Threonine dehydratase biosynthetic

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... 

L-Serine and L-threonine dehydratases dehydrate and subsequently deaminate the amino add to the corresponding a-keto add. These enzymes are known to require pjn-idoxal-S -phosphate as a coenzyme. They can function in a biosynthetic or catabolic marmer (99). Both enzymes can cause problems for the whole-cell-based production of L-serine (100). 

The properties of the biosynthetic enzymes isolated from multicellular plants are at least qualitatively similar to those of the analogous microbial threonine dehydratases. Measurements of the for threonine are complicated by allosteric kinetics in the presence of trace amounts of isoleucine (this amino acid is often required to stabilize the enzyme) and by a strong dependence of enzyme activity on pH (Section The enzymes are... 

The initial reaction associated with isoleucine biosynthesis is catalyzed by threonine dehydratase (18). In microorganisms and a few plants, both degra-dative and biosynthetic enzymes have been identified. The former are not regulated by amino acids, but some are activated by AMP or ADP. An apparently unregulated threonine dehydratase has been purified from seeds of Cus-cuta campestris (Madan and Nath, 1983). The lack of identifiable regulatory phenomena precludes identification of the function of this enzyme as either biosynthetic or degradative in nature. 

The essential role of threonine dehydratase in the biosynthesis of isoleucine has been confirmed by the identification of mutant plants that are auxotrophic for isoleucine and do not exhibit detectable dehydratase activity (Sidorov et al., 1981 Negrutiu et al., 1985). Biosynthetic enzymes classically exhibit high sensitivity to inhibition by the pathway product, and exogenous isoleucine... 

Figure 3 (a) The nisin biosynthetic gene cluster, (b) Posttranslational modifications during the biosynthesis of nisin. Dehydration of serine and threonine residues in the structural region of the precursor peptide NisA is performed by the dehydratase NisB. Then, the (Me)Lan rings are installed by the cyclase NisC. After secretion, the unmodified leader sequence is removed by the serine protease NisP, which generates the biologically active species, (c) The proposed cyclization mechanism for NisC. 

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