The tetrahydropyranopterin structure of the sulfur-free and metal-free molybdenum cofactor precursor.

The molybdenum cofactor (Moco), a highly conserved pterin compound coordinating molybdenum (Mo), is required for the activity of all Mo-dependent enzymes with the exception of nitrogenase. Moco is synthesized by a unique and evolutionary old multi-step pathway with two intermediates identified so far, the sulfur-free and metal-free pterin derivative precursor Z and molybdopterin, a pterin with an enedithiolate function essential for Mo ligation. The latter pterin component is believed to form a tetrahydropyranopterin similar to the one found for Moco in the crystal structure of Mo as well as tungsten (W) enzymes. Here we report the spectroscopic characterization and structure elucidation of precursor Z purified from Escherichia coli overproducing MoaA and MoaC, two proteins essential for bacterial precursor Z synthesis. We have shown that purified precursor Z is as active as precursor Z present in E. coli cell extracts, demonstrating that no modifications during the purification procedure have occurred. High resolution electrospray ionization mass spectrometry afforded a [M + H]+ ion compatible with a molecular formula of C10H15N5O8P. Consequently 1H NMR spectroscopy not allowed structural characterization of the molecule but confirmed that this intermediate undergoes direct oxidation to the previously well characterized non-productive follow-up product compound Z. The 1H chemical shift and coupling constant data are incompatible with previous structural proposals and indicate that precursor Z already is a tetrahydropyranopterin system and carries a geminal diol function in the C1' position.