Biochemical and crystallographic characterization of ferredoxin-NADP(+) reductase from nonphotosynthetic tissues

Distinct forms of ferredoxin-NADP(+) reductase are expressed in photosynthe tic and nonphotosynthetic plant tissues. Both enzymes catalyze electron tra nsfer between NADP(H) and ferredoxin; whereas in leaves the enzyme transfer s reducing equivalents from photoreduced ferredoxin to NADP(+) in photosynt hesis, in roots it has the opposite physiological role, reducing ferredoxin at the expense of NADPH mainly for use in nitrate assimilation. Here, stru ctural 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 p roteolysis, and an similar to 20 mV more positive redox potential. The 1.7 Angstrom resolution crystal structure is very similar to the structures of ferredoxin-NADP(+) reductases from photosynthetic tissues. Four distinct st ructural features of this root ferredoxin-NADP(+) reductases are an alterna te 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.