Structural basis of the catalytic role of Glu301 in Anabaena PCC 7119 ferredoxin-NADP+ reductase revealed by x-ray crystallography.

The three-dimensional crystal structure of the Glu301Ala site-directed mutant of ferredoxin-NADP+ reductase from Anabaena PCC 7119 has been determined at 1.8A resolution by x-ray diffraction. The overall folding of the Glu301Ala FNR mutant shows no significant differences with respect to that of the wild-type enzyme. However, interesting conformational changes are detected in the side chain of another glutamate residue, Glu139, which now points towards the FAD cofactor in the active center cavity. The new conformation of the Glu139 side chain is stabilized by a network of five hydrogen bonds to several water molecules, which seem to hold the carboxylate side chain in a rather fixed position. This interacting network connects the Glu139 side chain to the Ser80 side chain through a series of three water molecules. These observations are discussed in terms of the reactivity of Glu301Ala ferredoxin-NADP+ reductase towards its substrates, and the role of Glu301 in the catalysis is re-examined. Moreover, a structural explanation of the different reoxidation properties of this mutant is given on the basis of the reported structure by modeling the hypothetical flavin C(4a)-hydroperoxide intermediate. The model shows that the distal oxygen of the peroxide anion could be in an appropriate situation to act as the proton donor in the reoxidation process.