[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Proton pumping nicotinamide nucleotide transhydrogenase from Escherichia coli contains an alpha subunit with the NAD(H)-binding domain I and a beta subunit with the NADP(H)-binding domain III. The membrane domain (domain II) harbors the proton channel and is made up of the hydrophobic parts of the alpha and beta subunits. The interface in domain II between the alpha and the beta subunits has previously been investigated by cross-linking loops connecting the four transmembrane helices in the alpha subunit and loops connecting the nine transmembrane helices in the beta subunit. However, to investigate the organization of the nine transmembrane helices in the beta subunit, a split was introduced by creating a stop codon in the loop connecting transmembrane helices 9 and 10 by a single mutagenesis step, utilizing an existing downstream start codon. The resulting enzyme was composed of the wild-type alpha subunit and the two new peptides beta1 and beta2. As compared to other split membrane proteins, the new transhydrogenase was remarkably active and catalyzed activities for the reduction of 3-acetylpyridine-NAD(+) by NADPH, the cyclic reduction of 3-acetylpyridine-NAD(+) by NADH (mediated by bound NADP(H)), and proton pumping, amounting to about 50-107% of the corresponding wild-type activities. These high activities suggest that the alpha subunit was normally folded, followed by a concerted folding of beta1 + beta2. Cross-linking of a betaS105C-betaS237C double cysteine mutant in the functional split cysteine-free background, followed by SDS-PAGE analysis, showed that helices 9, 13, and 14 were in close proximity. This is the first time that cross-linking between helices in the same beta subunit has been demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Proton-translocating nicotinamide nucleotide transhydrogenases contain an NAD(H)-binding domain (dI), an NADP(H)-binding domain (dIII) and a membrane domain (dII) with the proton channel. Separately expressed and isolated dIII contains tightly bound NADP(H), predominantly in the oxidized form, possibly representing a so-called "occluded" intermediary state of the reaction cycle of the intact enzyme. Despite a K(d) in the micromolar to nanomolar range, this NADP(H) exchanges significantly with the bulk medium. Dissociated NADP(+) is thus accessible to added enzymes, such as NADP-isocitrate dehydrogenase, and can be reduced to NADPH. In the present investigation, dissociated NADP(H) was digested with alkaline phosphatase, removing the 2'-phosphate and generating NAD(H). Surprisingly, in the presence of dI, the resulting NADP(H)-free dIII catalyzed a rapid reduction of 3-acetylpyridine-NAD(+) by NADH, indicating that 3-acetylpyridine-NAD(+) and/or NADH interacts unspecifically with the NADP(H)-binding site. The corresponding reaction in the intact enzyme is not associated with proton pumping. It is concluded that there is a 2'-phosphate-binding region in dIII that controls tight binding of NADP(H) to dIII, which is not a required for fast hydride transfer. It is likely that this region is the Lys424-Arg425-Ser426 sequence and loops D and E. Further, in the intact enzyme, it is proposed that the same region/loops may be involved in the regulation of NADP(H) binding by an electrochemical proton gradent.
[Show abstract][Hide abstract] ABSTRACT: Proton-translocating nicotinamide nucleotide transhydrogenase is a membrane-bound protein composed of three domains: the hydrophilic NAD(H)-binding domain, the hydrophilic NADP(H)-binding domain, and the hydrophobic membrane domain. The latter harbors the proton channel. In Escherichia coli transhydrogenase, the membrane domain is composed of 13 transmembrane alpha helices, of which especially helices 13 and 14 contain conserved residues. To characterize the roles of the individual residues betaLeu240 to betaSer260 in helix 14, these were mutated as single mutants to cysteines in the cysteine-free background, and in the case of betaGly245, betaGly249, and betaGly252, also to leucines. In addition to the residues forming the helix, residues betaAsn238 and betaAsp239 were also mutated. Except for betaI242C, all mutants were normally expressed, purified, and characterized with respect to, e.g., catalytic activities and proton pumping. The results show that mutation of the conserved glycines betaGly245, betaGly249, and betaGly252, located on the same face of the helix, led to a general inhibition of all activities, especially in the case of betaGly252, suggesting a role of these glycines in helix-helix interactions. In contrast, mutation of the conserved serines betaSer250, betaSer251, and betaSer256 led to enhanced activities of all reactions, including the cyclic reaction which was mediated by bound NADP(H). Mutation of the remaining residues resulted in intermediate inhibitory effects. The results strongly support an important regulatory role of the membrane domain on the NADP(H)-binding site.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The two hydrophilic domains I and III of Escherichia coli transhydrogenase containing the binding sites for NAD(H) and NADP(H), respectively, are located on the cytosolic side of the membrane, whereas the hydrophobic domain II is composed of 13 transmembrane alpha-helices, and is responsible for proton transport. In the present investigation the segment betaC260-betaS266 connecting domain II and III was characterized primarily because of its assumed role in the bioenergetic coupling of the transhydrogenase reaction. Each residue of this segment was replaced by a cysteine in a cysteine-free background, and the mutated proteins analyzed. Except for betaS266C, binding studies of the fluorescent maleimide derivative MIANS to each cysteine in the betaC260-betaR266 region revealed an increased accessibility in the presence of NADP(H) bound to domain III; an opposite effect was observed for betaS266. A betaD213-betaR265 double cysteine mutant was isolated in a predominantly oxidized form, suggesting that the corresponding residues in the wild-type enzyme are closely located and form a salt bridge. The betaS260C, betaK261C, betaA262C, betaM263, and betaN264 mutants showed a pronounced inhibition of proton-coupled reactions. Likewise, several betaR265 mutants and the D213C mutant showed inhibited proton-coupled reactions but also markedly increased values. It is concluded that the mobile hinge region betaC260-betaS266 and the betaD213-betaR265 salt bridge play a crucial role in the communication between the proton translocation/binding events in domain II and binding/release of NADP(H) in domain III.