CRYSTAL-STRUCTURE OF TRYPTOPHANASE

The X-ray structure of tryptophanase (Tnase) reveals the interactions responsible for binding of the pyridoxal 5'-phosphate (PLP) and atomic details of the K+ binding site essential for catalysis. The structure of hole Tnase from Proteus vulgaris (space group P2(1)2(1)2(1) with a = 115.0 Angstrom, b = 118.2 Angstrom, c = 153.7 Angstrom) has been de termined at 2.1 Angstrom resolution by molecular replacement using tyr osine phenol-lyase (TPL) coordinates. The final model of Tnase, refine d to an R-factor of 18.7%, (R-free=22.8%) suggests that the PLP-enzyme form observed in the structure is a ketoenamine. PLP is bound in a cl eft formed by both the small and large domains of one subunit and the large domain of the adjacent subunit in the so-called ''catalytic'' di mer. The K+ cations are located on the interface of the subunits in th e dimer. The structure of the catalytic dimer and mode of PLP binding in Tnase resemble those found in aspartate aminotransferase, TPL, omeg a-amino acid pyruvate aminotransferase, dialkylglycine decarboxylase ( DGD), cystathionine beta-lyase and ornithine decarboxylase. No structu ral similarity has been detected between Tnase and the beta(2) dimer o f tryptophan synthase which catalyses the same beta-replacement reacti on. The single monovalent cation binding site of Tnase is similar to t hat of TPL, but differs from either of those in DGD. (C) 1998 Academic Press Limited.