Siderophore interacting proteins share the domain structure of the ferredoxin reductase like family. Siderophores are produced in various bacteria (and some plants) to extract iron from hosts. Binding constants are high, so iron can be pilfered from transferrin and lactoferrin for bacterial uptake, contributing to pathogen virulence. Ferredoxin reductase (FNR), an FAD and NAD(P) binding protein, was intially identified as a chloroplast reductase activity, catalyzing the electron transfer from reduced iron-sulfur protein ferredoxin to NADP+ as the final step in the electron transport mechanism of photosystem I. FNR transfers electrons from reduced ferredoxin to FAD (forming FADH2 via a semiquinone intermediate) and then transfers a hydride ion to convert NADP+ to NADPH. FNR has since been shown to utilize a variety of electron acceptors and donors and has a variety of physiological functions including nitrogen assimilation, dinitrogen fixation, steroid hydroxylation, fatty acid metabolism, oxygenase activity, and methane assimilation in a variety of organisms. FNR has an NAD(P)-binding sub-domain of the alpha/beta class and a discrete (usually N-terminal) flavin sub-domain which vary in orientation with respect to the NAD(P) binding domain. The N-terminal moeity may contain a flavin prosthetic group (as in flavoenzymes) or use flavin as a substrate. Because flavins such as FAD can exist in oxidized, semiquinone (one-electron reduced), or fully reduced hydroquinone forms, FNR can interact with one and two electron carriers. FNR has a strong preference for NADP(H) vs NAD(H).