Threonine-activating enzyme, purification

This enzyme, catalyzing Eq. (1), has been demonstrated in a number of plants (Smith and Thompson, 1%9 Ngo and Shargool, 1974 Ascano and Nicholas, 1977). The 54-fold purified enzyme from kidney bean seedlings (Smith and Thompson, 1971) had values of 6 x lO IW for serine and 2 x Qr M for acetyl-CoA. The enzyme does not catalyze the acetylation of homoserine or threonine. Activities with other acyl-CoA derivatives were not tested. During purification, this enzyme was separated fix>m cysteine synthase, in contrast to the serine acetyltransferase from SalmoneUa typhimurium which has been isolated predominantly complexed with cysteine synthase (Kredich et al., 1969). 

The final reaction in the biosynthesis of threonine involves a /8-y rearrangement and the loss of phosphate from O-phosphohomoserine (Fig. 2). Threonine synthases have been isolated from Lemna (Schnyder et al., 1975) radish, sugarbeet (Madison and Thompson, 1975), peas (Schnyder et al., 1975 Thoen et al., 1978b), and barley (Aames, 1978). None of these enzymes has been extensively characterized but a requirement for pyridoxyl-5 -phosphate was demonstrated after partial purification of the barley and pea enzymes. Unlike several other enzymes associated with threonine synthesis, the activity of threonine synthase was not stimulated by monovalent cations. However, all of the plant enzymes are strongly activated by 5-adeno-sylmethionine (Section III,B,5). 

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