title: Neurospora tryptophan synthase. Characterization of the pyridoxal phosphate binding site.
abstract: Tryptophan synthase, which catalyzes the final step of tryptophan biosynthesis, is a multifunctional protein that requires pyridoxal phosphate for two of its three distinct enzyme activities. Tryptophan synthase from Neurospora crassa, a homodimer of two 75-kDa subunits, was shown to bind 1 mol of pyridoxal phosphate/mol of subunit with a calculated dissociation constant for pyridoxal phosphate of 1.1 microM. The spectral properties of the holoenzyme, apoenzyme, and reconstituted holoenzyme were characterized and compared to those previously established for the heterotetrameric (alpha 2 beta 2) enzyme from Escherichia coli. The Schiff base formed between pyridoxal phosphate and the enzyme was readily reduced by sodium borohydride, but not sodium cyanoborohydride. The active site residue that binds pyridoxal phosphate, labeled by reduction of the Schiff base with tritium-labeled sodium borohydride, was determined to be lysine by high performance liquid chromatography analysis of the protein hydrolysate. A 5400-dalton peptide containing the reduced pyridoxal phosphate moiety was generated by cyanogen bromide treatment, purified and sequenced. The sequence is 85% homologous with the corresponding sequence obtained for yeast tryptophan synthase (Zalkin, H., and Yanofsky, C. (1982) J. Biol. Chem. 257, 1491-1500); the lysine derivatized by pyridoxal phosphate is located at the same relative position as that in the yeast and E. coli enzymes.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Texas Health Science Center, Houston 77225.
authors.forename: M L
authors.initials: ML
authors.lastname: Pratt
authors.forename: J A
authors.initials: JA
authors.lastname: DeMoss
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology; D008969:Molecular Sequence Data; D009491:Neurospora / Q000201:enzymology*; D009492:Neurospora crassa / Q000201:enzymology*; D011485:Protein Binding; D011732:Pyridoxal Phosphate / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D012689:Sequence Homology, Nucleic Acid; D013045:Species Specificity; D013053:Spectrophotometry; D014367:Tryptophan Synthase / Q000235:genetics / Q000378:metabolism*
chemical_list: D011732:Pyridoxal Phosphate; D014367:Tryptophan Synthase