The molybdopterin synthase complex is a heterotetrameric complex of two small and two large subunits. It catalyses sulfur transfer from the small, sulfur carrier subunit to cyclic pyranopterin monophosphate (aka 'precursor Z'), thereby converting it into molybdopterin. This is the second step of molybdenum cofactor (Moco) biosynthesis. (Adapted from
PMID:16669776.)