The organization of the membrane domain and its interaction with the NADP(H)-binding site in proton-translocating transhydrogenase from E. coli.

Proton-translocating nicotinamide nucleotide transhydrogenase is a conformationally driven pump which catalyzes the reversibel reduction of NADP(+) by NADH. Transhydrogenases contain three domains, i.e., the hydrophilic NAD(H)-binding domain I and the NADP(H)-binding domain III, and the hydrophobic domain II containing the proton channel. Domains I and III have been separately expressed and characterized structurally by, e.g. X-ray crystallography and NMR. These domains catalyze transhydrogenation in the absence of domain II. However, due to the absence of the latter domain, the reactions catalyzed by domains I and III differ significantly from those catalyzed by the intact enzyme. Mutagenesis of residues in domain II markedly affects the activity of the intact enzyme. In order to resolve the structure-function relationships of the intact enzyme, and the molecular mechanism of proton translocation, it is therefore essential to establish the structure and function of domain II and its interactions with domains I and III. This review describes some relevant recent results in this field of research.