Biochemical and Crystallographic Characterization of Ferredoxin−NADP+Reductase from Nonphotosynthetic Tissues,

Distinct forms of ferredoxin−NADP+ reductase are expressed in photosynthetic and nonphotosynthetic plant tissues. Both enzymes catalyze electron transfer between NADP(H) and ferredoxin; whereas in leaves the enzyme transfers reducing equivalents from photoreduced ferredoxin to NADP+ in photosynthesis, in roots it has the opposite physiological role, reducing ferredoxin at the expense of NADPH mainly for use in nitrate assimilation. Here, structural and kinetic properties of a nonphotosynthetic isoform were analyzed to define characteristics that may be related to tissue-specific function. Compared with spinach leaf ferredoxin−NADP+ reductase, the recombinant corn root isoform showed a slightly altered absorption spectrum, a higher pI, a >30-fold higher affinity for NADP+, greater susceptibility to limited proteolysis, and an ∼20 mV more positive redox potential. The 1.7 Å resolution crystal structure is very similar to the structures of ferredoxin−NADP+ reductases from photosynthetic tissues. Four distinct structural features of this root ferredoxin−NADP+ reductases are an alternate conformation of the bound FAD molecule, an alternate path for the amino-terminal extension, a disulfide bond in the FAD-binding domain, and changes in the surface that binds ferredoxin.