Identification and biochemical characterization of threonine dehydratase from the hyperthermophile Thermotoga maritima.

The peptidoglycan of the hyperthermophile Thermotoga maritima contains an unusual component, d-lysine (d-Lys), in addition to the typical d-alanine (d-Ala) and d-glutamate (d-Glu). In a previous study, we identified a Lys racemase that is presumably associated with d-Lys biosynthesis. However, our understanding of d-amino acid metabolism in T. maritima and other bacteria remains limited, although d-amino acids in the peptidoglycan are crucial for preserving bacterial cell structure and resistance to environmental threats. Herein, we characterized enzymatic and structural properties of TM0356 that shares a high amino acid sequence identity with serine (Ser) racemase. The results revealed that TM0356 forms a tetramer with each subunit containing a pyridoxal 5′-phosphate as a cofactor. The enzyme did not exhibit racemase activity toward various amino acids including Ser, and dehydratase activity was highest toward l-threonine (l-Thr). It also acted on l-Ser and l-allo-Thr, but not on the corresponding d-amino acids. The catalytic mechanism did not follow typical Michaelis–Menten kinetics; it displayed a sigmoidal dependence on substrate concentration, with highest catalytic efficiency (kcat/K0.5) toward l-Thr. Interestingly, dehydratase activity was insensitive to allosteric regulators l-valine and l-isoleucine (l-Ile) at low concentrations, while these l-amino acids are inhibitors at high concentrations. Thus, TM0356 is a biosynthetic Thr dehydratase responsible for the conversion of l-Thr to α-ketobutyrate and ammonia, which is presumably involved in the first step of the biosynthesis of l-Ile.