The Mechanism of Action of 5′-Adenylic Acid-activated Threonine Dehydrase

The site of AMP activation of the biodegradative l-threonine dehydrase of Escherichia coli was established by spectrophotometric and circular dichroic measurements, and by determining the Ki values in the presence and absence of AMP for a series of threonine analogs which undergo various partial reactions of the mechanism except dehydration. It was found that AMP greatly enhanced (a) the accumulation of an intermediate absorbing at 455 nm; (b) loss of circular dichroism; and (c) the binding of l-analogs of threonine including those which did not have an amino group. From observations with inhibitors and other published results, it was postulated that the intermediate absorbing at 455 nm is the Schiff base of α-aminocrotonate and pyridoxal phosphate. Similarly, it was concluded that loss of circular dichroism reflected the accumulation of Schiff bases between pyridoxal phosphate and l-threonine as well as with α-aminocrotonate. The major conclusion reached was that AMP enhances the first partial reaction of the sequence; namely, the noncovalent binding of threonine to the dehydrase.