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

Ferredoxin I (Fd I) from Spinacia oleracea is composed of 97 amino-acid res idues and a [2Fe-2S] cluster. The crystal structure of the E92K mutant of F d 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 a nd the active centre of spinach Fd is highly conserved with respect to ferr edoxins 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 efficien cy displayed by the E92K mutant. Inspection of the crystal packing reveals that the side chain of Lys92 is engaged in an intermolecular interaction wi th 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.