Structure of the mutant E92K of [2Fe-2S] ferredoxin I from Spinacia oleracea at 1.7 Å resolution

Ferredoxin I (Fd I) from Spinacia oleracea is composed of 97 amino-acid residues and a [2Fe-2S] cluster. The crystal structure of the E92K mutant of Fd I was solved by molecular replacement and refined to an R factor of 19.6% for 11 755 reflections at 1.7 Angstrom resolution. The overall structure and the active centre of spinach Fd is highly conserved with respect to ferredoxins of known structure. The E92K mutation appears to disturb a hydrogen-bond network which stabilizes the loop bearing the [2Fe-2S] cluster. This observation provides a rationale for the reduced electron-transfer efficiency displayed by the E92K mutant. Inspection of the crystal packing reveals that the side chain of Lys92 is engaged in an intermolecular interaction with Asp26 of a symmetry-related molecule. This feature may explain why only the mutant E92K and not wild-type Fd I could be successfully crystallized.