Metal ion requirement and tryptophan inhibition of normal and variant anthranilate synthase-anthranilate 5-phosphoribosylpyrophosphate phosphoribosyltransferase complexes from Salmonella tyrhimrium

Anthranilate phosphoribosyltransferase from the hyperthermophilic archaeon Thermococcus kodakarensis shows maximum activity with zinc and forms a unique dimeric structure

Anthranilate phosphoribosyltransferase (TrpD) is involved in tryptophan biosynthesis, catalyzing the transfer of a phosphoribosyl group to anthranilate, leading to the generation of phosphoribosyl anthranilate. TrpD belongs to the phosphoribosyltransferase (PRT) superfamily and is the only member of the structural class IV. X-ray structures of TrpD from seven species have been solved to date. Here, functional and structural characterization of a recombinant TrpD from hyperthermophilic archaeon Thermococcus kodakarensis KOD1 (TkTrpD) was carried out. Contrary to previously characterized Mg²⁺-dependent TrpD enzymes, TkTrpD was found to have a unique divalent cation dependency characterized by maximum activity in the presence of Zn²⁺ (1580 μmol·min^-1·mg^-1, the highest reported for any TrpD) followed by Ca²⁺ (948 μmol·min^-1·mg^-1) and Mg²⁺ (711 μmol·min^-1·mg^-1). TkTrpD displayed an unusually low thermostability compared to other previously characterized proteins from T. kodakarensis KOD1. The crystal structure of TkTrpD was determined in free form and in the presence of Zn²⁺ to 1.9 and 2.4 Å resolutions, respectively. TkTrpD structure displayed the typical PRT fold similar to other class IV PRTs, with a small N-terminal α-helical domain and a larger C-terminal α/β domain. Electron densities for Zn²⁺ were identified at the expected zinc-binding motif, DE(217-218), of the enzyme in each subunit of the dimer. Two additional Zn²⁺ were found at a new dimer interface formed in the presence of Zn²⁺. A fifth Zn²⁺ was found bound to Glu118 at crystal lattice contacts and a sixth one was ligated with Glu235. Based on the TkTrpD-Zn²⁺ structure, it is suggested that the formation of a new dimer may be responsible for the higher enzyme activity of TkTrpD in the presence of Zn²⁺ ions.

Expression and characterization of PhzE from P. aeruginosa PAO1: aminodeoxyisochorismate synthase involved in pyocyanin and phenazine-1-carboxylate production

PhzE from Pseudomonas aeruginosa catalyzes the first step in the biosynthesis of phenazine-1-carboxylic acid, pyocyanin, and other phenazines, which are virulence factors for Pseudomonas species. The reaction catalyzed converts chorismate into aminodeoxyisochorismate using ammonia supplied by a glutamine amidotransferase domain. It has structural and sequence homology to other chorismate-utilizing enzymes such as anthranilate synthase, isochorismate synthase, aminodeoxychorismate synthase, and salicylate synthase. Like these enzymes, it is Mg(2+) dependent and catalyzes a similar S(N)2" nucleophilic substitution reaction. PhzE catalyzes the addition of ammonia to C2 of chorismate, as does anthranilate synthase, yet unlike anthranilate synthase it does not catalyze elimination of pyruvate from enzyme-bound aminodeoxyisochorismate. Herein, the cloning of the phzE gene, high level expression of active enzyme in E. coli, purification, and kinetic characterization of the enzyme is presented, including temperature and pH dependence. Steady-state kinetics give K(chorismate)=20±4μM, K(Mg)(2+)=294±22μM, K(L-gln)=11±1mM, and k(cat)=2.2±0.2s(-1) for a random kinetic mechanism. PhzE can use NH(4)(+) as an alternative nucleophile, while Co(2+) and Mn(2+) are alternative divalent metals.

Anthranilate synthase without an LLES motif from a hyperthermophilic archaeon is inhibited by tryptophan

Tk-trpE and Tk-trpG, the genes that encode the two subunits of anthranilate synthase from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1, have been expressed independently in Escherichia coli. The anthranilate synthase complex (Tk-AS complex) was obtained by heat-treatment of the mixture of cell-free extracts containing each recombinant protein, Tk-TrpE (alpha subunit) and Tk-TrpG (beta subunit), at 85 degrees C for 10 min. Further purification of Tk-AS complex was carried out by anion-exchange chromatography followed by gel-filtration. Molecular mass estimations from gel-filtration chromatography indicated that Tk-AS complex was a heterodimer (alphabeta). The complex displayed both ammonia- and glutamine-dependent anthranilate synthase activities, and could not utilize asparagine as an ammonia donor. The optimal pH was pH 10.0 and the optimal temperature was 85 degrees C in both cases. Mg2+ was necessary for the anthranilate synthase activity. At 75 degrees C, the K(m) values of chorismate for ammonia- and glutamine-dependent activities were 13.8 and 3.4 microM, respectively. The K(m) value of Mg2+ was 20.5 microM. The K(m) values of glutamine and NH4Cl were 88 microM and 5.6 mM, respectively. Although Tk-TrpE displayed 47.6% similarity with TrpE of Salmonella typhimurium, conserved amino acid residues proven to be essential for inhibition of enzyme activity by L-tryptophan were not present in Tk-TrpE. Namely, residues corresponding to Glu39, Met293, and Cys465 in the enzyme from S. typhimurium were replaced by Arg28, Thr221, and Ala384 in Tk-TrpE. Nevertheless, significant inhibition by L-tryptophan was observed, with K(i) values of 5.25 and 74 microM for ammonia and glutamine-dependent activities, respectively. The inhibition was competitive with respect to chorismate. The results suggest that the amino acid residues involved in the feedback inhibition by L-tryptophan in the case of Tk-AS complex are distinct from previously reported anthranilate synthases.

Structural features of the phosphoribosyltransferases and their relationship to the human deficiency disorders of purine and pyrimidine metabolism

Similarities in the physical and chemical properties of the phosphoribosyltransferase family of enzymes suggest that they may share common structural features as observed in other functionally related proteins. The unusually high incidence of structural gene mutations of these enzymes in man are associated with several metabolic diseases of purine and pyrimidine metabolism. It is proposed that these disorders are the consequence of structural mutations to an architectural domain common to all of the phosphoribosyltransferases.