Structures of TtuA in complex with the 4Fe-4S cluster and AMPPNP. (A) Overall structure of the Z-shaped TtuA dimer. Two protomers are shown in cyan and blue. The PP-loop is colored orange. The bound [4Fe-4S] cluster and AMPPNP are shown as balls and sticks (yellow indicates sulfate; brown, iron; green, carbon; blue, nitrogen; red, oxygen; pink, phosphorous). (B) Close-up view of the catalytic center of TtuA. The Fe-S cluster and AMPPNP are shown as balls and sticks. The PP-loop binding with AMPPNP is colored orange.