Stereochemistry at C-3 in nucleophilic 3-replacements and a,3-eliminations

A similar stereochemical question as in the /8-replacement reactions can be asked in the a, /8-eliminations where the group X is replaced by a hydrogen, i.e., is the proton added at C-/8 of the PLP-aminoacrylate on the same face from which X departed or on the opposite face This question has been answered for a number of enzymes which generate either a-ketobutyrate or pyruvate as the keto acid product. Crout and coworkers [119,120] determined the steric course of proton addition in the a,/8-elimination of L-threonine by biosynthetic L-threonine dehydratase and of D-threonine by an inducible D-threonine dehydratase, both in Serratia marcescens. Either substrate, deuterated at C-3, was converted in vivo into isoleucine, which was compared by proton NMR to a sample prepared from (3S)-2-amino[3-2H]butyric acid. With both enzymes the hydroxyl group at C-3 was replaced by a proton in a retention mode. Although this has not been established with certainty, it is likely that both enzymes, like other bacterial threonine dehydratases [121], contain PLP as cofactor. Sheep liver L-threonine dehydratase, on the other hand, is not a PLP enzyme but contains an a-ketobutyrate moiety at the active site [122], It replaces the hydroxyl group of L-threonine with H in a retention mode, but that of L-allothreonine in an inversion mode [123]. Snell and coworkers [124] established that the replacement of OH by H in the a, /8-elimination of D-threonine catalyzed by the PLP-containing D-serine dehydratase from E. coli also proceeds in a retention mode. They 

Steric course at C-/J in nucleophilic / -replacement reactions catalyzed by PLP enzymes 

---