Abstract
Two enzymes which catalyse threonine synthesis from homoserine have been separated from Baker's yeast extract by ammonium sulfate fractionation. It has become apparent that two distinct reactions are involved in the over-all reaction. With Enzyme I, associated with the fraction precipitating at lower salt concentration, a stable precursor for threonine is synthesized from homoserine and ATP in the presence of Mg++, then the product acts as specific substrate for Enzyme II, which is associated with the fraction precipitating at higher concentration of the salt, resulting in the formation of threonine even in the absence of ATP.
