Interdomain hydride transfer in proton-translocating transhydrogenase

NADPH-generating systems in bacteria and archaea

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an essential electron donor in all organisms. It provides the reducing power that drives numerous anabolic reactions, including those responsible for the biosynthesis of all major cell components and many products in biotechnology. The efficient synthesis of many of these products, however, is limited by the rate of NADPH regeneration. Hence, a thorough understanding of the reactions involved in the generation of NADPH is required to increase its turnover through rational strain improvement. Traditionally, the main engineering targets for increasing NADPH availability have included the dehydrogenase reactions of the oxidative pentose phosphate pathway and the isocitrate dehydrogenase step of the tricarboxylic acid (TCA) cycle. However, the importance of alternative NADPH-generating reactions has recently become evident. In the current review, the major canonical and non-canonical reactions involved in the production and regeneration of NADPH in prokaryotes are described, and their key enzymes are discussed. In addition, an overview of how different enzymes have been applied to increase NADPH availability and thereby enhance productivity is provided.

Metabolic engineering and transhydrogenase effects on NADPH availability in Escherichia coli

The synthesis of several industrially useful compounds are cofactor-dependent, requiring reducing equivalents like NADPH in enzymatic reactions leading up to the synthesis of high-value compounds like polymers, chiral alcohols, and antibiotics. However, NADPH is costly and has limited intracellular availability. This study focuses on the study of the effect of the two transhydrogenase enzymes of Escherichia coli, PntAB and UdhA (SthA) on reducing equivalents-dependent biosynthesis. The production of (S)-2-chloropropionate from 2-chloroacrylate is used as a model system for monitoring NADPH availability because 2-haloacrylate reductase, the enzyme catalyzing the one-step conversion to (S)-2-chloropropionate in the synthesis pathway, requires NADPH as a cofactor. Results suggest that the presence of UdhA increases product yield and NADPH availability while the presence of PntAB has the opposite effect. A maximum product yield of 1.4 mol product/mol glucose was achieved aerobically in a pnt-deletion strain with udhA overexpression, a 150% improvement over the wild-type control strain.

A review of the binding-change mechanism for proton-translocating transhydrogenase

Proton-translocating transhydrogenase is found in the inner membranes of animal mitochondria, and in the cytoplasmic membranes of many bacteria. It catalyses hydride transfer from NADH to NADP(+) coupled to inward proton translocation. Evidence is reviewed suggesting the enzyme operates by a "binding-change" mechanism. Experiments with Escherichia coli transhydrogenase indicate the enzyme is driven between "open" and "occluded" states by protonation and deprotonation reactions associated with proton translocation. In the open states NADP(+)/NADPH can rapidly associate with, or dissociate from, the enzyme, and hydride transfer is prevented. In the occluded states bound NADP(+)/NADPH cannot dissociate, and hydride transfer is allowed. Crystal structures of a complex of the nucleotide-binding components of Rhodospirillum rubrum transhydrogenase show how hydride transfer is enabled and disabled at appropriate steps in catalysis, and how release of NADP(+)/NADPH is restricted in the occluded state. Thermodynamic and kinetic studies indicate that the equilibrium constant for hydride transfer on the enzyme is elevated as a consequence of the tight binding of NADPH relative to NADP(+). The protonation site in the translocation pathway must face the outside if NADP(+) is bound, the inside if NADPH is bound. Chemical shift changes detected by NMR may show where alterations in protein conformation resulting from NADP(+) reduction are initiated. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Zinc ions selectively inhibit steps associated with binding and release of NADP(H) during turnover of proton-translocating transhydrogenase

Transhydrogenase couples the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. In membrane vesicles from Escherichia coli and Rhodospirillum rubrum, the transhydrogenase reaction (measured in the direction driving inward proton translocation) was inhibited by Zn(2+) and Cd(2+). However, depending on pH, the metal ions either had no effect on, or stimulated, "cyclic" transhydrogenation. They must, therefore, interfere specifically with steps involving binding/release of NADP(+)/NADPH: the steps thought to be associated with proton translocation. It is suggested that Zn(2+) and Cd(2+) bind in the proton-transfer pathway and block inter-conversion of states responsible for changing NADP(+)/NADPH binding energy.

Conformational diversity in NAD(H) and interacting transhydrogenase nicotinamide nucleotide binding domains

Transhydrogenase (TH) couples direct and stereospecific hydride transfer between NAD(H) and NADP(H), bound within soluble domains I and III, respectively, to proton translocation across membrane bound domain II. The cocrystal structure of Rhodospirillum rubrum TH domains I and III has been determined in the presence of limiting NADH, under conditions in which the subunits reach equilibrium during crystallization. The crystals contain three heterotrimeric complexes, dI(2)dIII, in the asymmetric unit. Multiple conformations of loops and side-chains, and NAD(H) cofactors, are observed in domain I pertaining to substrate/product exchange, and highlighting electrostatic interactions during the hydride transfer. Two interacting NAD(H)-NADPH pairs are observed where alternate conformations of the NAD(H) phosphodiester and conserved arginine side-chains are correlated. In addition, the stereochemistry of one NAD(H)-NADPH pair approaches that expected for nicotinamide hydride transfer reactions. The cocrystal structure exhibits non-crystallographic symmetry that implies another orientation for domain III, which could occur in dimeric TH. Superposition of the "closed" form of domain III (PDB 1PNO, chain A) onto the dI(2)dIII complex reveals a severe steric conflict of highly conserved loops in domains I and III. This overlap, and the overlap with a 2-fold related domain III, suggests that motions of loop D within domain III and of the entire domain are correlated during turnover. The results support the concept that proton pumping in TH is driven by the difference in binding affinity for oxidized and reduced nicotinamide cofactors, and in the absence of a difference in redox potential, must occur through conformational effects.

The soluble and membrane-bound transhydrogenases UdhA and PntAB have divergent functions in NADPH metabolism of Escherichia coli

Pentose phosphate pathway and isocitrate dehydrogenase are generally considered to be the major sources of the anabolic reductant NADPH. As one of very few microbes, Escherichia coli contains two transhydrogenase isoforms with unknown physiological function that could potentially transfer electrons directly from NADH to NADP+ and vice versa. Using defined mutants and metabolic flux analysis, we identified the proton-translocating transhydrogenase PntAB as a major source of NADPH in E. coli. During standard aerobic batch growth on glucose, 35-45% of the NADPH that is required for biosynthesis was produced via PntAB, whereas pentose phosphate pathway and isocitrate dehydrogenase contributed 35-45% and 20-25%, respectively. The energy-independent transhydrogenase UdhA, in contrast, was essential for growth under metabolic conditions with excess NADPH formation, i.e. growth on acetate or in a phosphoglucose isomerase mutant that catabolized glucose through the pentose phosphate pathway. Thus, both isoforms have divergent physiological functions: energy-dependent reduction of NADP+ with NADH by PntAB and reoxidation of NADPH by UdhA. Expression appeared to be modulated by the redox state of cellular metabolism, because genetic and environmental manipulations that increased or decreased NADPH formation down-regulated pntA or udhA transcription, respectively. The two transhydrogenase isoforms provide E. coli primary metabolism with an extraordinary flexibility to cope with varying catabolic and anabolic demands, which raises two general questions: why do only a few bacteria contain both isoforms, and how do other organisms manage NADPH metabolism?

Properties of a proton-translocating nicotinamide nucleotide transhydrogenase from Escherichia coli with alpha and beta subunits linked through fused transmembrane helices

Proton-translocating nicotinamide nucleotide transhydrogenase from Escherichia coli is composed of an alpha and a beta subunit, whereas the homologues mitochondrial enzyme contains a single polypeptide. As compared to the latter transhydrogenase, using a 14-helix model for its membrane topology, the point of fusion is between the transmembrane helices 4 and 6 where the fusion linker provides the extra transmembrane helix 5. In order to clarify the potential role of this extra helix/linker, the alpha and the beta subunits were fused using three connecting peptides of different lengths, one (pAX9) involving essentially a direct coupling, a second (pKM) with a linking peptide of 18 residues, and a third (pKMII) with a linking peptide of 32 residues, as compared to the mitochondrial extra peptide of 27 residues. The results demonstrate that the plasma membrane-bound and purified pAX9 enzyme with the short linker was partly misfolded and strongly inhibited with regard to both catalytic activities and proton translocation, whereas the properties of pKM and pKMII with longer linkers were similar to those of wild-type E. coli transhydrogenase but partly different from those of the mitochondrial enzyme although pKMII generally gave higher activities. It is concluded that a mitochondrial-like linking peptide is required for proper folding and activity of the E. coli fused transhydrogenase, and that differences between the catalytic properties of the E. coli and the mitochondrial enzymes are unrelated to the linking peptide. This is the first time that larger subunits of a membrane protein with multiple transmembrane helices have been fused with retained activity.

A change in ionization of the NADP(H)-binding component (dIII) of proton-translocating transhydrogenase regulates both hydride transfer and nucleotide release

Transhydrogenase couples the transfer of hydride-ion equivalents between NAD(H) and NADP(H) to proton translocation across a membrane. The enzyme has three components: dI binds NAD(H), dIII binds NADP(H) and dII spans the membrane. Coupling between transhydrogenation and proton translocation involves changes in the binding of NADP(H). Mixtures of isolated dI and dIII from Rhodospirillum rubrum transhydrogenase catalyse a rapid, single-turnover burst of hydride transfer between bound nucleotides; subsequent turnover is limited by NADP(H) release. Stopped-flow experiments showed that the rate of the hydride transfer step is decreased at low pH. Single Trp residues were introduced into dIII by site-directed mutagenesis. Two mutants with similar catalytic properties to those of the wild-type protein were selected for a study of nucleotide release. The way in which Trp fluorescence was affected by nucleotide occupancy of dIII was different in the two mutants, and hence two different procedures for determining the rate of nucleotide release were developed. The apparent first-order rate constants for NADP(+) release and NADPH release from isolated dIII increased dramatically at low pH. It is concluded that a single ionisable group in dIII controls both the rate of hydride transfer and the rate of nucleotide release. The properties of the protonated and unprotonated forms of dIII are consistent with those expected of intermediates in the NADP(H)-binding-change mechanism. The ionisable group might be a component of the proton-translocation pathway in the complete enzyme.

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

The pyridine nucleotide transhydrogenase of Escherichia coli carries out transmembrane proton translocation coupled to transfer of a hydride ion equivalent between NAD(+) and NADP(+). The membrane domain (domain II) of the enzyme is composed of 13 transmembrane helices. Previous studies (N. A. Glavas et al., Biochemistry 34, 7694-7702, 1995) have suggested that betaHis91 in transmembrane helix 9 is involved in the translocation pathway of protons across the membrane. In this study we have replaced amino acid residues on the same face of helix 9 as betaHis91 by single cysteine residues. We then examined the effect of the sulfhydryl inhibitors N-ethylmaleimide (NEM) and p-chloromercuriphenylsulfonate (pCMPS) on enzyme activity and, in the case of [(14)C]NEM, as an enzyme label. The pattern of enzyme inhibition and labelling is consistent with the presence of an aqueous cavity through domain II from the cytosolic surface to the region of betaHis91. Residue betaAsn222 in helix 13, which appears also to be involved in the proton pathway across domain II, may interface with this aqueous cavity. A further series of mutants of betaGlu124 on helix 10 confirms the proposal (P. D. Bragg and C. Hou, Arch. Biochem. Biophys. 363, 182-190, 1999) that this residue is involved in passive permeation of protons across domain II.

Crosslinking between alpha and beta subunits defines the orientation and spatial relationship of some of the transmembrane helices of the proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli

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 either eight or nine transmembrane helices at the amino-terminal region of the beta subunit. We have introduced pairs of cysteine residues into a cysteine-free transhydrogenase by site-directed mutagenesis. Disulfide bond formation between some of these cysteine residues occurred spontaneously or on treatment with cupric 1, 10-phenanthrolinate. Analysis of crosslinked products confirmed that there are nine transmembrane helices in the domain II region of the beta subunit. The proximity to one another of several of the transmembrane helices was determined. Thus, helices 2 and 4 are close to helix 6 (nomenclature of Meuller and Rydström, J. Biol. Chem. 274, 19072-19080, 1999), and helix 3 and the carboxyl-terminal eight residues of the alpha subunit are close to helix 7. In the alpha(2)beta(2) tetramer, helices 2 and 4 of one alpha subunit are close to the same pair of transmembrane helices of the other alpha subunit, and helix 6 of one beta subunit is close to helix 6 of the other beta subunit.

Stopped-flow reaction kinetics of recombinant components of proton-translocating transhydrogenase with physiological nucleotides

New information on the high resolution structure of the membrane proton pump, transhydrogenase, now provides a framework for understanding kinetic descriptions of the enzyme. Here, we have studied redox reactions catalyzed by mixtures of the recombinant NAD(H)-binding component (dI) of Rhodospirillum rubrum transhydrogenase, and the recombinant NADP(H)-binding component (dIII) of either the R. rubrum enzyme or the human enzyme. By recording changes in the fluorescence emission of native and engineered Trp residues, the rates of the redox reaction with physiological nucleotides have been measured under stopped-flow conditions, for the first time. Rate constants for the binding reaction between NAD(+)/NADH and the R. rubrum dI.dIII complex are much greater than those between nucleotide and isolated dI. For the redox step between the physiological nucleotides on the R. rubrum dI. dIII complex, the rate constant in the forward direction, k(f) approximately 2900 s(-1), and that for the reverse reaction, k(r) approximately 110 s(-1). Comparisons with reactions involving an analogue of NAD(H) indicate that the rate constants at this step are strongly affected by the redox driving force.

Interactions between the soluble domain I of nicotinamide nucleotide transhydrogenase from Rhodospirillum rubrum and transhydrogenase from Escherichia coli. Effects on catalytic and H+-pumping activities

Nicotinamide nucleotide transhydrogenase from Escherichia coli is composed of two subunits, the alpha and the beta subunits, each of which contains a hydrophilic domain, domain I and III, respectively, as well as several transmembrane helices, collectively denoted domain II. The interactions between domain I from Rhodospirillum rubrum (rrI) and the intact or the protease-treated enzyme from E. coli was investigated using the separately expressed and purified domain I from R. rubrum, and His-tagged intact and trypsin-treated E. coli transhydrogenase. Despite harsh treatments with, e.g. detergents and denaturing agents, the alpha and beta subunits remained tightly associated. A monoclonal antibody directed towards the alpha subunit was strongly inhibitory, an effect that was relieved by added rrI. In addition, rrI also reactivated the trypsin-digested E. coli enzyme in which domain I had been partly removed. This suggests that the hydrophilic domains I and III are not in permanent contact but are mobile during catalysis while being anchored to domain II. Replacement of domain I of intact, as well as trypsin-digested, E. coli transhydrogenase with rrI resulted in a markedly different pH dependence of the cyclic reduction of 3-acetyl-pyridine-NAD+ by NADH in the presence of NADP(H), suggesting that the protonation of one or more protonable groups in domain I is controlling this reaction. The reverse reaction and proton pumping showed a less pronounced change in pH dependence, demonstrating the regulatory role of domain II in these reactions.

Proton translocating nicotinamide nucleotide transhydrogenase from E. coli. Mechanism of action deduced from its structural and catalytic properties

Transhydrogenase couples the stereospecific and reversible transfer of hydride equivalents from NADH to NADP(+) to the translocation of proton across the inner membrane in mitochondria and the cytoplasmic membrane in bacteria. Like all transhydrogenases, the Escherichia coli enzyme is composed of three domains. Domains I and III protrude from the membrane and contain the binding site for NAD(H) and NADP(H), respectively. Domain II spans the membrane and constitutes at least partly the proton translocating pathway. Three-dimensional models of the hydrophilic domains I and III deduced from crystallographic and NMR data and a new topology of domain II are presented. The new information obtained from the structures and the numerous mutation studies strengthen the proposition of a binding change mechanism, as a way to couple the reduction of NADP(+) by NADH to proton translocation and occurring mainly at the level of the NADP(H) binding site.

The high-resolution structure of the NADP(H)-binding component (dIII) of proton-translocating transhydrogenase from human heart mitochondria

BACKGROUND

Transhydrogenase, located in the inner membranes of animal mitochondria and the cytoplasmic membranes of bacteria, couples the transfer of reducing equivalents between NAD(H) and NADP(H) to proton pumping. The protein comprises three subunits termed dI, dII and dIII. The dII component spans the membrane. The dI component, which contains the binding site for NAD(+)/NADH, and the dIII component, which has the binding site for NADP(+)/NADPH, protrude from the membrane. Proton pumping is probably coupled to changes in the binding affinities of dIII for NADP(+) and NADPH.

RESULTS

The first X-ray structure of the NADP(H)-binding component, dIII, of human heart transhydrogenase is described here at 2.0 A resolution. It comprises a single domain resembling the classical Rossmann fold, but NADP(+) binds to dIII with a reversed orientation. The first betaalphabetaalphabeta motif of dIII contains a Gly-X-Gly-X-X-Ala/Val 'fingerprint', but it has a different function to that in the classical Rossmann structure. The nicotinamide ring of NADP(+) is located on a ridge where it is exposed to interaction with NADH on the dI subunit. Two distinctive features of the dIII structure are helix D/loop D, which projects from the beta sheet, and loop E, which forms a 'lid' over the bound NADP(+).

CONCLUSIONS

Helix D/loop D interacts with the bound nucleotide and loop E, and probably interacts with the membrane-spanning dII. Changes in ionisation and conformation in helix D/loop D, resulting from proton translocation through dII, are thought to be responsible for the changes in affinity of dIII for NADP(+) and NADPH that drive the reaction.

Structure and mechanism of proton-translocating transhydrogenase

Recent developments have led to advances in our understanding of the structure and mechanism of action of proton-translocating (or AB) transhydrogenase. There is (a) a high-resolution crystal structure, and an NMR structure, of the NADP(H)-binding component (dIII), (b) a homology-based model of the NAD(H)-binding component (dI) and (c) an emerging consensus on the position of the transmembrane helices (in dII). The crystal structure of dIII, in particular, provides new insights into the mechanism by which the energy released in proton translocation across the membrane is coupled to changes in the binding affinities of NADP(+) and NADPH that drive the chemical reaction.

Effect of NBD chloride (4-chloro-7-nitrobenzo-2-oxa-1,3-diazole) on the pyridine nucleotide transhydrogenase of Escherichia coli

Pyridine nucleotide transhydrogenases of bacterial cytosolic membranes and mitochondrial inner membranes are proton pumps in which hydride transfer between NADP(+) and NAD(+) is coupled to proton translocation across cytosolic or mitochondrial membranes. The pyridine nucleotide transhydrogenase of Escherichia coli is composed of two subunits (alpha and beta). Three domains are recognized. The extrinsic cytosolic domain 1 of the amino-terminal region of the alpha subunit bears the NAD(H)-binding site. The NADP(H)-binding site is present in domain 3, the extrinsic cytosolic carboxyl-terminal region of the beta subunit. Domain 2 is composed of the membrane-intrinsic carboxyl-terminal region of the alpha subunit and the membrane-intrinsic amino-terminal region of the beta subunit. Treatment of the transhydrogenase of E. coli with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD chloride) inhibited enzyme activity. Analysis of inhibition revealed that several sites on the enzyme were involved. NBD chloride modified two (betaCys-147 and betaCys-260) of the seven cysteine residues present in the transhydrogenase. Modification of betaCys-260 in domain 2 resulted in inhibition of enzyme activity. Modification of residues other than cysteine residues also resulted in inhibition of transhydrogenation as shown by use of a cysteine-free mutant enzyme. The beta subunit was modified by NBD chloride to a greater extent than the alpha subunit. Reaction of domain 2 and domain 3 was prevented by NADPH. Modification of domain 3 is probably not associated with inhibition of enzyme activity. Modification of domain 2 of the beta subunit resulted in a decreased binding affinity for NADPH at its binding site in domain 3. The product resulting from the reaction of NBD chloride with NADPH was a very effective inhibitor of transhydrogenation. In experiments with NBD chloride in the presence of NADPH it is likely that all of the sites of reaction described above will contribute to the inhibition observed. The NBD-NADPH adduct will likely be more useful than NBD chloride in investigations of the pyridine nucleotide transhydrogenase.

Evidence for the stabilization of NADPH relative to NADP(+) on the dIII components of proton-translocating transhydrogenases from Homo sapiens and from Rhodospirillum rubrum by measurement of tryptophan fluorescence

A unique Trp residue in the recombinant dIII component of transhydrogenase from human heart mitochondria (hsdIII), and an equivalent Trp engineered into the dIII component of Rhodospirillum rubrum transhydrogenase (rrdIII.D155W), are more fluorescent when NADP(+) is bound to the proteins, than when NADPH is bound. We have used this to determine the occupancy of the binding site during transhydrogenation reactions catalysed by mixtures of recombinant dI from the R. rubrum enzyme and either hsdIII or rrdIII.D155W. The standard redox potential of NADP(+)/NADPH bound to the dIII proteins is some 60-70 mV higher than that in free solution. This results in favoured reduction of NADP(+) by NADH at the catalytic site, and supports the view that changes in affinity at the nucleotide-binding site of dIII are central to the mechanism by which transhydrogenase is coupled to proton translocation across the membrane.

The mobile loop region of the NAD(H) binding component (dI) of proton-translocating nicotinamide nucleotide transhydrogenase from Rhodospirillum rubrum: complete NMR assignment and effects of bound nucleotides

The dI component of transhydrogenase binds NAD+ and NADH. A mobile loop region of dI plays an important role in the nucleotide binding process, and mutations in this region result in impaired hydride transfer in the complete enzyme. We have previously employed one-dimensional 1H-NMR spectroscopy to study wild-type and mutant dI proteins of Rhodospirillum rubrum and the effects of nucleotide binding. Here, we utilise two- and three-dimensional NMR experiments to assign the signals from virtually all of the backbone and side-chain protons of the loop residues. The mobile loop region encompasses 17 residues: Asp223-Met239. The assignments also provide a much strengthened basis for interpreting the structural changes occurring upon nucleotide binding, when the loop closes down onto the surface of the protein and loses mobility. The role of the mobile loop region in catalysis is discussed with particular reference to a newly-developed model of the dI protein, based on its homology with alanine dehydrogenase.

A catalytically active complex formed from the recombinant dI protein of Rhodospirillum rubrum transhydrogenase, and the recombinant dIII protein of the human enzyme

Transhydrogenase is a proton pump. It has three components: dI and dIII protrude from the membrane and contain the binding sites for NAD(H) and NADP(H), respectively, and dII spans the membrane. We have expressed dIII from Homo sapiens transhydrogenase (hsdIII) in Escherichia coli. The purified protein was associated with stoichiometric amounts of NADP(H) bound to the catalytic site. The NADP+ and NADPH were released only slowly from the protein, supporting the suggestion that nucleotide-binding by dIII is regulated by the membrane-spanning dII. HsdIII formed a catalytically active complex with recombinant dI from Rhodospirillum rubrum (rrdI), even in the absence of dII. The rates of forward and reverse transhydrogenation catalysed by this complex are probably limited by slow release from dIII of NADPH and NADP+, respectively. The hybrid complex also catalysed high rates of 'cyclic' transhydrogenation, indicating that hydride transfer, and exchange of nucleotides with dI, are rapid. Stopped-flow experiments revealed a rapid, monoexponential, single-turnover burst of reverse transhydrogenation in pre-steady-state. The apparent first-order rate constant of the burst increased with the concentration of rrdI. A deuterium isotope effect (kH/kD approximately 2 at 27 degrees C) was observed when [4B-1H]NADPH was replaced with [4B-2H]NADPH. The characteristics of the burst of transhydrogenation with rrdI:hsdIII differed from those previously reported for rrdI:rrdIII (J.D. Venning et al., Eur. J. Biochem. 257 (1998) 202-209), but the differences are readily explained by a greater dissociation constant of the hybrid complex. The steady-state rate of reverse transhydrogenation by the rrdI:hsdIII complex was almost independent of pH, but there was a single apparent pKa ( approximately 9.1) associated with the cyclic reaction. The reactions of the dI:dIII complex probably proceed independently of those protonation/deprotonation reactions which, in the complete enzyme, are associated with H+ translocation.

Structural changes in the recombinant, NADP(H)-binding component of proton translocating transhydrogenase revealed by NMR spectroscopy

We have analysed 1H, 15N-HSQC spectra of the recombinant, NADP(H)-binding component of transhydrogenase in the context of the emerging three dimensional structure of the protein. Chemical shift perturbations of amino acid residues following replacement of NADP+ with NADPH were observed in both the adenosine and nicotinamide parts of the dinucleotide binding site and in a region which straddles the protein. These observations reflect the structural changes resulting from hydride transfer. The interactions between the recombinant, NADP(H)-binding component and its partner, NAD(H)-binding protein, are complicated. Helix B of the recombinant, NADP(H)-binding component may play an important role in the binding process.