The NADP(H)-binding component (dIII) of human heart transhydrogenase: crystallization and preliminary crystallographic analysis

Transhydrogenase is a membrane protein which uses the energy of the proton motive force to drive the reduction of NADP⁺ by NADH. The enzyme has three domains: dII spans the membrane, while dI and dIII protrude from the membrane and contain the binding sites for NAD(H) and NADP(H), respectively. DIII from human heart transhydrogenase has been expressed in Escherichia coli. The purified protein has been crystallized with bound NADP⁺ using the hanging-drop vapour-diffusion method with ammonium sulfate as a precipitant. The crystals belong to the tetragonal space group P4₁22 or P4₃22, with unit-cell parameters a = b = 58.1, c = 251.0 Å. A 2.1 Å resolution native data set has been collected with an R_merge of 6.8%.