In a structural model, the sequence also contains a hydrophobic N-terminus which can be predicted to loop back and forth across the membrane probably three times. The small subunit has a single histidine which is conserved across species, and this residue has been proposed to cooperate with the large subunit to form one of the heme binding sites. This arrangement places the proline-rich C-terminus on the cytosolic side of the membrane where it can interact with p47phox.