Acts as a sulfur carrier required for molybdopterin biosynthesis. Component of the molybdopterin synthase complex that catalyzes the conversion of precursor Z into molybdopterin by mediating the incorporation of 2 sulfur atoms into precursor Z to generate a dithiolene group. In the complex, serves as sulfur donor by being thiocarboxylated (-COSH) at its C-terminus by MOCS3. After interaction with MOCS2B, the sulfur is then transferred to precursor Z to form molybdopterin. Cofactor biosynthesis; molybdopterin biosynthesis. Heterotetramer; composed of 2 small (MOCS2A) and 2 large (MOCS2B) subunits. Widely expressed. Highest levels are found in heart and skeletal muscle. Lower levels are present in brain, kidney and pancreas. Very low levels are found in lung and peripheral blood leukocytes. C-terminal thiocarboxylation occurs in 2 steps, it is first acyl-adenylated (-COAMP) via the hesA/moeB/thiF part of MOCS3, then thiocarboxylated (-COSH) via the rhodanese domain of MOCS3. The disease is caused by variants affecting the gene represented in this entry. This protein is produced by a bicistronic gene which also produces the large subunit (MOCS2B) from an overlapping reading frame. Expression of these 2 proteins are related since a mutation that removes the start codon of the small subunit (MOCS2A) also impairs expression of the large subunit (MOCS2B). Belongs to the MoaD family. MOCS2A subfamily.