The presence of an aqueous cavity in the proton-pumping pathway of the pyridine nucleotide transhydrogenase of Escherichia coli is suggested by the reaction of the enzyme with sulfhydryl inhibitors

Energy transfer between the nicotinamide nucleotide transhydrogenase and ATP synthase of Escherichia coli

Membrane bound nicotinamide nucleotide transhydrogenase (TH) catalyses the hydride transfer from NADH to NADP⁺. Under physiological conditions, this reaction is endergonic and must be energized by the pmf, coupled to transmembrane proton transport. Recent structures of transhydrogenase holoenzymes suggest new mechanistic details, how the long-distance coupling between hydride transfer in the peripheral nucleotide binding sites and the membrane-localized proton transfer occurs that now must be tested experimentally. Here, we provide protocols for the efficient expression and purification of the Escherichia coli transhydrogenase and its reconstitution into liposomes, alone or together with the Escherichia coli F₁F₀ ATP synthase. We show that E. coli transhydrogenase is a reversible enzyme that can also work as a NADPH-driven proton pump. In liposomes containing both enzymes, NADPH driven H⁺-transport by TH is sufficient to instantly fuel ATP synthesis, which adds TH to the pool of pmf generating enzymes. If the same liposomes are energized with ATP, NADPH production by TH is stimulated > sixfold both by a pH gradient or a membrane potential. The presented protocols and results reinforce the tight coupling between hydride transfer in the peripheral nucleotide binding sites and transmembrane proton transport and provide powerful tools to investigate their coupling mechanism.

Proton-translocating transhydrogenase: an update of unsolved and controversial issues

Proton-translocating transhydrogenases, reducing NADP(+) by NADH through hydride transfer, are membrane proteins utilizing the electrochemical proton gradient for NADPH generation. The enzymes have important physiological roles in the maintenance of e.g. reduced glutathione, relevant for essentially all cell types. Following X-ray crystallography and structural resolution of the soluble substrate-binding domains, mechanistic aspects of the hydride transfer are beginning to be resolved. However, the structure of the intact enzyme is unknown. Key questions regarding the coupling mechanism, i.e., the mechanism of proton translocation, are addressed using the separately expressed substrate-binding domains. Important aspects are therefore which functions and properties of mainly the soluble NADP(H)-binding domain, but also the NAD(H)-binding domain, are relevant for proton translocation, how the soluble domains communicate with the membrane domain, and the mechanism of proton translocation through the membrane domain.

Cross-linking of transmembrane helices in proton-translocating nicotinamide nucleotide transhydrogenase from Escherichia coli: implications for the structure and function of the membrane domain

Proton-pumping nicotinamide nucleotide transhydrogenase from Escherichia coli contains an alpha and a beta subunit of 54 and 49 kDa, respectively, and is made up of three domains. Domain I (dI) and III (dIII) are hydrophilic and contain the NAD(H)- and NADP(H)-binding sites, respectively, whereas the hydrophobic domain II (dII) contains 13 transmembrane alpha-helices and harbours the proton channel. Using a cysteine-free transhydrogenase, the organization of dII and helix-helix distances were investigated by the introduction of one or two cysteines in helix-helix loops on the periplasmic side. Mutants were subsequently cross-linked in the absence and presence of diamide and the bifunctional maleimide cross-linker o-PDM (6 A), and visualized by SDS-PAGE. In the alpha(2)beta(2) tetramer, alphabeta cross-links were obtained with the alphaG476C-betaS2C, alphaG476C-betaT54C and alphaG476C-betaS183C double mutants. Significant alphaalpha cross-links were obtained with the alphaG476C single mutant in the loop connecting helix 3 and 4, whereas betabeta cross-links were obtained with the betaS2C, betaT54C and betaS183C single mutants in the beginning of helix 6, the loop between helix 7 and 8 and the loop connecting helix 11 and 12, respectively. In a model based on 13 mutants, the interface between the alpha and beta subunits in the dimer is lined along an axis formed by helices 3 and 4 from the alpha subunit and helices 6, 7 and 8 from the beta subunit. In addition, helices 2 and 4 in the alpha subunit together with helices 6 and 12 in the beta subunit interact with their counterparts in the alpha(2)beta(2) tetramer. Each beta subunit in the alpha(2)beta(2) tetramer was concluded to contain a proton channel composed of the highly conserved helices 9, 10, 13 and 14.

Essential glycine in the proton channel of Escherichia coli transhydrogenase

The nicotinamide nucleotide transhydrogenases of mitochondria and bacteria are proton pumps that couple hydride ion transfer between NAD(H) and NADP(H) bound, respectively, to extramembranous domains I and III, to proton translocation by the membrane-intercalated domain II. Previous experiments have established the involvement of three conserved domain II residues in the proton pumping function of the enzyme: His91, Ser139, and Asn222, located on helices 9, 10, and 13, respectively. Eight highly conserved domain II glycines in helices 9, 10, 13, and 14 were mutated to alanine, and the mutant enzymes were assayed for hydride transfer between domains I and III and for proton translocation by domain II. One of the glycines on helix 14, Gly252, was further mutated to Cys, Ser, Thr, and Val, expression levels of the mutant enzymes were evaluated, and each was purified and assayed. The results show that Gly252 is essential for function and support a model for the proton channel composed of helices 9, 10, 13, and 14. Gly252 would allow spatial proximity of His91, Ser139, and Asn222 for proton conductance within the channel. Gly252 mutants are distinguished by high levels of cyclic transhydrogenation activity in the absence of added NADP(H) and by complete loss of proton pumping activity. The purified G252A mutant has <1% proton translocation and reverse transhydrogenation activity, retains 0.9 mol of NADP(H) per domain III, and has 96% intrinsic cyclic transhydrogenation activity, which does not exceed 100% upon the addition of NADP(H). These properties imply that Gly252 mutants exhibit a native-like domain II conformation while blocking proton translocation and coupled exchange of NADP(H) in domain III.

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.

Identification of cysteine residues responsible for oxidative cross-linking and chemical inhibition of human nucleoside-triphosphate diphosphohydrolase 3

Cysteine-to-serine mutations were constructed to test the functional and structural significance of the three non-extracellular cysteine residues in ecto-nucleoside-triphosphate diphosphohydrolase 3 (eNTPDase3). None of these cysteines were found to be essential for enzyme activity. However, Cys(10), located on the short N-terminal cytoplasmic tail, was found to be responsible for dimer formation occurring via oxidation during membrane preparation as well as for dimer cross-linking resulting from exogenously added sulfhydryl-specific cross-linking agents. The resistance to further cross-linking of these dimers into higher order oligomers by lysine-specific cross-linkers suggests that this enzyme may form its native tetrameric structure as a "dimer of dimers" with nonequivalent interactions between subunits. Cys(501), located in the hydrophobic C-terminal membrane-spanning domain of eNTPDase3, was found to be the site of chemical modification by a sulfhydryl-specific reagent, p-chloromercuriphenylsulfonic acid (pCMPS), leading to inhibition of enzyme activity. The effect of pCMPS was negligible after dissociation of the enzyme into monomers by Triton X-100, suggesting that the mechanism of inhibition is dependent on the oligomeric structure. Because Cys(501) is accessible for modification by the membrane-impermeant reagent pCMPS, we hypothesize that eNTPDase3 (and possibly other eNTPDases) contains a water-filled crevice allowing access of water and hydrophilic compounds to at least part of the protein's C-terminal membrane-spanning helix.

Characterization of mutants of beta histidine91, beta aspartate213, and beta asparagine222, possible components of the energy transduction pathway of the proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli

The roles of three residues (betaHis91, betaAsp213, and betaAsn222) implicated in energy transduction in the membrane-spanning domain II of the proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli have been examined using site-directed mutagenesis. All mutations affected transhydrogenation and proton pumping activities, although to various extents. Replacing betaHis91 or betaAsn222 of domain II by the basic residues lysine or arginine resulted in occlusion of NADP(H) at the NADP(H)-binding site of domain III. This was not seen with betaD213K or betaD213R mutants. It is suggested that betaHis91 and betaAsn222 interact with betaAsp392, a residue probably involved in initiating conformational changes at the NADP(H)-binding site in the normal catalytic cycle of the enzyme (M. Jeeves et al. (2000) Biochim. Biophys. Acta 1459, 248-257). The introduced positive charges in the betaHis91 and betaAsn222 mutants might stabilize the carboxyl group of betaAsp392 in its anionic form, thus locking the NADP(H)-binding site in the occluded conformation. In comparison with the nonmutant enzyme, and those of mutants of betaAsp213, most mutant enzymes at betaHis91 and betaAsn222 bound NADP(H) more slowly at the NADP(H)-binding site. This is consistent with the effect of these two residues on the binding site. We could not demonstrate by mutation or crosslinking or through the formation of eximers with pyrene maleimide that betaHis91 and betaAsn222 were in proximity in domain II.

Intersubunit crosslinking of the heterotetrameric proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli defines intersubunit contacts between transmembrane helices of the beta subunits

The proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli is composed of two types of subunits, alpha and beta, organized as an alpha(2)beta(2) tetramer. The protein contains three recognizable domains, of which domain II is the transmembrane region of the molecule containing the pathway for proton translocation. Domain II is composed of four transmembrane helices at the carboxyl-terminus of the alpha subunit and nine transmembrane helices at the amino-terminal region of the beta subunit. We have introduced pairs of cysteine residues into all of the loops connecting the transmembrane helices of domain II of the beta subunit. Crosslinking between the two beta subunits of the tetramer was induced spontaneously, or by treatment with cupric 1,10-phenanthrolinate or o-phenylenedimaleimide. Crosslinks between pairs of betaA114C, betaS183C, and betaA262C residues were observed, suggesting that pairs of domain II transmembrane helices 11, 12, and 14 were in proximity. These results, together with previous data (Bragg and Hou (2000) Biochem. Biophys. Res. Commun. 273, 955-959) suggest that the transhydrogenase tetramer is formed by apposition of alpha(2) and beta(2) dimers. Crosslinking between pairs of cysteine residues in the same beta subunit was not observed, possibly because the interhelical loops of the domain II region of the beta subunit were too short to allow correct orientation of the sulfhydryl groups for crosslinking.