The Threonine‐Sensitive Homoserine Dehydrogenase and Aspartokinase Activities of Escherichia coli K 12

Aspartokinase I‐homoserine dehydrogeanase I is an allosteric protein with six subunits. Conformation changes upon addition of a substrate, l‐aspartate, and of an activator, K+, or an inhibitor, l‐threonine, are demonstrated by a study of the ultraviolet absorption and of the fluorescence emission of the protein. The effects of the ligands are compatible with a two‐state model and a three‐by‐three concerted transition of the subunits from the inactive, threonine binding conformation, to the active conformation binding aspartate and the potassium ions.