NMR-detected hydrogen/deuterium (H/D) exchange of amide protons is a powerful way for investigating the residuebased conformational stability and dynamics of proteins in solution. Maize ferredoxin-NADP ؉ reductase (FNR) is a relatively large protein with 314 amino acid residues, consisting of flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADP ؉ )-binding domains. To address the structural stability and dynamics of FNR, H/D exchange of amide protons was performed using heteronuclear NMR at pD r values 8.0 and 6.0, physiologically relevant conditions mimicking inside of chloroplasts. At both pD r values, the exchange rate varied widely depending on the residues. The profiles of protected residues revealed that the highly protected regions matched well with the hydrophobic cores suggested from the crystal structure, and that the NADP ؉ -binding domain can be divided into two subdomains. The global stability of FNR obtained by H/D exchange with NMR was higher than that by chemical denaturation, indicating that H/D exchange is especially useful for analyzing the residue-based conformational stability of large proteins, for which global unfolding is mostly irreversible. Interestingly, more dynamic conformation of the C-terminal subdomain of the NADP ؉ -binding domain at pD r 8.0, the daytime pH in chloroplasts, than at pD r 6.0 is likely to be involved in the increased binding of NADP ؉ for elevating the activity of FNR. In light of photosynthesis, the present study provides the first structure-based relationship of dynamics with function for the FNR-type family in solution.Protein conformations as shown by x-ray crystallography or nuclear magnetic resonance spectroscopy are virtually dynamic entities over various time ranges in solution (1, 2). Clarifying such conformational motions at the level of the atom or residue is essential for understanding the structural stabilities of proteins and the relationships between protein structures and functions (3-5). The hydrogen/deuterium (H/D) 4 exchange of amide protons in backbones has become an important way to address the motions of a protein from small or relatively large scale motions involved in biological functions such as binding of a substrate or releasing a product to much more large scale motions like a global unfolding process (4, 6). Among several approaches, the H/D exchange combined with heteronuclear NMR spectroscopy is the most convenient and powerful way because it can provide residue-specific information for most residues (7). For many globular proteins, this approach has identified protected cores, which are often composed of secondary structures buried inside the molecules and the cooperative interactions with partner molecules (4, 8), leading to allostery (9). Detailed analyses of exchanges in the native state of cytochrome c in the presence of various concentrations of denaturants suggested a pathway to unfolding, in which groups of secondary structural elements (i.e. foldons) are unfolded sequentially through distin...