The main external alternative NAD(P)H dehydrogenase of Neurospora crassa mitochondria

Physiological relevance, localization and substrate specificity of the alternative (type II) mitochondrial NADH dehydrogenases of Ogataea parapolymorpha

Mitochondria from Ogataea parapolymorpha harbor a branched electron-transport chain containing a proton-pumping Complex I NADH dehydrogenase and three Type II NADH dehydrogenases (NDH-2). To investigate the physiological role, localization and substrate specificity of these enzymes, the growth of various NADH dehydrogenase knockout mutants was quantitatively characterized in shake-flask and chemostat cultures, followed by oxygen-uptake experiments with isolated mitochondria. NAD(P)H:quinone oxidoreduction of the three NDH-2 were individually assessed. Our findings reveal that the O. parapolymorpha respiratory chain contains an internal NADH-accepting NDH-2 (Ndh2-1/OpNdi1), at least one external NAD(P)H-accepting enzyme, and likely additional mechanisms for respiration-linked oxidation of cytosolic NADH. Metabolic regulation appears to prevent competition between OpNdi1 and Complex I for mitochondrial NADH. With the exception of OpNdi1, the respiratory chain of O. parapolymorpha exhibits metabolic redundancy and tolerates deletion of multiple NADH-dehydrogenase genes without compromising fully respiratory metabolism.

Mutations in the Second Alternative Oxidase Gene: A New Approach to Group Aspergillus niger Strains

Alternative oxidase is a terminal oxidase in the branched mitochondrial electron transport chain of most fungi including Aspergillus niger (subgenus Circumdati, section Nigri). A second, paralogous aox gene (aoxB) is extant in some A. niger isolates but also present in two divergent species of the subgenus Nidulantes-A. calidoustus and A. implicatus-as well as in Penicillium swiecickii. Black aspergilli are cosmopolitan opportunistic fungi that can cause diverse mycoses and acute aspergillosis in immunocompromised individuals. Amongst the approximately 75 genome-sequenced A. niger strains, aoxB features considerable sequence variation. Five mutations were identified that rationally affect transcription or function or terminally modify the gene product. One mutant allele that occurs in CBS 513.88 and A. niger neotype strain CBS 554.65 involves a chromosomal deletion that removes exon 1 and intron 1 from aoxB. Another aoxB allele results from retrotransposon integration. Three other alleles result from point mutations: a missense mutation of the start codon, a frameshift, and a nonsense mutation. A. niger strain ATCC 1015 has a full-length aoxB gene. The A. niger sensu stricto complex can thus be subdivided into six taxa according to extant aoxB allele, which may facilitate rapid and accurate identification of individual species.

MDH2 produced OAA is a metabolic switch rewiring the fuelling of respiratory chain and TCA cycle

The mitochondrial respiratory chain (RC) enables many metabolic processes by regenerating both mitochondrial and cytosolic NAD⁺ and ATP. The oxidation by the RC of the NADH metabolically produced in the cytosol involves redox shuttles as the malate-aspartate shuttle (MAS) and is of paramount importance for cell fate. However, the specific metabolic regulations allowing mitochondrial respiration to prioritize NADH oxidation in response to high NADH/NAD⁺ redox stress have not been elucidated. The recent discovery that complex I (NADH dehydrogenase), and not complex II (Succinate dehydrogenase), can assemble with other respiratory chain complexes to form functional entities called respirasomes, led to the assumption that this supramolecular organization would favour NADH oxidation. Unexpectedly, characterization of heart and liver mitochondria demonstrates that the RC systematically favours electrons provided by the 'respirasome free' complex II. Our results demonstrate that the preferential succinate driven respiration is tightly controlled by OAA levels, and that OAA feedback inhibition of complex II rewires RC fuelling increasing NADH oxidation capacity. This new regulatory mechanism synergistically increases RC's NADH oxidative capacity and rewires MDH2 driven anaplerosis of the TCA, preventing malate production from succinate to favour oxidation of cytosolic malate. This regulatory mechanism synergistically adjusts RC and TCA fuelling in response to extramitochondrial malate produced by the MAS.

Alternative Type II NAD(P)H Dehydrogenases in the Mitochondria of Protists and Fungi

Plants, fungi, and some protists possess a more branched electron transport chain in their mitochondria compared to canonical one. In these organisms, the electron transport chain contains several rotenone-insensitive NAD(P)H dehydrogenases. Some are located on the outer surface, and others are located on the inner surface of the inner mitochondrial membrane. The putative role of these enzymes still remains elusive, but they may prevent the overreduction of the electron transport chain components and decrease the production of reaction oxygen species as a consequence. The last two decades resulted in the discovery of alternative rotenone-insensitive NAD(P)H dehydrogenases present in representatives of fungi and protozoa. The aim of this review is to gather and focus on current information concerning molecular and functional properties, regulation, and the physiological role of fungal and protozoan alternative NAD(P)H dehydrogenases.

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.

A deletion variant partially complements a porin-less strain of Neurospora crassa

Mitochondrial porin, the voltage-dependent anion channel, plays an important role in metabolism and other cellular functions within eukaryotic cells. To further the understanding of porin structure and function, Neurospora crassa wild-type porin was replaced with a deletion variant lacking residues 238–242 (238porin). 238porin was assembled in the mitochondrial outer membrane, but the steady state levels were only about 3% of those of the wild-type protein. The strain harbouring 238porin displayed cytochrome deficiencies and expressed alternative oxidase. Nonetheless, it exhibited an almost normal linear growth rate. Analysis of mitochondrial proteomes from a wild-type strain FGSC9718, a strain lacking porin (ΔPor-1), and one expressing only 238porin, revealed that the major differences between the variant strains were in the levels of subunits of the NADH:ubiquinone oxidoreductase (complex I) of the electron transport chain, which were reduced only in the ΔPor-1 strain. These, and other proteins related to electron flow and mitochondrial biogenesis, are differentially affected by relative porin levels.

Glucose Starvation Alters Heat Shock Response, Leading to Death of Wild Type Cells and Survival of MAP Kinase Signaling Mutant

A moderate heat shock induces Neurospora crassa to synthesize large quantities of heat shock proteins that are protective against higher, otherwise lethal temperatures. However, wild type cells do not survive when carbohydrate deprivation is added to heat shock. In contrast, a mutant strain defective in a stress-activated protein kinase does survive the combined stresses. In order to understand the basis for this difference in survival, we have determined the relative levels of detected proteins in the mutant and wild type strain during dual stress, and we have identified gene transcripts in both strains whose quantities change in response to heat shock or dual stress. These data and supportive experimental evidence point to reasons for survival of the mutant strain. By using alternative respiratory mechanisms, these cells experience less of the oxidative stress that proves damaging to wild type cells. Of central importance, mutant cells recycle limited resources during dual stress by undergoing autophagy, a process that we find utilized by both wild type and mutant cells during heat shock. Evidence points to inappropriate activation of TORC1, the central metabolic regulator, in wild type cells during dual stress, based upon behavior of an additional signaling mutant and inhibitor studies.

Mitochondrial type II NAD(P)H dehydrogenases in fungal cell death

During aerobic respiration, cells produce energy through oxidative phosphorylation, which includes a specialized group of multi-subunit complexes in the inner mitochondrial membrane known as the electron transport chain. However, this canonical pathway is branched into single polypeptide alternative routes in some fungi, plants, protists and bacteria. They confer metabolic plasticity, allowing cells to adapt to different environmental conditions and stresses. Type II NAD(P)H dehydrogenases (also called alternative NAD(P)H dehydrogenases) are non-proton pumping enzymes that bypass complex I. Recent evidence points to the involvement of fungal alternative NAD(P)H dehydrogenases in the process of programmed cell death, in addition to their action as overflow systems upon oxidative stress. Consistent with this, alternative NAD(P)H dehydrogenases are phylogenetically related to cell death - promoting proteins of the apoptosis-inducing factor (AIF)-family.

External NAD(P)H dehydrogenases in Acanthamoeba castellanii mitochondria

The mitochondrial respiratory chain of plants and some fungi contains multiple rotenone-insensitive NAD(P)H dehydrogenases, of which at least two are located on the outer surface of the inner membrane (i.e., external NADH and external NADPH dehydrogenases). Annotated sequences of the putative alternative NAD(P)H dehydrogenases of the protozoan Acanthamoeba castellanii demonstrated similarity to plant and fungal sequences. We also studied activity of these dehydrogenases in isolated A. castellanii mitochondria. External NADPH oxidation was observed for the first time in protist mitochondria. The coupling parameters were similar for external NADH oxidation and external NADPH oxidation, indicating similar efficiencies of ATP synthesis. Both external NADH oxidation and external NADPH oxidation had an optimal pH of 6.8 independent of relevant ubiquinol-oxidizing pathways, the cytochrome pathway or a GMP-stimulated alternative oxidase. The maximal oxidizing activity with external NADH was almost double that with external NADPH. However, a lower Michaelis constant (K(M)) value for external NADPH oxidation was observed compared to that for external NADH oxidation. Stimulation by Ca(2+) was approximately 10 times higher for external NADPH oxidation, while NADH dehydrogenase(s) appeared to be slightly dependent on Ca(2+). Our results indicate that external NAD(P)H dehydrogenases similar to those in plant and fungal mitochondria function in mitochondria of A. castellanii.

The mitochondrial complex I of trypanosomatids--an overview of current knowledge

The contribution of trypanosomatid mitochondrial complex I for energy transduction has long been debated. Herein, we summarize current knowledge on the composition and relevance of this enzyme. Bioinformatic and proteomic analyses allowed the identification of many conserved and trypanosomatid-specific subunits of NADH:ubiquinone oxidoreductase, revealing a multifunctional enzyme capable of performing bioenergetic activities and possibly, also of functioning in fatty acid metabolism. A multimeric structure organized in 5 domains of more than 2 MDa is predicted, in contrast to the 1 MDa described for mammalian complex I. The relevance of mitochondrial complex I within the Trypanosomatidae family is quite diverse with its NADH oxidation activity being dispensable for both procyclic and bloodstream Trypanosoma brucei, whereas in Phytomonas serpens the enzyme is the only respiratory complex able to sustain membrane potential. Aside from complex I, trypanosomatid mitochondria contain a type II NADH dehydrogenase and a NADH-dependent fumarate reductase as alternative electron entry points into the respiratory chain and thus, some trypanosomatids may have bypassed the need for complex I. The involvement of each of these enzymes in the maintenance of the mitochondrial redox balance in trypanosomatids is still an open question and requires further investigation.

Characterization of apoptosis-related oxidoreductases from Neurospora crassa

The genome from Neurospora crassa presented three open reading frames homologous to the genes coding for human AIF and AMID proteins, which are flavoproteins with oxidoreductase activities implicated in caspase-independent apoptosis. To investigate the role of these proteins, namely within the mitochondrial respiratory chain, we studied their cellular localization and characterized the respective null mutant strains. Efficiency of the respiratory chain was analyzed by oxygen consumption studies and supramolecular organization of the OXPHOS system was assessed through BN-PAGE analysis in the respective null mutant strains. The results demonstrate that, unlike in mammalian systems, disruption of AIF in Neurospora does not affect either complex I assembly or function. Furthermore, the mitochondrial respiratory chain complexes of the mutant strains display a similar supramolecular organization to that observed in the wild type strain. Further characterization revealed that N. crassa AIF appears localized to both the mitochondria and the cytoplasm, whereas AMID was found exclusively in the cytoplasm. AMID2 was detected in both mitochondria and cytoplasm of the amid mutant strain, but was barely discernible in wild type extracts, suggesting overlapping functions for the two proteins.

Inactivating alternative NADH dehydrogenases: enhancing fungal bioprocesses by improving growth and biomass yield?

Debate still surrounds the physiological roles of the alternative respiratory enzymes found in many fungi and plants. It has been proposed that alternative NADH:ubiquinone oxidoreductases (NADH dehydrogenases) may protect against oxidative stress, conversely, elevated activity of these enzymes has been linked to senescence. Here we show that inhibition of these enzymes in a fungal protein expression system (Aspergillus niger) leads to significantly enhanced specific growth rate, substrate uptake, carbon dioxide evolution, higher protein content, and more efficient use of substrates. These findings are consistent with a protective role of the NADH dehydrogenases against oxidative stress, thus, when electron flow via these enzymes is blocked, flux through the main respiratory pathway rises, leading to enhanced ATP generation. We anticipate that our findings will stimulate further studies in fungal and plant cultures leading to significant improvements in these expression systems, and to deeper insights into the cellular roles of alternative respiration.

Disruption of alternative NAD(P)H dehydrogenases leads to decreased mitochondrial ROS in Neurospora crassa

Mitochondria are a main providers of high levels of energy, but also a major source of reactive oxygen species (ROS) during normal oxidative metabolism. The involvement of Neurospora crassa alternative NAD(P)H dehydrogenases in mitochondrial ROS production was evaluated. The growth responses of a series of respiratory mutants to several stress conditions revealed that disrupting alternative dehydrogenases leads to an increased tolerance to the redox cycler paraquat, with a mutant devoid of the external NDE1 and NDE2 enzymes being significantly more resistant. The nde1nde2 mutant mitochondria show a significant decrease in ROS generation in the presence and absence of paraquat, regardless of the respiratory substrate used, and an intrinsic increase in catalase activity. Analysis of ROS production by a complex I mutant (nuo51) indicates that, as in other organisms, paraquat-derived ROS in Neurospora mitochondria occur mainly at the level of complex I. We propose that disruption of the external NAD(P)H dehydrogenases NDE1 and NDE2 leads to a synergistic effect diminishing ROS generation by the mitochondrial respiratory chain. This, in addition to a robust increase in scavenging capacity, provides the mutant strain with an improved ability to withstand paraquat treatment.

The roles played by Aspergillus nidulans apoptosis-inducing factor (AIF)-like mitochondrial oxidoreductase (AifA) and NADH-ubiquinone oxidoreductases (NdeA-B and NdiA) in farnesol resistance

Farnesol (FOH) is a nonsterol isoprenoid produced by dephosphorylation of farnesyl pyrophosphate, a catabolite of the cholesterol biosynthetic pathway. These isoprenoids inhibit proliferation and induce apoptosis. Here, we show that Aspergillus nidulans AifA encoding the apoptosis-inducing factor (AIF)-like mitochondrial oxidoreductase plays a role in the function of the mitochondrial Complex I. Additionally, we demonstrated that ndeA-B and ndiA encode external and internal alternative NADH dehydrogenases, respectively, that have a function in FOH resistance. When exposed to FOH, the ΔaifA and ΔndeA strains have increased ROS production while ΔndeB, ΔndeA ΔndeB, and ΔndiA mutant strains showed the same ROS accumulation than in the absence of FOH. We observed several compensatory mechanisms affecting the differential survival of these mutants to FOH.

Molecular gene therapy: overexpression of the alternative NADH dehydrogenase NDI1 restores overall physiology in a fungal model of respiratory complex I deficiency

Defects in oxidative phosphorylation lie at the heart of a wide variety of degenerative disorders, cancer, and aging. Here, we show, using the fungal model Podospora anserina, that the overexpression of the native mitochondrial matrix-faced type II NADH dehydrogenase NDI1, paralogue of the human apoptosis inducing factor AIF1, can fully restore all physiological consequences of respiratory complex I deficiency. We disrupted the 19.3-kDa subunit of the complex I catalytic core, orthologue of the human PSST subunit, leading to a complete absence of the complex without affecting the assembly and/or stability of the rest of the respiratory chain. This disruption caused a several-fold life span extension at the expense of both male and female fertility. The effect was generally similar but markedly milder than that caused by defects in the complex III/IV-dependent pathway and not associated with a clear reduction in the steady-state level of mitochondrial reactive oxygen species. Whereas the native expression of NDI1 was sufficient to overcome lethality, only the artificial, constitutive overexpression of NDI1 could fully remedy this deficiency: The latter strikingly restored both life span and fertility to levels indistinguishable from wild type, thus demonstrating its unique potential in molecular gene therapy.

Oxidative stress in industrial fungi

Fungi are amongst the most industrially important microorganisms in current use within the biotechnology industry. Most such fungal cultures are highly aerobic in nature, a character that has been frequently referred to in both reactor design and fungal physiology. The most fundamentally significant outcome of the highly aerobic growth environment in fermenter vessels is the need for the fungal culture to effectively combat in the intracellular environment the negative consequences of high oxygen transfer rates. The use of oxygen as the respiratory substrate is frequently reported to lead to the development of oxidative stress, mainly due to oxygen-derived free radicals, which are collectively termed as reactive oxygen species (ROS). Recently, there has been extensive research on the occurrence, extent, and consequences of oxidative stress in microorganisms, and the underlying mechanisms through which cells prevent and repair the damage caused by ROS. In the present study, we critically review the current understanding of oxidative stress events in industrially relevant fungi. The review first describes the current state of knowledge of ROS concisely, and then the various antioxidant strategies employed by fungal cells to counteract the deleterious effects, together with their implications in fungal bioprocessing are also discussed. Finally, some recommendations for further research are made.

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.

Ca2+-binding and Ca2+-independent respiratory NADH and NADPH dehydrogenases of Arabidopsis thaliana

Type II NAD(P)H:quinone oxidoreductases are single polypeptide proteins widespread in the living world. They bypass the first site of respiratory energy conservation, constituted by the type I NADH dehydrogenases. To investigate substrate specificities and Ca(2+) binding properties of seven predicted type II NAD(P)H dehydrogenases of Arabidopsis thaliana we have produced them as T7-tagged fusion proteins in Escherichia coli. The NDB1 and NDB2 enzymes were found to bind Ca(2+), and a single amino acid substitution in the EF hand motif of NDB1 abolished the Ca(2+) binding. NDB2 and NDB4 functionally complemented an E. coli mutant deficient in endogenous type I and type II NADH dehydrogenases. This demonstrates that these two plant enzymes can substitute for the NADH dehydrogenases in the bacterial respiratory chain. Three NDB-type enzymes displayed distinct catalytic profiles with substrate specificities and Ca(2+) stimulation being considerably affected by changes in pH and substrate concentrations. Under physiologically relevant conditions, the NDB1 fusion protein acted as a Ca(2+)-dependent NADPH dehydrogenase. NDB2 and NDB4 fusion proteins were NADH-specific, and NDB2 was stimulated by Ca(2+). The observed activity profiles of the NDB-type enzymes provide a fundament for understanding the mitochondrial system for direct oxidation of cytosolic NAD(P)H in plants. Our findings also suggest different modes of regulation and metabolic roles for the analyzed A. thaliana enzymes.

The external alternative NAD(P)H dehydrogenase NDE3 is localized both in the mitochondria and in the cytoplasm of Neurospora crassa

The filamentous fungus Neurospora crassa has a branched respiratory chain. Several alternative dehydrogenases, aside from the canonical complex I enzyme, are involved in the oxidation of NAD(P)H substrates. Based on homology searches in the fungal genome, we have tentatively identified one of these proteins. The corresponding gene was inactivated by the generation of repeat-induced point mutations and a null-mutant strain was isolated. This mutant is deficient in the oxidation of cytosolic NADH, and to a lesser extent NADPH. Thus, a fourth mitochondrial alternative NAD(P)H dehydrogenase, named NDE3, was recognized in N. crassa. Interestingly, a combination of Western blot analysis of cell fractions and the in vivo detection of the protein fused to the green fluorescent protein revealed that it is also located in the fungal cytoplasm. In contrast to the other NAD(P)H dehydrogenases, expression of the nde-3 gene is up-regulated in the late exponential growth phase of N. crassa. The absence of the protein results in an up-regulation of the nde-2 transcript in this phase of growth, suggesting that the proteins are important in specific stages of fungal development. The identification of the proteins responsible for the entry point of electrons from NAD(P)H into the respiratory chain of N. crassa is likely completed.

Substrate-inducible versions of internal alternative NADH: ubiquinone oxidoreductase from Yarrowia lipolytica

In standard laboratory strains of the obligate aerobic yeast Yarrowia lipolytica, respiratory chain complex I (proton-translocating NADH : ubiquinone oxidoreductase) is an essential enzyme, since alternative NADH dehydrogenase activity is located exclusively at the external face of the mitochondrial inner membrane. Deletions and other loss-of-function mutations in genes for nuclear coded subunits of complex I can be obtained only when an internal version of the latter enzyme, termed NDH2i, is introduced. In contrast to recent findings with Neurospora crassa, external alternative NADH dehydrogenase activity is dispensable in complex I deletion strains of Y. lipolytica. We used regulable promoters to create strains which express internal alternative NADH dehydrogenase in a substrate-dependent manner. The ability to switch between complex I-dependent and -independent mode of growth simply by changing the carbon source is an important prerequisite for screens for both loss-of-function and inhibitor resistance mutation. The isocitrate lyase promoter (pICL1), in combination with a NDH2i allele that results in reduced expression and activity, was most promising. In the presence of complex I inhibitors, this construct allowed growth on acetate, but not on glucose minimal media. A somewhat higher background was observed with the acyl-CoA oxidase 2 (pPOX2) promoter on glucose minimal media.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Reoxidation of cytosolic NADPH in Kluyveromyces lactis

Saccharomyces cerevisiae and Kluyveromyces lactis are considered to be the prototypes of two distinct metabolic models of facultatively-aerobic yeasts: Crabtree-positive/fermentative and Crabtree-negative/respiratory, respectively. Our group had previously proposed that one of the molecular keys supporting this difference lies in the mechanisms involved in the reoxidation of the NADPH produced as a consequence of the activity of the pentose phosphate pathway. It has been demonstrated that a significant part of this reoxidation is carried out in K. lactis by mitochondrial external alternative dehydrogenases which use NADPH, the enzymes of S. cerevisiae being NADH-specific. Moreover, the NADPH-dependent pathways of response to oxidative stress appear as a feasible alternative that might co-exist with direct mitochondrial reoxidation.

Characterization of the second external alternative dehydrogenase from mitochondria of the respiratory yeast Kluyveromyces lactis

The mitochondria of the respiratory yeast Kluyveromyces lactis are able to reoxidize cytosolic NADPH. Previously, we characterized an external alternative dehydrogenase, KlNde1p, having this activity. We now characterize the second external alternative dehydrogenase of K. lactis mitochondria, KlNde2p. We examined its role in cytosolic NADPH reoxidation by studying heterologous expression of KlNDE2 in Saccharomyces cerevisiae mutants and by constructing Deltaklnde1 and Deltaklnde2 mutants. KlNde2p uses NADH or NADPH as substrates, its activity in isolated mitochondria is not regulated by exogenously added calcium and it is not down-regulated when the cells grow in glucose versus lactate. KlNde2p shows lower affinity for NADPH than KlNde1p. Both enzymes show similar pH optimum.

Metabolic maps and functions of the Plasmodium mitochondrion

The mitochondrion of Plasmodium species is a validated drug target. However, very little is known about the functions of this organelle. In this review, we utilize data available from the Plasmodium falciparum genome sequencing project to piece together putative metabolic pathways that occur in the parasite, comparing this with the existing biochemical and cell biological knowledge. The Plasmodium mitochondrion contains both conserved and unusual features, including an active electron transport chain and many of the necessary enzymes for coenzyme Q and iron-sulphur cluster biosynthesis. It also plays an important role in pyrimidine metabolism. The mitochondrion participates in an unusual hybrid haem biosynthesis pathway, with enzymes localizing in both the mitochondrion and plastid organelles. The function of the tricarboxylic acid cycle in the mitochondrion is unclear. We discuss directions for future research into this fascinating, yet enigmatic, organelle.

Isolation and bioenergetic characterization of mitochondria from Pichia pastoris

Pichia pastoris is a methylotrophic yeast of high biotechnological interest. The bioenergetic properties of mitochondria from Pichia pastoris have not yet been determined. We report on a protocol for the isolation of the mitochondria in a state that shows good energy coupling. Analysis of Pichia pastoris growth and bioenergetic properties of the isolated mitochondria reveals that glycerol is the carbon source that yields the best results. Under our growth conditions, mitochondria oxidize external NADH but do not possess an alternative oxidase. Finally, Pichia pastoris mitochondria also lack the nucleotide-stimulated uncoupling pathway previously identified in Saccharomyces cerevisiae.

Electron transfer ability from NADH to menaquinone and from NADPH to oxygen of type II NADH dehydrogenase of Corynebacterium glutamicum

Type II NADH dehydrogenase of Corynebacterium glutamicum (NDH-2) was purified from an ndh overexpressing strain. Purification conferred 6-fold higher specific activity of NADH:ubiquinone-1 oxidoreductase with a 3.5-fold higher recovery than that previously reported (K. Matsushita et al., 2000). UV-visible and fluorescence analyses of the purified enzyme showed that NDH-2 of C. glutamicum contained non-covalently bound FAD but not covalently bound FMN. This enzyme had an ability to catalyze electron transfer from NADH and NADPH to oxygen as well as various artificial quinone analogs at neutral and acidic pHs respectively. The reduction of native quinone of C. glutamicum, menaquinone-2, with this enzyme was observed only with NADH, whereas electron transfer to oxygen was observed more intensively with NADPH. This study provides evidence that C. glutamicum NDH-2 is a source of the reactive oxygen species, superoxide and hydrogen peroxide, concomitant with NADH and NADPH oxidation, but especially with NADPH oxidation. Together with this unique character of NADPH oxidation, phylogenetic analysis of NDH-2 from various organisms suggests that NDH-2 of C. glutamicum is more closely related to yeast or fungal enzymes than to other prokaryotic enzymes.

New insights into type II NAD(P)H:quinone oxidoreductases

Type II NAD(P)H:quinone oxidoreductases (NDH-2) catalyze the two-electron transfer from NAD(P)H to quinones, without any energy-transducing site. NDH-2 accomplish the turnover of NAD(P)H, regenerating the NAD(P)(+) pool, and may contribute to the generation of a membrane potential through complexes III and IV. These enzymes are usually constituted by a nontransmembrane polypeptide chain of approximately 50 kDa, containing a flavin moiety. There are a few compounds that can prevent their activity, but so far no general specific inhibitor has been assigned to these enzymes. However, they have the common feature of being resistant to the complex I classical inhibitors rotenone, capsaicin, and piericidin A. NDH-2 have particular relevance in yeasts like Saccharomyces cerevisiae and in several prokaryotes, whose respiratory chains are devoid of complex I, in which NDH-2 keep the balance and are the main entry point of electrons into the respiratory chains. Our knowledge of these proteins has expanded in the past decade, as a result of contributions at the biochemical level and the sequencing of the genomes from several organisms. The latter showed that most organisms contain genes that potentially encode NDH-2. An overview of this development is presented, with special emphasis on microbial enzymes and on the identification of three subfamilies of NDH-2.

The nuclear genes encoding the internal (KlNDI1) and external (KlNDE1) alternative NAD(P)H:ubiquinone oxidoreductases of mitochondria from Kluyveromyces lactis

Cloning, sequence and functional analyses of the Kluyveromyces lactis genes KlNDI1 and KlNDE1 are reported. These genes encode for proteins with high homology to the mitochondrial internal (Ndi1p) and external (Nde1p) alternative NADH:ubiquinone oxidoreductases from Saccharomyces cerevisiae and complement the respective mutations. Analysis of KlNDI1 transcriptional regulation showed that expression of this gene is lower in 2% glucose than in 0.5% glucose or non-fermentable carbon sources. Beta-galactosidase activity values, shown by lacZ fusions of KlNDI1 promoter deletions, suggested that two Adr1p binding sites mediate this carbon source regulation of KlNDI1. The expression of the KlNDE1 gene in S. cerevisiae mutant strains and measurement of respiration with isolated mitochondria showed that the protein encoded by KlNDE1 oxidizes NADPH, this being an important difference with respect to the conventional yeast S. cerevisiae. Moreover, Northern blot experiments using a phosphoglucose isomerase mutant showed that KlNDE1 gene transcription increases with glucose metabolism through the pentose phosphate pathway.