title: L-serine and L-threonine dehydratase from Clostridium propionicum. Two enzymes with different prosthetic groups.
abstract: L-Serine dehydratase from the Gram-positive bacterium Peptostreptococcus asaccharolyticus is novel in the group of enzymes deaminating 2-hydroxyamino acids in that it is an iron-sulfur protein and lacks pyridoxal phosphate [Grabowski, R. and Buckel, W. (1991) Eur. J. Biochem. 199, 89-94]. It was proposed that this type of L-serine dehydratase is widespread among bacteria but has escaped intensive characterization due to its oxygen lability. Here, we present evidence that another Gram-positive bacterium, Clostridium propionicum, contains both an iron-sulfur-dependent L-serine dehydratase and a pyridoxal-phosphate-dependent L-threonine dehydratase. These findings support the notion that two independent mechanisms exist for the deamination of 2-hydroxyamino acids. L-Threonine dehydratase was purified 400-fold to apparent homogeneity and revealed as being a tetramer of identical subunits (m = 39 kDa). The purified enzyme exhibited a specific activity of 5 mu kat/mg protein and a Km for L-threonine of 7.7 mM. L-Serine (Km = 380 mM) was also deaminated, the V/Km ratio, however, being 118-fold lower than the one for L-threonine. L-Threonine dehydratase was inactivated by borohydride, hydroxylamine and phenylhydrazine, all known inactivators of pyridoxal-phosphate-containing enzymes. Incubation with NaB3H4 specifically labelled the enzyme. Activity of the phenylhydrazine-inactivated enzyme could be restored by pyridoxal phosphate. L-Serine dehydratase was also purified 400-fold, but its extreme instability did not permit purification to homogeneity. The enzyme was specific for L-serine (Km = 5 mM) and was inhibited by L-cysteine (Ki = 0.5 mM) and D-serine (Ki = 8 mM). Activity was insensitive towards borohydride, hydroxylamine and phenylhydrazine but was rapidly lost upon exposure to air. Fe2+ specifically reactivated the enzyme. L-Serine dehydratase was composed of two different subunits (alpha, m = 30 kDa; beta, m = 26 kDa), their apparent molecular masses being similar to the ones of the two subunits of the iron-sulfur-dependent enzyme from P. asaccharolyticus. Moreover, the N-terminal sequences of the small subunits from these two organisms were found to be 47% identical. In addition, 38% identity with the N-terminus of one of the two L-serine dehydratases of Escherichia coli was detected.
authors.affiliation: Laboratorium für Mikrobiologie des Fachbereichs Biologie der Philipps-Universität, Marburg, Germany.
authors.forename: A E
authors.initials: AE
authors.lastname: Hofmeister
authors.forename: R
authors.initials: R
authors.lastname: Grabowski
authors.forename: D
authors.initials: D
authors.lastname: Linder
authors.forename: W
authors.initials: W
authors.lastname: Buckel
mesh_terms: D000595:Amino Acid Sequence; D003013:Clostridium / Q000201:enzymology* / Q000254:growth & development; D003470:Culture Media; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D004790:Enzyme Induction; D007501:Iron / Q000032:analysis; D012695:L-Serine Dehydratase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D008969:Molecular Sequence Data; D011732:Pyridoxal Phosphate / Q000032:analysis; D016415:Sequence Alignment; D013913:Threonine Dehydratase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D003470:Culture Media; D011732:Pyridoxal Phosphate; D007501:Iron; D012695:L-Serine Dehydratase; D013913:Threonine Dehydratase