Crystal structure of molybdopterin synthase and its evolutionary relationship to ubiquitin activation

Molybdenum cofactor (Moco) biosynthesis is an evolutionarily conserved path way present in eubacteria, archaea and eukaryotes, including humans. Geneti c deficiencies of enzymes involved in Moco biosynthesis in humans lead to a severe and usually fatal disease. Moco contains a tricyclic pyranopterin, termed molybdopterin (MPT), that bears the cis-dithiolene group responsible for molybdenum ligation. The dithiolene group of MPT is generated by MPT s ynthase, which consists of a large and small subunits. The 1.45 Angstrom re solution crystal structure of MPT synthase reveals a heterotetrameric prote in in which the C-terminus of each small subunit is inserted into a large s ubunit to form the active site. In the activated form of the enzyme this C- terminus is present as a thiocarboxylate. In the structure of a covalent co mplex of MPT synthase, an isopeptide bond is present between the C-terminus of the small subunit and a Lys side chain in the large subunit. The strong structural similarity between the small subunit of MPT synthase and ubiqui tin provides evidence for the evolutionary antecedence of the Moco biosynth etic pathway to the ubiquitin dependent protein degradation pathway.