Methionine-repressible homoserine dehydrogenase of serratia marcescens: purification and properties

Abstract

Serratia marcescens Sa-3 possesses two homoserine dehydrogenases and neither has any aspartokinase activity unlike the case of Es-cherichia coli enzymes. The two enzymes have been separated. One of them is active with either NAD^- or NADP⁺ and has been purified about 180-fold to homogeneity. This enzyme is completely repressed by the presence of 1mm methionine or homoserine in the growth medium, but its activity is unaffected by any amino acid of the aspartate family either singly or together. In many of its properties (such as pH optimum, K_m for substrate and cofactors), it resembles its counterpart in E. coli K₁₂. Potassium ions stabilize the enzyme but are not essential for activity. Its molecular weight is around 155,000 as determined by gel filtration and approximately 76,000 by SDS-polyacrylamide gel electrophoresis. This suggests that the enzyme has two subunits (polypeptide chains) in the molecule: 8M urea has no effect on enzyme activity. This enzyme represents approximately 30% of the total homoserine dehydrogenase activity of S. marcescens unlike in Salmonella typhimurium and E. coli K₁₂ where it is a minor or a negligible component.