Structural studies of molybdopterin synthase provide insights into its catalytic mechanism

Molybdenum cofactor biosynthesis is an evolutionarily conserved pathway present in eubacteria, archaea, and eukaryotes, including humans. Genetic deficiencies of enzymes involved in cofactor biosynthesis in humans lead to a severe and usually fatal disease. The molybdenum cofactor contains a tricyclic pyranopterin, termed molybdopterin, that bears the cis-dithiolene group responsible for molybdenum ligation. The dithiolene group of molybdopterin is generated by molybdopterin synthase, which consists of a large (MoaE) and small (MoaD) subunit. The crystal structure of molybdopterin synthase revealed a heterotetrameric enzyme in which the C terminus of each MoaD subunit is deeply inserted into a MoaE subunit to form the active site. In the activated form of the enzyme, the MoaD C terminus is present as a thiocarboxylate. The present study identified the position of the thiocarboxylate sulfur by exploiting the anomalous signal originating from the sulfur atom. The structure of molybdopterin synthase in a novel crystal form revealed a binding pocket for the terminal phosphate of molybdopterin, the product of the enzyme, and suggested a binding site for the pterin moiety present in precursor Z and molybdopterin. Finally, the crystal structure of the MoaE homodimer provides insights into the conformational changes accompanying binding of the MoaD subunit.