Division of labor in transhydrogenase by alternating proton translocation and hydride transfer

摘要

NADPH/NADP~+ (the reduced form of NADPVnicotinamide adenine dinucleotide phosphate) homeostasis is critical for countering oxidative stress in cells. Nicotinamide nucleotide transhydrogenase (TH), a membrane enzyme present in both bacteria and mitochondria, couples the proton motive force to the generation of NADPH. We present the 2.8 A crystal structure of the transmembrane proton channel domain of TH from Thermus thermophilus and the 6.9 A crystal structure of the entire enzyme (holo-TH). The membrane domain crystallized as a symmetric dimer, with each protomer containing a putative proton channel. The holo-TH is a highly asymmetric dimer with the NADP(H)-binding domain (dⅢ) in two different orientations. This unusual arrangement suggests a catalytic mechanism in which the two copies of dⅢ alternatively function in proton translocation and hydride transfer.