at the carboxyl terminus of the protein and contains an alpha helix-beta strand fold. This terminal domain is where the NADP binds. The active site for the enzyme occurs at the interface between the two domains. Binding of the enzyme to the thylakoid membrane involves a polyproline type II helix created between two FNR monomers and several proline rich integral membrane proteins. As of late 2007, 54 structures had been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , and . Function Ferredoxin: NADP reductase is the last enzyme in the transfer of electrons during photosynthesis from photosystem I to NADPH. The NADPH is then used as a reducing equivalent in the reactions of the Calvin cycle. Electron cycling from ferredoxin to NADPH only occurs in the light in part because FNR activity is inhibited in the dark. In nonphotosynthetic organisms, the FNR primarily works in reverse to provide reduced ferredoxin for various metabolic pathways. These pathways include nitrogen fixation, terpenoid biosynthesis, steroid metabolism, oxidative stress response, and iron–sulfur protein biogenesis. FNR is a soluble protein that is found both free in the chloroplast stroma and bound to the thylakoid membrane. This binding occurs opposite to the active site of the enzyme and does not seem to affect the structure of the active site or have a significant impact on the enzyme's activity. When bound to the thylakoid membrane, the enzyme exists as a dimer, but when it is free in the stroma, it is monomeric. The binding of the FNR to the integral membrane proteins on the thylakoid membrane is enhanced under acidic conditions, so recruitment and binding of FNR to the thylakoid membrane may be a method of storing and stabilizing the enzyme in the dark when photosynthesis is not occurring. The chloroplast stroma varies from being slightly acidic in the dark to more alkaline in the light. Therefore, in the dark, more FNRs would be recruited and bound to the thylakoid membrane, and in the light, more FNRs would dissociate from the membrane and be free in the stroma. Evolution Ferredoxin NADP reductases