveal an unexpected effect of magnesium ions, which might be the basis for the unusual high Mg2+ requirement for optimal enzyme reaction