High resolution crystal structure of a Mg2+-dependent porphobilinogen synthase

Common to the biosynthesis of all known tetrapyrroles is the condensation o f two molecules of 5-aminolevulinic acid to the pyrrole porphobilinogen cat alyzed by the enzyme porphobilinogen synthase (PBGS). Two major classes of PBGS are known. Zn2+-dependent PBGSs are found in mammals, yeast and some b acteria including Escherichia coli, while Mg2+-dependent PBGSs are present mainly in plants and other bacteria. The crystal structure of the Mg2+-depe ndent PEGS from the human pathogen Pseudomonas aeruginosa in complex with t he competitive inhibitor levulinic acid (LA) solved at 1.67 Angstrom resolu tion shows a homooctameric enzyme that consists of four asymmetric dimers. The monomers in each dimer differ from each other by having a "closed" and an "open" active site pocket. In the closed subunit, the active site is com pletely shielded from solvent by a well-defined lid that is partially disor dered in the open subunit. A single molecule of LA binds to a mainly hydrop hobic pocket in each monomer where it is covalently attached via a Schiff b ase to an active site lysine residue. Whereas no metal ions are found in th e active site of both monomers, a single well-defined and highly hydrated M g2+ is present only in the closed form about 14 Angstrom away from the Schi ff base forming nitrogen atom of the active site lysine. We conclude that t he observed differences in the active sites of both monomers might be induc ed by Mg2+-binding to this remote site and propose a structure-based mechan ism for this allosteric Mg2+ in rate enhancement. (C) 1999 Academic Press.