Structural investigations of the environment of the iron-sulfur cluster of the 2-iron ferredoxins.

Ferredoxins are a class of proteins involved in electron transfer reactions. Their distinguishing characteristics is the presence of sulfur-bound (“non-heme”) iron atoms through which electrons are carried. These proteins include the rubredoxins (1), which contain ~50 residues and a single iron, coordinated by 4 cysteines, as shown in Fig. 1A; the “clostridial” ferredoxins (2), possess ~60 residues and 2 active sites of the type shown in Fig. 1B; and the photosynthetic plant and algal ferredoxins (3) which have ~100 residues and an active site containing 2 irons, 2 labile sulfurs, and 4 cysteines. Adrenodoxin (4) and putidaredoxin (5) have similar iron-sulfur centers, though their sequences are quite dissimilar. The sequences of 7 of the 2-iron ferredoxins have been determined (6–12) and are illustrated in Fig. 2. Difference matrices, shown in Fig. 3, reveal a high degree of invariance and conservatism in these sequences. Horsetail (13) and Suringer (14) ferredoxins have also been partially sequenced, and both lack cysteine 18, indicating that cysteines 39, 44, 47, and 77 coordinate the iron-sulfur center.