4.

GRE small subunits. (A) Overall BSS architecture (PDB ID: 9PZJ) showing β (dark sea green), γ (sea green), and GRD (green). (B) Overall architecture of the HPAD heterotetramer (α₂γ₂) (PDB ID = 2YAJ) showing γ (steel blue) and GRD (dark cyan). (C) Overall MASS architecture (PDB ID: 9O8U) showing β (indigo), γ (medium purple), δ (rebecca purple), and GRD (purple). (D) HiPIP protein (PDB ID: 1ISU). (E) Rubredoxin protein (PDB ID: 6RXN). (F) BSS β and γ subunits harbor [4Fe-4S] clusters. (G) HPAD γ subunit harbors two [4Fe-4S] clusters, one ligated by four Cys, and one ligated by three Cys and one His. (H) MASS β and γ each harbor a [4Fe-4S] cluster, whereas MASS δ harbors a mononuclear nonheme Fe cofactor. All Fe atoms are colored orange. All S atoms are colored yellow.