Transhydrogenase catalyzes the reduction of NADP+ by NADH coupled to the translocation of protons across a membrane. The polypeptide composition of the enzyme in Rhodospirillum rubrum is unique in that the NAD(H)-binding domain (called Ths) exists as a separate polypeptide. Ths was expressed in Escherichia coli and purified. The binding of nucleotide substrates and analogues to Ths was examined by one-dimensional proton nuclear magnetic resonance (NMR) spectroscopy and by measuring the quenching of fluorescence of its lone Trp residue. NADH and reduced acetylpyridine adenine dinucleotide bound tightly to Ths, whereas NAD+, oxidized acetylpyridine adenine dinucleotide, deamino-NADH, 5'-AMP and adenosine bound less tightly. Reduced nicotinamide mononucleotide, NADPH and 2'-AMP bound only very weakly to Ths. The difference in the binding affinity between NADH and NAD+ indicates that there may be an energy requirement for the transfer of reducing equivalents into this site in the complete enzyme under physiological conditions. Earlier results had revealed a mobile loop at the surface of Ths (Diggle, C., Cotton, N. P. J., Grimley, R. L., Quirk, P. G., Thomas, C. M., and Jackson, J. B. (1995) Eur. J. Biochem. 232, 315-326); the loop loses mobility when Ths binds nucleotide; the reaction involves two steps. This was more clearly evident, even for tight-binding nucleotides, when experiments were carried out at higher temperatures (37 degrees C), where the resonances of the mobile loop were substantially narrower. The binding of adenosine was sufficient to initiate loop closure; the presence of a reduced nicotinamide moiety in the dinucleotide apparently serves to tighten the binding. Two-dimensional 1H NMR spectroscopy of the Ths-5'-AMP complex revealed nuclear Overhauser effect interactions between protons of amino acid residues in the mobile loop (including those in a Tyr residue) and the nucleotide. This suggests that, in the complex, the loop has closed down to within 0.5 nm of the nucleotide.
