Modification of substrate specificity in single point mutants of Agrobacterium tumefaciens type II NADH dehydrogenase

The Auxiliary NADH Dehydrogenase Plays a Crucial Role in Redox Homeostasis of Nicotinamide Cofactors in the Absence of the Periplasmic Oxidation System in Gluconobacter oxydans NBRC3293

Gluconobacter oxydans has the unique property of a glucose oxidation system in the periplasmic space, where glucose is oxidized incompletely to ketogluconic acids in a nicotinamide cofactor-independent manner. Elimination of the gdhM gene for membrane-bound glucose dehydrogenase, the first enzyme for the periplasmic glucose oxidation system, induces a metabolic change whereby glucose is oxidized in the cytoplasm to acetic acid. G. oxydans strain NBRC3293 possesses two molecular species of type II NADH dehydrogenase (NDH), the primary and auxiliary NDHs that oxidize NAD(P)H by reducing ubiquinone in the cell membrane. The substrate specificities of the two NDHs are different from each other: primary NDH (p-NDH) oxidizes NADH specifically but auxiliary NDH (a-NDH) oxidizes both NADH and NADPH. We constructed G. oxydans NBRC3293 derivatives defective in the ndhA gene for a-NDH, in the gdhM gene, and in both. Our ΔgdhM derivative yielded higher cell biomass on glucose, as reported previously, but grew at a lower rate than the wild-type strain. The ΔndhA derivative showed growth behavior on glucose similar to that of the wild type. The ΔgdhM ΔndhA double mutant showed greatly delayed growth on glucose, but its cell biomass was similar to that of the ΔgdhM strain. The double mutant accumulated intracellular levels of NAD(P)H and thus shifted the redox balance to reduction. Accumulated NAD(P)H levels might repress growth on glucose by limiting oxidative metabolisms in the cytoplasm. We suggest that a-NDH plays a crucial role in redox homeostasis of nicotinamide cofactors in the absence of the periplasmic oxidation system in G. oxydans IMPORTANCE Nicotinamide cofactors NAD⁺ and NADP⁺ mediate redox reactions in metabolism. Gluconobacter oxydans, a member of the acetic acid bacteria, oxidizes glucose incompletely in the periplasmic space-outside the cell. This incomplete oxidation of glucose is independent of nicotinamide cofactors. However, if the periplasmic oxidation of glucose is abolished, the cells oxidize glucose in the cytoplasm by reducing nicotinamide cofactors. Reduced forms of nicotinamide cofactors are reoxidized by NADH dehydrogenase (NDH) on the cell membrane. We found that two kinds of NDH in G. oxydans have different substrate specificities: the primary enzyme is NADH specific, and the auxiliary one oxidizes both NADH and NADPH. Inactivation of the latter enzyme in G. oxydans cells in which we had induced cytoplasmic glucose oxidation resulted in elevated intracellular levels of NAD(P)H, limiting cell growth on glucose. We suggest that the auxiliary enzyme is important if G. oxydans grows independently of the periplasmic oxidation system.

Comparative Genomic Analysis of Closely Related Acetobacter pasteurianus Strains Provides Evidence of Horizontal Gene Transfer and Reveals Factors Necessary for Thermotolerance

Acetobacter pasteurianus is an industrial strain used for the vinegar production. Many A. pasteurianus strains with different phenotypic characteristics have been isolated so far. To understand the genetic background underpinning these phenotypes, a comparative genomic analysis of A. pasteurianus strains was conducted. Based on bioinformatics and experimental results, we report the following. (i) The gene repertoire related to the respiratory chains showed that several horizontal gene transfer events occurred after the divergence of these strains, indicating that the respiratory chain in A. pasteurianus has the diversity to adapt to its environment. (ii) There is a clear difference in thermotolerance even between 12 closely related strains. NBRC 3279, NBRC 3284, and NBRC 3283, in particular, which have only 55 mutations in total, showed differences in thermotolerance. The Na⁺/H⁺ antiporter gene nhaK2 was mutated in the thermosensitive NBRC 3279 and NBRC 3284 strains and not in the thermotolerant NBRC 3283 strain. The Na⁺/H⁺ antiporter activity of the three strains and expression of nhaK2 gene from NBRC 3283 in the two thermosensitive strains showed that these mutations are critical for thermotolerance. These results suggested that horizontal gene transfer events and several mutations have affected the phenotypes of these closely related strains.IMPORTANCE Acetobacter pasteurianus, an industrial vinegar-producing strain, exhibits diverse phenotypic differences such as respiratory activity related to acetic acid production, acetic acid resistance, or thermotolerance. In this study, we investigated the correlations between genome sequences and phenotypes among closely related A. pasteurianus strains. The gene repertoire related to the respiratory chains showed that the respiratory components of A. pasteurianus has a diversity caused by several horizontal gene transfers and mutations. In three closely related strains with clear differences in their thermotolerances, we found that the insertion or deletion that occurred in the Na⁺/H⁺ antiporter gene nhaK2 is directly related to their thermotolerance. Our study suggests that a relatively quick mutation has occurred in the closely related A. pasteurianus due to its genetic instability and that this has largely affected its phenotype.

Switching the substrate specificity from NADH to NADPH by a single mutation of NADH oxidase from Lactobacillus rhamnosus

Enzymatic NADP⁺ regeneration is a promising approach to produce valuable chemicals under economic conditions. Among all the enzymatic routes, using water-forming NADH oxidase is an ideal one because there is no by-product. However, most NADH oxidases have a low specific activity to NADPH. In this work, a thermostable NADH oxidase from Lactobacillus rhamnosus (LrNox) was rationally engineered to switch its specificity from NADH to NADPH. The results show that mutants D177A, G178R, D177A/G178R, D177A/G178R/L179S improved the NADPH activity by a factor of 4-6. The highest NADPH catalytic efficiency (K_cat/K_m 223.71 S^-1 μm^-1, 47.6-fold higher than wild-type LrNox) and 51% of NADH activity retention were achieved by replacing the single amino acid Leu179 for serine (L179S) in LrNox. Modeling of L179S-NADPH complex reveals that the phosphate group of NADPH interacts with the hydroxyl of Ser179 with a strong hydrogen bond and several shorter hydrogen bonds with the amino group of Lys185 could stabilize the binding of NADPH in the L179S mutant. This work provides an efficient method for converting NAD(P)H specificity and shows that L179S mutant is a potential and efficient auxiliary enzyme for NADP⁺ regeneration.

Diversity of NADH dehydrogenases in acetic acid bacteria: adaptation to modify their phenotype through gene expansions and losses and neo-functionalization

NADH dehydrogenase plays an important role in the central metabolism of almost all organisms, including acetic acid bacteria (AAB). In this study, the gene diversity of the NADH dehydrogenases in AAB was investigated. The distribution of the genes of the type I and type II NADH dehydrogenases in AAB was mostly congruent with their phylogenetic relationships. There are two phylogenetically distinct type I NADH dehydrogenase complexes, complex IA and complex IE. Complex IA', which lacks the nuoM gene from complex IA, was only conserved in the genera Acetobacter, Gluconacetobacter and Komagataeibacter, which all have the ability to perform acetic acid fermentation, whereas the complex IE gene cluster was found randomly in several species of AAB. Almost all AAB, excluding the early-diverged species, had the type II NADH dehydrogenase, while some of the species also had the homologue with an amino acid replacement at the residue responsible for NADPH oxidation ability. Thus, the gene repertoire of NADH dehydrogenases shows a history of adaptation towards their habitats through gene expansions and losses and neo-functionalization in AAB.

Steric hindrance controls pyridine nucleotide specificity of a flavin-dependent NADH:quinone oxidoreductase

The crystal structure of the NADH:quinone oxidoreductase PA1024 has been solved in complex with NAD⁺ to 2.2 Å resolution. The nicotinamide C4 is 3.6 Å from the FMN N5 atom, with a suitable orientation for facile hydride transfer. NAD⁺ binds in a folded conformation at the interface of the TIM-barrel domain and the extended domain of the enzyme. Comparison of the enzyme-NAD⁺ structure with that of the ligand-free enzyme revealed a different conformation of a short loop (75-86) that is part of the NAD⁺ -binding pocket. P78, P82, and P84 provide internal rigidity to the loop, whereas Q80 serves as an active site latch that secures the NAD⁺ within the binding pocket. An interrupted helix consisting of two α-helices connected by a small three-residue loop binds the pyrophosphate moiety of NAD⁺ . The adenine moiety of NAD⁺ appears to π-π stack with Y261. Steric constraints between the adenosine ribose of NAD⁺ , P78, and Q80, control the strict specificity of the enzyme for NADH. Charged residues do not play a role in the specificity of PA1024 for the NADH substrate.

Expression of alternative NADH dehydrogenases (NDH-2) in the phytopathogenic fungus Ustilago maydis

Type 2 alternative NADH dehydrogenases (NDH‐2) participate indirectly in the generation of the electrochemical proton gradient by transferring electrons from NADH and NADPH into the ubiquinone pool. Due to their structural simplicity, alternative NADH dehydrogenases have been proposed as useful tools for gene therapy of cells with defects in the respiratory complex I. In this work, we report the presence of three open reading frames, which correspond to NDH‐2 genes in the genome of Ustilago maydis. These three genes were constitutively transcribed in cells cultured in YPD and minimal medium with glucose, ethanol, or lactate as carbon sources. Proteomic analysis showed that only two of the three NDH‐2 were associated with isolated mitochondria in all culture media. Oxygen consumption by permeabilized cells using NADH or NADPH was different for each condition, opening the possibility of posttranslational regulation. We confirmed the presence of both external and internal NADH dehydrogenases, as well as an external NADPH dehydrogenase insensitive to calcium. Higher oxygen consumption rates were observed during the exponential growth phase, suggesting that the activity of NADH and NADPH dehydrogenases is coupled to the dynamics of cell growth.

The mechanism of catalysis by type-II NADH:quinone oxidoreductases

Type II NADH:quinone oxidoreductase (NDH-2) is central to the respiratory chains of many organisms. It is not present in mammals so may be exploited as an antimicrobial drug target or used as a substitute for dysfunctional respiratory complex I in neuromuscular disorders. NDH-2 is a single-subunit monotopic membrane protein with just a flavin cofactor, yet no consensus exists on its mechanism. Here, we use steady-state and pre-steady-state kinetics combined with mutagenesis and structural studies to determine the mechanism of NDH-2 from Caldalkalibacillus thermarum. We show that the two substrate reactions occur independently, at different sites, and regardless of the occupancy of the partner site. We conclude that the reaction pathway is determined stochastically, by the substrate/product concentrations and dissociation constants, and can follow either a ping-pong or ternary mechanism. This mechanistic versatility provides a unified explanation for all extant data and a new foundation for the development of therapeutic strategies.

The evolution of substrate specificity-associated residues and Ca(2+) -binding motifs in EF-hand-containing type II NAD(P)H dehydrogenases

Most eukaryotic organisms, except some animal clades, have mitochondrial alternative electron transport enzymes that allow respiration to bypass the energy coupling in oxidative phosphorylation. The energy bypass enzymes in plants include the external type II NAD(P)H dehydrogenases (DHs) of the NDB family, which are characterized by an EF-hand domain for Ca(2+) binding. Here we investigate these plant enzymes by combining molecular modeling with evolutionary analysis. Molecular modeling of the Arabidopsis thaliana AtNDB1 with the yeast ScNDI1 as template revealed distinct similarities in the core catalytic parts, and highlighted the interaction between the pyridine nucleotide and residues correlating with NAD(P)H substrate specificity. The EF-hand domain of AtNDB1 has no counterpart in ScNDI1, and was instead modeled with Ca(2+) -binding signal transducer proteins. Combined models displayed a proximity of the AtNDB1 EF-hand domain to the substrate entrance side of the catalytic part. Evolutionary analysis of the eukaryotic NDB-type proteins revealed ancient and recent reversions between the motif observed in proteins specific for NADH (acidic type) and NADPH (non-acidic type), and that the clade of enzymes with acidic motifs in angiosperms derives from non-acidic-motif NDB-type proteins present in basal plants, fungi and protists. The results suggest that Ca(2+) -dependent external NADPH oxidation is an ancient process, indicating that it has a fundamental importance for eukaryotic cellular redox metabolism. In contrast, the external NADH DHs in plants are products of a recent expansion, mirroring the expansion of the alternative oxidase family.

Type II NADH:quinone oxidoreductase family: phylogenetic distribution, structural diversity and evolutionary divergences

Type II NADH:quinone oxidoreductases (NDH-2s) are membrane proteins, crucial for the catabolic metabolism, because they contribute to the maintenance of the NADH/NAD⁺ balance. In several pathogenic bacteria and protists, NDH-2s are the only enzymes performing respiratory NADH:quinone oxidoreductase activity. For this reason and for being considered absent in mammals, NDH-2s were proposed as suitable targets for novel antimicrobial therapies. We selected all sequences of genes encoding NDH-2s from fully sequenced genomes present in the KEGG database. These genes were present in 61% of the 1805 species belonging to Eukarya (83%), Bacteria (60%) and Archaea (32%). Notably sequences from mammal species including humans were retrieved in our selection as NDH-2s. The data obtained and the already available information allowed systematizing several properties of NDH-2s: (i) the existence of additional sequence motifs with putative regulatory functions, (ii) specificity towards NADH or NADPH and (iii) the type of quinone binding motif. We observed that NDH-2 family distribution is not congruent with the taxonomic tree, suggesting different origins for the eukaryotic sequences and possible lateral gene transfer among prokaryotes. We note the absence of genes coding for NDH-2 in anaerobic phyla and the presence of multiple copies in several genomes, specifically in cyanobacteria. These observations inspired us to propose a metabolic hypothesis for the appearance of NDH-2s.

NDH-1 and NDH-2 Plastoquinone Reductases in Oxygenic Photosynthesis

Oxygenic photosynthesis converts solar energy into chemical energy in the chloroplasts of plants and microalgae as well as in prokaryotic cyanobacteria using a complex machinery composed of two photosystems and both membrane-bound and soluble electron carriers. In addition to the major photosynthetic complexes photosystem II (PSII), cytochrome b6f, and photosystem I (PSI), chloroplasts also contain minor components, including a well-conserved type I NADH dehydrogenase (NDH-1) complex that functions in close relationship with photosynthesis and likewise originated from the endosymbiotic cyanobacterial ancestor. Some plants and many microalgal species have lost plastidial ndh genes and a functional NDH-1 complex during evolution, and studies have suggested that a plastidial type II NADH dehydrogenase (NDH-2) complex substitutes for the electron transport activity of NDH-1. However, although NDH-1 was initially thought to use NAD(P)H as an electron donor, recent research has demonstrated that both chloroplast and cyanobacterial NDH-1s oxidize reduced ferredoxin. We discuss more recent findings related to the biochemical composition and activity of NDH-1 and NDH-2 in relation to the physiology and regulation of photosynthesis, particularly focusing on their roles in cyclic electron flow around PSI, chlororespiration, and acclimation to changing environments.

Effect of ADH II deficiency on the intracellular redox homeostasis in Zymomonas mobilis

Mutant strain of the facultatively anaerobic, ethanol-producing bacterium Zymomonas mobilis, deficient in the Fe-containing alcohol dehydrogenase isoenzyme (ADH II), showed impaired homeostasis of the intracellular NAD(P)H during transition from anaerobic to aerobic conditions, and also in steady-state continuous cultures at various oxygen supplies. At the same time, ADH II deficiency in aerobically grown cells was accompanied by a threefold increase of catalase activity and by about 50% increase of hydrogen peroxide excretion. It is concluded that ADH II under aerobic conditions functions to maintain intracellular redox homeostasis and to protect the cells from endogenous hydrogen peroxide.

Characterization of Nda2, a plastoquinone-reducing type II NAD(P)H dehydrogenase in chlamydomonas chloroplasts

Electron transfer pathways associated to oxygenic photosynthesis, including cyclic electron flow around photosystem I and chlororespiration, rely on non-photochemical reduction of plastoquinones (PQs). In higher plant chloroplasts, a bacterial-like NDH complex homologous to complex I is involved in PQ reduction, but such a complex is absent from Chlamydomonas plastids where a type II NAD(P)H dehydrogenase activity has been proposed to operate. With the aim to elucidate the nature of the enzyme-supporting non-photochemical reduction of PQs, one of the type II NAD(P)H dehydrogenases identified in the Chlamydomonas reinhardtii genome (Nda2) was produced as a recombinant protein in Escherichia coli and further characterized. As many type II NAD(P)H dehydrogenases, Nda2 uses NADH as a preferential substrate, but in contrast to the eukaryotic enzymes described so far, contains non-covalently bound FMN as a cofactor. When expressed at a low level, Nda2 complements growth of an E. coli lacking both NDH-1 and NDH-2, but is toxic at high expression levels. Using an antibody raised against the recombinant protein and based on its mass spectrometric identification, we show that Nda2 is localized in thylakoid membranes. Chlorophyll fluorescence measurements performed on thylakoid membranes show that Nda2 is able to interact with thylakoid membranes of C. reinhardtii by reducing PQs from exogenous NADH or NADPH. We discuss the possible involvement of Nda2 in cyclic electron flow around PSI, chlororespiration, and hydrogen production.