Why are two different types of pyridine nucleotide transhydrogenase found in living organisms?

Versatile selective evolutionary pressure using synthetic defect in universal metabolism

The non-natural needs of industrial applications often require new or improved enzymes. The structures and properties of enzymes are difficult to predict or design de novo. Instead, semi-rational approaches mimicking evolution entail diversification of parent enzymes followed by evaluation of isolated variants. Artificial selection pressures coupling desired enzyme properties to cell growth could overcome this key bottleneck, but are usually narrow in scope. Here we show diverse enzymes using the ubiquitous cofactors nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) can substitute for defective NAD regeneration, representing a very broadly-applicable artificial selection. Inactivation of Escherichia coli genes required for anaerobic NAD regeneration causes a conditional growth defect. Cells are rescued by foreign enzymes connected to the metabolic network only via NAD or NADP, but only when their substrates are supplied. Using this principle, alcohol dehydrogenase, imine reductase and nitroreductase variants with desired selectivity modifications, and a high-performing isopropanol metabolic pathway, are isolated from libraries of millions of variants in single-round experiments with typical limited information to guide design.

An interspecies malate-pyruvate shuttle reconciles redox imbalance in an anaerobic microbial community

Microbes in ecosystems often develop coordinated metabolic interactions. Therefore, understanding metabolic interdependencies between microbes is critical to deciphering ecosystem function. In this study, we sought to deconstruct metabolic interdependencies in organohalide-respiring consortium ACT-3 containing Dehalobacter restrictus using a combination of metabolic modeling and experimental validation. D. restrictus possesses a complete set of genes for amino acid biosynthesis yet when grown in isolation requires amino acid supplementation. We reconciled this discrepancy using flux balance analysis considering cofactor availability, enzyme promiscuity, and shared protein expression patterns for several D. restrictus strains. Experimentally, ¹³C incorporation assays, growth assays, and metabolite analysis of D. restrictus strain PER-K23 cultures were performed to validate the model predictions. The model resolved that the amino acid dependency of D. restrictus resulted from restricted NADPH regeneration and predicted that malate supplementation would replenish intracellular NADPH. Interestingly, we observed unexpected export of pyruvate and glutamate in parallel to malate consumption in strain PER-K23 cultures. Further experimental analysis using the ACT-3 transfer cultures suggested the occurrence of an interspecies malate-pyruvate shuttle reconciling a redox imbalance, reminiscent of the mitochondrial malate shunt pathway in eukaryotic cells. Altogether, this study suggests that redox imbalance and metabolic complementarity are important driving forces for metabolite exchange in anaerobic microbial communities.

The soluble transhydrogenase UdhA affecting the glutamate-dependent acid resistance system of Escherichia coli under acetate stress

The soluble transhydrogenase (UdhA) is one of two transhydrogenases that play a role in maintaining the balance between NAD(H) pools and NADP(H) pools in Escherichia coli. Although UdhA has been extensively used in metabolic engineering and biocatalysis for cofactor regeneration, its role in acid resistance has not been reported. Here we used DNA microarray to explore the impact of UdhA on transcript levels. We demonstrated that during growth on acetate, the expression of genes involved in the respiratory chain and Gad acid resistance system was inhibited in the udhA-knockout strain. The deletion of udhA significantly repressed the expression of six genes (gadA, gadB, gadC, gadE, hdeA and hdeB) which are involved in Gad acid resistance and resulted in low survival of the bacterium at a low pH of 4.9. Moreover, UdhA was essential for NADH production which is important for the adaptive growth of E. coli on acetate, while NADH concentration in the udhA-knockout strain was quite low and supplemental NADH significantly increased the expression of acid resistance genes and survival of the udhA-knockout strain. These results demonstrated that UdhA is an important source of NADH of E. coli growth on acetate and affects Gad acid resistance system under acetate stress.

Summary: UdhA function stated in this study helps us to understand the physiological roles of UdhA affecting NADH production and Gad acid resistance system in E.coli in acetate environment.

Review of NAD(P)H-dependent oxidoreductases: Properties, engineering and application

NAD(P)H-dependent oxidoreductases catalyze the reduction or oxidation of a substrate coupled to the oxidation or reduction, respectively, of a nicotinamide adenine dinucleotide cofactor NAD(P)H or NAD(P)⁺. NAD(P)H-dependent oxidoreductases catalyze a large variety of reactions and play a pivotal role in many central metabolic pathways. Due to the high activity, regiospecificity and stereospecificity with which they catalyze redox reactions, they have been used as key components in a wide range of applications, including substrate utilization, the synthesis of chemicals, biodegradation and detoxification. There is great interest in tailoring NAD(P)H-dependent oxidoreductases to make them more suitable for particular applications. Here, we review the main properties and classes of NAD(P)H-dependent oxidoreductases, the types of reactions they catalyze, some of the main protein engineering techniques used to modify their properties and some interesting examples of their modification and application.

Pyridine nucleotide transhydrogenases enable redox balance of Pseudomonas putida during biodegradation of aromatic compounds

The metabolic versatility of the soil bacterium Pseudomonas putida is reflected by its ability to execute strong redox reactions (e.g., mono- and di-oxygenations) on aromatic substrates. Biodegradation of aromatics occurs via the pathway encoded in the archetypal TOL plasmid pWW0, yet the effect of running such oxidative route on redox balance against the background metabolism of P. putida remains unexplored. To answer this question, the activity of pyridine nucleotide transhydrogenases (that catalyze the reversible interconversion of NADH and NADPH) was inspected under various physiological and oxidative stress regimes. The genome of P. putida KT2440 encodes a soluble transhydrogenase (SthA) and a membrane-bound, proton-pumping counterpart (PntAB). Mutant strains, lacking sthA and/or pntAB, were subjected to a panoply of genetic, biochemical, phenomic and functional assays in cells grown on customary carbon sources (e.g., citrate) versus difficult-to-degrade aromatic substrates. The results consistently indicated that redox homeostasis is compromised in the transhydrogenases-defective variant, rendering the mutant sensitive to oxidants. This metabolic deficiency was, however, counteracted by an increase in the activity of NADP⁺ -dependent dehydrogenases in central carbon metabolism. Taken together, these observations demonstrate that transhydrogenases enable a redox-adjusting mechanism that comes into play when biodegradation reactions are executed to metabolize unusual carbon compounds.

Identification of the Elusive Pyruvate Reductase of Chlamydomonas reinhardtii Chloroplasts

Under anoxic conditions the green alga Chlamydomonas reinhardtii activates various fermentation pathways leading to the creation of formate, acetate, ethanol and small amounts of other metabolites including d-lactate and hydrogen. Progress has been made in identifying the enzymes involved in these pathways and their subcellular locations; however, the identity of the enzyme involved in reducing pyruvate to d-lactate has remained unclear. Based on sequence comparisons, enzyme activity measurements, X-ray crystallography, biochemical fractionation and analysis of knock-down mutants, we conclude that pyruvate reduction in the chloroplast is catalyzed by a tetrameric NAD(+)-dependent d-lactate dehydrogenase encoded by Cre07.g324550. Its expression during aerobic growth supports a possible function as a 'lactate valve' for the export of lactate to the mitochondrion for oxidation by cytochrome-dependent d-lactate dehydrogenases and by glycolate dehydrogenase. We also present a revised spatial model of fermentation based on our immunochemical detection of the likely pyruvate decarboxylase, PDC3, in the cytoplasm.

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.

Characterization of a Cr(VI)-sensitive Pseudomonas corrugata 28 mutant impaired in a pyridine nucleotide transhydrogenase gene

Bacteria are known to adopt complex metabolic strategies in an effort to counteract the impact of numerous toxic compounds. In this study, a Cr(VI)-sensitive mutant of the Cr(VI)-hyperresistant bacterium Pseudomonas corrugata 28, obtained by insertional mutagenesis using the EZ-Tn5™ <R6Kγori/KAN-2>Tnp, was employed to gain a greater understanding of Cr(VI) resistance in bacteria. The insertion of the transposon, which occurred 16 bp upstream from the start codon of an ORF encoding a soluble pyridine nucleotide transhydrogenase (STH), negatively affected expression of the sth gene. The compromised expression of the sth gene in the mutant had two main effects on the pyridine nucleotide pools: (i) a decrease in NADPH and NADH fractions with a consequent shift in the redox state toward oxidation; and (ii) a decrease in the total concentration of the pyridine nucleotides. In the absence of a suitable pool of NADPH, the mutant failed to sustain an effective defense against the oxidative stress induced by Cr(VI).

Metabolic systems maintain stable non-equilibrium via thermodynamic buffering

Here, we analyze how the set of nucleotides in the cell is equilibrated and how this generates simple rules that help the cell to organize itself via maintenance of a stable non-equilibrium state. A major mechanism operating to achieve this state is thermodynamic buffering via high activities of equilibrating enzymes such as adenylate kinase. Under stable non-equilibrium, the ratios of free and Mg-bound adenylates, Mg(2+) and membrane potentials are interdependent and can be computed. The adenylate status is balanced with the levels of reduced and oxidized pyridine nucleotides through regulated uncoupling of the pyridine nucleotide pool from ATP production in mitochondria, and through oxidation of substrates non-coupled to NAD(+) reduction in peroxisomes. The set of adenylates and pyridine nucleotides constitutes a generalized cell energy status and determines rates of major metabolic fluxes. As the result, fluxes of energy and information become organized spatially and temporally, providing conditions for self-maintenance of metabolism.

Responses of the central metabolism in Escherichia coli to phosphoglucose isomerase and glucose-6-phosphate dehydrogenase knockouts

The responses of Escherichia coli central carbon metabolism to knockout mutations in phosphoglucose isomerase and glucose-6-phosphate (G6P) dehydrogenase genes were investigated by using glucose- and ammonia-limited chemostats. The metabolic network structures and intracellular carbon fluxes in the wild type and in the knockout mutants were characterized by using the complementary methods of flux ratio analysis and metabolic flux analysis based on [U-(13)C]glucose labeling and two-dimensional nuclear magnetic resonance (NMR) spectroscopy of cellular amino acids, glycerol, and glucose. Disruption of phosphoglucose isomerase resulted in use of the pentose phosphate pathway as the primary route of glucose catabolism, while flux rerouting via the Embden-Meyerhof-Parnas pathway and the nonoxidative branch of the pentose phosphate pathway compensated for the G6P dehydrogenase deficiency. Furthermore, additional, unexpected flux responses to the knockout mutations were observed. Most prominently, the glyoxylate shunt was found to be active in phosphoglucose isomerase-deficient E. coli. The Entner-Doudoroff pathway also contributed to a minor fraction of the glucose catabolism in this mutant strain. Moreover, although knockout of G6P dehydrogenase had no significant influence on the central metabolism under glucose-limited conditions, this mutation resulted in extensive overflow metabolism and extremely low tricarboxylic acid cycle fluxes under ammonia limitation conditions.

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?

Regulation of NAD- and NADP-dependent isocitrate dehydrogenases by reduction levels of pyridine nucleotides in mitochondria and cytosol of pea leaves

Regulation of NAD- and NADP-dependent isocitrate dehydrogenases (NAD-ICDH, EC 1.1.1.41, and NADP-ICDH, EC 1.1.1.42) by the level of reduced and oxidized pyridine nucleotides has been investigated in pea (Pisum sativum L.) leaves. The affinities of mitochondrial and cytosolic ICDH enzymes to substrates and inhibitors were determined on partially purified preparations in forward and reverse directions. From the kinetic data, it follows that NADP(+)- and NAD(+)-dependent isocitrate dehydrogenases in mitochondria represent a system strongly responding to the intramitochondrial NADPH and NADH levels. The NADPH, NADP(+), NADH and NAD(+) concentrations were determined by subcellular fractionation of pea leaf protoplasts using membrane filtration in mitochondria and cytosol in darkness and in the light under saturating and limiting CO(2) conditions. The cytosolic NADPH/NADP ratio was about 1 and almost constant both in darkness and in the light. In mitochondria, the NADPH/NADP ratio was low in darkness (0.2) and increased in the light, reaching 3 in limiting CO(2) conditions compared to 1 in saturating CO(2). At high reduction levels of NADP and NAD observed at limiting CO(2) in the light, i.e. when photorespiratory glycine is the main mitochondrial substrate, isocitrate oxidation in mitochondria will be suppressed and citrate will be transported to the cytosol ('citrate valve'), where the cytosolic NADP-ICDH supplies 2-oxoglutarate for the photorespiratory ammonia refixation.

Metabolic adaptation of Escherichia coli during temperature-induced recombinant protein production: 2. Redirection of metabolic fluxes

The impact of temperature-induced synthesis of human basic fibroblast growth factor (hFGF-2) in high-cell-density cultures of recombinant Escherichia coli was studied by estimating metabolic flux variations. Metabolic flux distributions in E. coli were calculated by means of a stoichiometric network and linear programming. After the temperature upshift, a substantially elevated energy demand for synthesis of hFGF-2 and heat shock proteins resulted in a redirection of metabolic fluxes. Catabolic pathways like the Embden-Meyerhof-Parnas pathway and the tricarboxylic acid (TCA) cycle showed significantly enhanced activities, leading to reduced flux to growth-associated pathways like the pentose phosphate pathway and other anabolic pathways. Upon temperature upshift, an excess of NADPH was produced in the TCA cycle by isocitrate dehydrogenase. The metabolic model predicted the involvement of a transhydrogenase generating additional NADH from NADPH, thereby increasing ATP regeneration in the respiratory chain. The influence of the temperature upshift on the host's metabolism was investigated by means of a control strain harboring the "empty" parental expression vector. The metabolic fluxes after the temperature upshift were redirected similarly to the production strain; the effects, however, were observed to a lesser extent and with different time profiles.

Characterization of a nicotinamide nucleotide transhydrogenase gene from the green alga Acetabularia acetabulum and comparison of its structure with those of the corresponding genes in mouse and Caenorhabditis elegans

Proton-pumping nicotinamide nucleotide transhydrogenase (Nnt) is a membrane-bound enzyme that catalyzes the reversible reduction of NADP(+) by NADH. This reaction is linked to proton translocation across the membrane. Depending on metabolic conditions, the enzyme may be involved in NADPH generation, e.g., for detoxification of peroxides and/or free radicals and protection from ischemic damage. Nnt exists in most prokaryotes and in animal mitochondria. It is composed of 2-3 subunits in bacteria and of a single polypeptide in mitochondria. An open question is whether Nnt exists in any photosynthetic eukaryotes and if so, to which class it belongs. In the present study it is demonstrated that, by cloning and sequencing cDNA and genomic copies of its NNT gene, an ancient alga, Acetabularia acetabulum (Chlorophyta, Dasycladales), contains a nuclear-encoded Nnt. In contrast to photosynthetic bacteria, this algal Nnt is composed of a single polypeptide of the class found in animal mitochondria. Excluding a poly(A) tail, NNT cDNA from A. acetabulum is 3688 bp long, consists of eight exons and spans 17 kb. The NNT gene from mouse was also characterized. Subsequently, the gene organization of the A. acetabulum NNT was compared to those of the homologous mouse (100 kb and 21 exons) and Caenorhabditis elegans (5.1 kb and 18 exons) genes.

Expression of a cytoplasmic transhydrogenase in Saccharomyces cerevisiae results in formation of 2-oxoglutarate due to depletion of the NADPH pool

The intracellular redox state of a cell is to a large extent defined by the concentration ratios of the two pyridine nucleotide systems NADH/NAD(+) and NADPH/NADP(+) and has a significant influence on product formation in microorganisms. The enzyme pyridine nucleotide transhydrogenase, which can catalyse transfer of reducing equivalents between the two nucleotide systems, occurs in several organisms, but not in yeasts. The purpose of this work was to analyse how metabolism during anaerobic growth of Saccharomyces cerevisiae might be altered when transfer of reducing equivalents between the two systems is made possible by expression of a cytoplasmic transhydrogenase from Azotobacter vinelandii. We therefore cloned sth, encoding this enzyme, and expressed it under the control of a S. cerevisiae promoter in a strain derived from the industrial model strain S. cerevisiae CBS8066. Anaerobic batch cultivations in high-performance bioreactors were carried out in order to allow quantitative analysis of the effect of transhydrogenase expression on product formation and on the intracellular concentrations of NADH, NAD(+), NADPH and NADP(+). A specific transhydrogenase activity of 4.53 U/mg protein was measured in the extracts from the strain expressing the sth gene from A. vinelandii, while no transhydrogenase activity could be detected in control strains without the gene. Production of the transhydrogenase caused a significant increase in formation of glycerol and 2-oxoglutarate. Since NADPH is used to convert 2-oxoglutarate to glutamate while glycerol formation increases when excess NADH is formed, this suggested that transhydrogenase converted NADH and NADP(+) to NAD(+) and NADPH. This was further supported by measurements of the intracellular nucleotide concentrations. Thus, the (NADPH/NADP(+)):(NADH/NAD(+)) ratio was reduced from 35 to 17 by the transhydrogenase. The increased formation of 2-oxoglutarate was accompanied by a two-fold decrease in the maximal specific growth rate. Also the biomass and ethanol yields were significantly lowered by the transhydrogenase.

Characterization of a second methylene tetrahydromethanopterin dehydrogenase from Methylobacterium extorquens AM1

Cell extracts of Methylobacterium extorquens AM1 were recently found to catalyze the dehydrogenation of methylene tetrahydromethanopterin (methylene H4MPT) with NAD+ and NADP+. The purification of a 32-kDa NADP-specific methylene H4MPT dehydrogenase (MtdA) was described already. Here we report on the characterization of a second methylene H4MPT dehydrogenase (MtdB) from this aerobic alpha-proteobacterium. Purified MtdB with an apparent molecular mass of 32 kDa was shown to catalyze the oxidation of methylene H4MPT to methenyl H4MPT with NAD+ and NADP+ via a ternary complex catalytic mechanism. The Km for methylene H4MPT was 50 microM with NAD+ (Vmax = 1100 U x mg(-1) and 100 microM with NADP+ (Vmax = 950 U x mg(-1). The Km value for NAD+ was 200 microM and for NADP+ 20 microM. In contrast to MtdA, MtdB could not catalyze the dehydrogenation of methylene tetrahydrofolate. Via the N-terminal amino-acid sequence, the MtdB encoding gene was identified to be orfX located in a cluster of genes whose translated products show high sequence identities to enzymes previously found only in methanogenic and sulfate reducing archaea. Despite its location, MtdB did not show sequence similarity to archaeal enzymes. The highest similarity was to MtdA, whose encoding gene is located outside of the archaeal island. Mutants defective in MtdB were unable to grow on methanol and showed a pronounced sensitivity towards formaldehyde. On the basis of the mutant phenotype and of the kinetic properties, possible functions of MtdB and MtdA are discussed. We also report that both MtdB and MtdA can be heterologously overproduced in Escherichia coli making these two enzymes readily available for structural analysis.

Inactivation of isocitrate lyase leads to increased production of medium-chain-length poly(3-hydroxyalkanoates) in Pseudomonas putida

Medium-chain-length (mcl) poly(3-hydroxyalkanoates) (PHAs) are storage polymers that are produced from various substrates and accumulate in Pseudomonas strains belonging to rRNA homology group I. In experiments aimed at increasing PHA production in Pseudomonas strains, we generated an mcl PHA-overproducing mutant of Pseudomonas putida KT2442 by transposon mutagenesis, in which the aceA gene was knocked out. This mutation inactivated the glyoxylate shunt and reduced the in vitro activity of isocitrate dehydrogenase, a rate-limiting enzyme of the citric acid cycle. The genotype of the mutant was confirmed by DNA sequencing, and the phenotype was confirmed by biochemical experiments. The aceA mutant was not able to grow on acetate as a sole carbon source due to disruption of the glyoxylate bypass and exhibited two- to fivefold lower isocitrate dehydrogenase activity than the wild type. During growth on gluconate, the difference between the mean PHA accumulation in the mutant and the mean PHA accumulation in the wild-type strain was 52%, which resulted in a significant increase in the amount of mcl PHA at the end of the exponential phase in the mutant P. putida KT217. On the basis of a stoichiometric flux analysis we predicted that knockout of the glyoxylate pathway in addition to reduced flux through isocitrate dehydrogenase should lead to increased flux into the fatty acid synthesis pathway. Therefore, enhanced carbon flow towards the fatty acid synthesis pathway increased the amount of mcl PHA that could be accumulated by the mutant.

Expression of the Escherichia coli pntA and pntB genes, encoding nicotinamide nucleotide transhydrogenase, in Saccharomyces cerevisiae and its effect on product formation during anaerobic glucose fermentation

We studied the physiological effect of the interconversion between the NAD(H) and NADP(H) coenzyme systems in recombinant Saccharomyces cerevisiae expressing the membrane-bound transhydrogenase from Escherichia coli. Our objective was to determine if the membrane-bound transhydrogenase could work in reoxidation of NADH to NAD+ in S. cerevisiae and thereby reduce glycerol formation during anaerobic fermentation. Membranes isolated from the recombinant strains exhibited reduction of 3-acetylpyridine-NAD+ by NADPH and by NADH in the presence of NADP+, which demonstrated that an active enzyme was present. Unlike the situation in E. coli, however, most of the transhydrogenase activity was not present in the yeast plasma membrane; rather, the enzyme appeared to remain localized in the membrane of the endoplasmic reticulum. During anaerobic glucose fermentation we observed an increase in the formation of 2-oxoglutarate, glycerol, and acetic acid in a strain expressing a high level of transhydrogenase, which indicated that increased NADPH consumption and NADH production occurred. The intracellular concentrations of NADH, NAD+, NADPH, and NADP+ were measured in cells expressing transhydrogenase. The reduction of the NADPH pool indicated that the transhydrogenase transferred reducing equivalents from NADPH to NAD+.

The udhA gene of Escherichia coli encodes a soluble pyridine nucleotide transhydrogenase

The udhA gene of Escherichia coli was cloned and expressed in E. coli and found to encode an enzyme with soluble pyridine nucleotide transhydrogenase activity. The N-terminal end of the enzyme contains the fingerprint motif of a dinucleotide binding domain, not present in published E. coli genome sequences due to a sequencing error. E. coli is hereby the first organism reported to possess both a soluble and a membrane-bound pyridine nucleotide transhydrogenase.

Cloning, sequence, and properties of the soluble pyridine nucleotide transhydrogenase of Pseudomonas fluorescens

The gene encoding the soluble pyridine nucleotide transhydrogenase (STH) of Pseudomonas fluorescens was cloned and expressed in Escherichia coli. STH is related to the flavoprotein disulfide oxidoreductases but lacks one of the conserved redox-active cysteine residues. The gene is highly similar to an E. coli gene of unknown function.

Proton-translocating transhydrogenase and NAD- and NADP-linked isocitrate dehydrogenases operate in a substrate cycle which contributes to fine regulation of the tricarboxylic acid cycle activity in mitochondria

H(+)-transhydrogenase (H(+)-Thase) and NADP-linked isocitrate dehydrogenase (NADP-ICDH) are very active in animal mitochondria but their physiological function is only poorly understood. This is especially so in the case of the heart and muscle, where there are no major consumers of NADPH. We propose here that H(+)-Thase and NADP-ICDH have a combined function in the fine regulation of the activity of the tricarboxylic acid (TCA) cycle, providing enhanced sensitivity to changes in energy demand. This is achieved through cycling of substrates by NAD-linked ICDH, NADP-linked ICDH and H(+)-Thase. It is proposed that NAD-ICDH operates in the forward direction of the TCA cycle, but NADP-ICDH is driven in reverse by elevated levels of NADPH resulting from the action of the transmembrane proton electrochemical potential gradient (delta p) on H(+)-Thase. This has the effect of increasing the sensitivity to allosteric modifiers of NAD-ICDH (NADH, ADP, ATP, Ca2+ etc), potentially giving rise to large changes in the net flux from iso-citrate to alpha-ketoglutarate. Furthermore, changes in the level of delta p resulting from changes in the demand for ATP would, via H(+)-Thase, shift the redox state of the NADP pool and this, in turn, would lead to a change in the rate of the reaction catalysed by NADP-ICDH and hence to an additional and complementary effect on the net metabolic flux from isocitrate to alpha-ketoglutarate. Other consequences of this substrate cycle are, (i) the production of heat at the expense of delta p, which may contribute to thermoregulation in the animal, and (ii) an increased rate of dissipation of delta p (leak).

Expression of the cloned subunits of Escherichia coli transhydrogenase from separate replicons

The pntA and pntB genes of Escherichia coli, encoding the alpha- and beta-subunits of the pyridine nucleotide transhydrogenase, were cloned individually in two different compatible plasmids into Escherichia coli mutants lacking transhydrogenase activity. Energy-linked and non-energy-linked transhydrogenase activities were produced only in cells carrying both plasmids thus showing that the products of both genes are required for the formation of an active enzyme. ATP-energized transhydrogenase activity was not increased in cells containing amplified levels of the transhydrogenase when the cell membrane ATPase was also amplified. It is suggested that the excess transhydrogenase is effectively uncoupled from the ATPase by compartmentalization in the cell.

Studies on the mechanism of electron transport to nitrogenase in Azotobacter vinelandii

The involvement of the cytoplasmic membrane in electron transport to nitrogenase has been studied. Evidence shows that nitrogenase activity in Azotobacter vinelandii is coupled to the flux of electrons through the respiratory chain. To obtain information about proteins involved, the changes occurring in A. vinelandii cells transferred to nitrogen-free medium after growth on NH4Cl (depression of nitrogenase activity) were studied. Synthesis of the nitrogenase polypeptides was detectable 5 min after transfer to nitrogen-free medium. No nitrogenase activity could be detected until t = 20 min, whereupon a linear increase of nitrogenase activity with time was observed. Synthesis of nitrogenase was accompanied by synthesis of flavodoxin II and two membrane-bound polypeptides of Mr 29,000 and 30,000. Analysis with respect to changes in membrane-bound NAD(P)H dehydrogenase activities revealed the induction of an NADPH dehydrogenase activity, which was not detectable in membranes isolated from cells grown in the presence of NH4OAc. This induced activity was associated with the appearance of a polypeptide of Mr 29,000 in the NADPH dehydrogenase complex.

Purification and properties of reconstitutively active nicotinamide nucleotide transhydrogenase of Escherichia coli

The nicotinamide nucleotide transhydrogenase of Escherichia coli has been purified from cytoplasmic membranes by pre-extraction of the membranes with sodium cholate and Triton X-100, solubilization of the enzyme with sodium deoxycholate in the presence of 1 M potassium chloride, and centrifugation through a 1.1 M sucrose solution. The purified enzyme consists of two subunits, alpha and beta, of apparent Mr 50000 and 47000. During transhydrogenation between NADPH and 3-acetylpyridine adenine dinucleotide by both the purified enzyme reconstituted into liposomes and the membrane-bound enzyme, a pH gradient is established across the membrane as indicated by the quenching of the fluorescence of 9-aminoacridine. Treatment of transhydrogenase with N,N'-dicyclohexylcarbodiimide results in an inhibition of proton pump activity and transhydrogenation, suggesting that proton translocation and catalytic activities are obligatory linked. NADH protected the enzyme against inhibition by N,N'-dicyclohexylcarbodiimide, while NADP, and to a lesser extent NADPH, appeared to increase the rate of inhibition. [14C]Dicyclohexylcarbodiimide preferentially labelled the 50000-Mr subunit of the transhydrogenase enzyme. The presence of an allosteric binding site which reacts with NADH, but not with reduced 3-acetylpyridine adenine dinucleotide, has been demonstrated.

Cloning and expression of the transhydrogenase gene of Escherichia coli

Based on the rationale that Escherichia coli cells harboring plasmids containing the pnt gene would contain elevated levels of enzyme, we have isolated three clones bearing the transhydrogenase gene from the Clarke and Carbon colony bank. The three plasmids were subjected to restriction endonuclease analysis. A 10.4-kilobase restriction fragment which overlapped all three plasmids was cloned into the PstI site of plasmid pUC13. Examination of several deletion derivatives of the resulting plasmid and subsequent treatment with exonuclease BAL 31 revealed that enhanced transhydrogenase expression was localized within a 3.05-kilobase segment. This segment was located at 35.4 min in the E. coli genome. Plasmid pDC21 conferred on its host 70-fold overproduction of transhydrogenase. The protein products of plasmids carrying the pnt gene were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membranes from cells containing the plasmids. Two polypeptides of molecular weights 50,000 and 47,000 were coded by the 3.05-kilobase fragment of pDC11. Both polypeptides were required for expression of transhydrogenase activity.

NAD+ kinase--a review

NAD+ kinase catalyzes the only (known) biochemical reaction leading to the production of NADP+ from NAD+. Most evidence indicates it is found in the cytoplasm, but reports of its presence in (other) cell bodies can not be discounted. Viewed as a protein, our knowledge of NADK composition and architecture is rudimentary. Though recognized as a large multimeric protein, no agreement is evident for the molecular weight (Mr = approximately 4-65 X 10(4] of the native protein. Is calmodulin an integral subunit of (some, all) NAD+ kinases (analogous to phosphorylase kinase in skeletal muscle)? Or is it an external modulator? Consensus is evident that a subunit of molecular weight 30-35 X 10(3) is a component of the mammalian and yeast kinase. In one case (rabbit liver) two types of subunits are reported to give rise to oligomers differing in molecular weight and catalytic activities. Viewed as an enzyme it is not known why such a complex aggregate is needed for what might otherwise appear to a routine phosphorylation reaction. Rapid equilibrium random (for pigeon liver and C. utilis preparations) and ping-pong (for A. vinelandii kinase) mechanisms have been proposed for the reaction, with multiple reactant binding sites indicated for the random cases. From the perspective of enzyme modulation, the demonstration that green plant and sea urchin egg kinases are targets for calmodulin regulation by intracellular Ca2+ links NADP+ production in these sources to the multi-level discriminatory control functions inherent to this Ca2+-protein complex. Significant questions arise from the results of various investigators considered in this review. These queries offer fertile ground for the selective design of key experiments directed to a better understanding of NAD+ kinase function and pyridine nucleotide biochemistry.