Acts as a sulfur carrier required for molybdopterin biosynthesis (PubMed:22453920). 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 (PubMed:25709896, PubMed:12732628, PubMed:22453920). After interaction with MOCS2B, the sulfur is then transferred to precursor Z to form molybdopterin (PubMed:22453920). {ECO:0000255|HAMAP-Rule:MF_03051, ECO:0000269|PubMed:12732628, ECO:0000269|PubMed:22453920, ECO:0000269|PubMed:25709896}. Catalytic subunit of the molybdopterin synthase complex, a complex that catalyzes the conversion of precursor Z into molybdopterin. Acts by mediating the incorporation of 2 sulfur atoms from thiocarboxylated MOCS2A into precursor Z to generate a dithiolene group (By similarity) (PubMed:12732628, PubMed:15073332, PubMed:25709896). Together with MBIP, inhibits the activity of stress kinase EIF2AK2/PKR; this may suppress JNK activation and subsequent stress-responsive transcription, or suppress eIF2a phosphorylation to favor translation (PubMed:26705305). {ECO:0000255|HAMAP-Rule:MF_03052, ECO:0000269|PubMed:12732628, ECO:0000269|PubMed:15073332, ECO:0000269|PubMed:25709896, ECO:0000269|PubMed:26705305}. This is the function of MOCS2 (molybdenum cofactor synthesis 2, ENSG00000164172).