Kynureninase [E.C. 3.7.1.3] is a pyridoxal-5'-phosphate (PLP)-dependent enzyme that catalyzes the hydrolytic cleavage of l-kynurenine to anthranilic acid and l-alanine. Sequence alignment with other PLP-dependent enzymes indicated that kynureninase is in subgroup IVa of the aminotransferases, along with nifS, CsdB, and serine-pyruvate aminotransferase, which suggests that kynureninase has an aminotransferase fold. Crystals of Pseudomonas fluorescens kynureninase were obtained, and the structure was solved by molecular replacement using the CsdB coordinates combined with multiple isomorphous heavy atom replacement. The coordinates were deposited in the PDB (ID code 1QZ9). The structure, refined to an R factor of 15.5% to 1.85 A resolution, is dimeric and has the aminotransferase fold. The structure also confirms the prediction from sequence alignment that Lys-227 is the PLP-binding residue in P. fluorescens kynureninase. The conserved Asp-201, expected for an aminotransferase fold, is located near the PLP nitrogen, but Asp-132 is also strictly conserved and at a similar distance from the pyridinium nitrogen. Mutagenesis of both conserved aspartic acids shows that both contribute equally to PLP binding, but Asp-201 has a greater role in catalysis. The structure shows that Tyr-226 donates a hydrogen bond to the phosphate of PLP. Unusual among PLP-dependent enzymes, Trp-256, which is also strictly conserved in kynureninases from bacteria to humans, donates a hydrogen bond to the phosphate through the indole N1-hydrogen.

The ®nal step in lysine biosynthesis in bacteria, the conversion of meso-diaminopimelate to l-lysine, is catalyzed by the only known d-amino-acid decarboxylase, diaminopimelate decarboxylase (DDC). The Escherichia coli DDC has been cloned, overexpressed in E. coli with a carboxy-terminal polyhistidine puri®cation tag and crystallized from lithium sulfate. The protein is intensely yellow, owing to the pyridoxal-5 H -phosphate cofactor, and is enzymatically active. Large well ordered crystals, belonging to space group P6 1 22 with unit-cell parameters a = b = 98.6, c = 177 A Ê , make high-resolution X-ray diffraction studies possible to characterize the residues important in stereospeci®c decarboxylation and reprotonation during catalytic turnover.

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