The crystal structure of the C24A mutant of Azotobacter vinelandii 7Fe ferredoxin (FdI) has been solved and refined at 2.0-A resolution. The structure is isomorphous to native FdI except at the site of mutation where A24 moves toward the [4Fe-4S] cluster. In spite of this inefficient packing results: three of five van der Waals contacts from the S gamma of C24 in native FdI are lost and the remaining two become longer. Consequently, the [4Fe-4S] cluster is either disordered or has a higher temperature factor (B factor) compared to the rest of the C24A FdI molecule. In addition, the entire C24A FdI structure has a higher overall B factor than native FdI. Therefore, in comparison to native FdI, the C24A mutant is isomorphous but exhibits large differences in B factor, especially at the [4Fe-4S] cluster. In contrast, the C20A FdI structure (Martin, A. G., Burgess, B. K., Stout, C. D., Cash, V. L., Dean, D. R., Jensen, G. M., and Stephens, P. J. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 598-602), which contains large structural rearrangements in the vicinity of the [4Fe-4S] cluster, exhibits essentially no change in B factor. The conformational change observed at residue 24 is similar in both C24A and C20A FdI structures. The solvent accessibility of the Fe atoms in the [3Fe-4S] and [4Fe-4S] clusters is similar in C24A, C20A, and native FdI.