Interactions between glucose metabolism and oxidative phosphorylations on respiratory-competent Saccharomyces cerevisiae cells

The Warburg Effect in Yeast: Repression of Mitochondrial Metabolism Is Not a Prerequisite to Promote Cell Proliferation

O. Warburg conducted one of the first studies on tumor energy metabolism. His early discoveries pointed out that cancer cells display a decreased respiration and an increased glycolysis proportional to the increase in their growth rate, suggesting that they mainly depend on fermentative metabolism for ATP generation. Warburg's results and hypothesis generated controversies that are persistent to this day. It is thus of great importance to understand the mechanisms by which cancer cells can reversibly regulate the two pathways of their energy metabolism as well as the functioning of this metabolism in cell proliferation. Here, we made use of yeast as a model to study the Warburg effect and its eventual function in allowing an increased ATP synthesis to support cell proliferation. The role of oxidative phosphorylation repression in this effect was investigated. We show that yeast is a good model to study the Warburg effect, where all parameters and their modulation in the presence of glucose can be reconstituted. Moreover, we show that in this model, mitochondria are not dysfunctional, but that there are fewer mitochondria respiratory chain units per cell. Identification of the molecular mechanisms involved in this process allowed us to dissociate the parameters involved in the Warburg effect and show that oxidative phosphorylation repression is not mandatory to promote cell growth. Last but not least, we were able to show that neither cellular ATP synthesis flux nor glucose consumption flux controls cellular growth rate.

Nucleo-mitochondrial interaction of yeast in response to cadmium sulfide quantum dot exposure

Cell sensitivity to quantum dots (QDs) has been attributed to a cascade triggered by oxidative stress leading to apoptosis. The role and function of mitochondria in animal cells are well understood but little information is available on the complex genetic networks that regulate nucleo-mitochondrial interaction. The effect of CdS QD exposure in yeast Saccharomyces cerevisiae was assessed under conditions of limited lethality (<10%), using cell physiological and morphological endpoints. Whole-genomic array analysis and the screening of a deletion mutant library were also carried out. The results showed that QDs: increased the level of reactive oxygen species (ROS) and decreased the level of reduced vs oxidized glutathione (GSH/GSSG); reduced oxygen consumption and the abundance of respiratory cytochromes; disrupted mitochondrial membrane potentials and affected mitochondrial morphology. Exposure affected the capacity of cells to grow on galactose, which requires nucleo-mitochondrial involvement. However, QDs exposure did not materially induce respiratory deficient (RD) mutants but only RD phenocopies. All of these cellular changes were correlated with several key nuclear genes, including TOM5 and FKS1, involved in the maintenance of mitochondrial organization and function. The consequences of these cellular effects are discussed in terms of dysregulation of cell function in response to these "pathological mitochondria".

Proton pumping complex I increases growth yield in Candida utilis

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and cellular maintenance processes. A crucial parameter for growth evaluation is its yield, i.e. the efficiency of the transformation processes. The yeast Candida utilis is of peculiar interest since its mitochondria exhibit a complex I that is proposed to pump protons but also an external NADH dehydrogenase that do not pump protons. Here, we show that in C. utilis cells grown on non-fermentable media, growth yield is 30% higher as compared to that of Saccharomyces cerevisiae that do not exhibit a complex I. Moreover, ADP/O determination in C. utilis shows that electrons coming from internal NADH dehydrogenase go through proton pumping complex I, whereas electrons coming from external NADH dehydrogenases do not go through proton pumping complex I. Furthermore, we show that electron competition strictly depends on extra-mitochondrial NADH concentration, i.e. the higher the extra-mitochondrial NADH concentration, the higher the competition process with a right way for electrons coming from external NADH dehydrogenases. Such a complex regulation in C. utilis allows an increase in growth yield when cytosolic NADH is not plentiful but still favors the cytosolic NADH re-oxidation at high NADH, favoring biomass generation metabolic pathways.

2-Butanol and butanone production in Saccharomyces cerevisiae through combination of a B12 dependent dehydratase and a secondary alcohol dehydrogenase using a TEV-based expression system

2-Butanol and its chemical precursor butanone (methyl ethyl ketone – MEK) are chemicals with potential uses as biofuels and biocommodity chemicals. In order to produce 2-butanol, we have demonstrated the utility of using a TEV-protease based expression system to achieve equimolar expression of the individual subunits of the two protein complexes involved in the B₁₂-dependent dehydratase step (from the pdu-operon of Lactobacillus reuterii), which catalyze the conversion of meso-2,3-butanediol to butanone. We have furthermore identified a NADH dependent secondary alcohol dehydrogenase (Sadh from Gordonia sp.) able to catalyze the subsequent conversion of butanone to 2-butanol. A final concentration of 4±0.2 mg/L 2-butanol and 2±0.1 mg/L of butanone was found. A key factor for the production of 2-butanol was the availability of NADH, which was achieved by growing cells lacking the GPD1 and GPD2 isogenes under anaerobic conditions.

In Saccharomyces cerevisiae fructose-1,6-bisphosphate contributes to the Crabtree effect through closure of the mitochondrial unspecific channel

In Saccharomyces cerevisiae addition of glucose inhibits oxygen consumption, i.e. S. cerevisiae is Crabtree-positive. During active glycolysis hexoses-phosphate accumulate, and probably interact with mitochondria. In an effort to understand the mechanism underlying the Crabtree effect, the effect of two glycolysis-derived hexoses-phosphate was tested on the S. cerevisiae mitochondrial unspecific channel (ScMUC). Glucose-6-phosphate (G6P) promoted partial opening of ScMUC, which led to proton leakage and uncoupling which in turn resulted in, accelerated oxygen consumption. In contrast, fructose-1,6-bisphosphate (F1,6BP) closed ScMUC and thus inhibited the rate of oxygen consumption. When added together, F1,6BP reverted the mild G6P-induced effects. F1,6BP is proposed to be an important modulator of ScMUC, whose closure contributes to the "Crabtree effect".

Unsuspected pyocyanin effect in yeast under anaerobiosis

The blue–green phenazine, Pyocyanin (PYO), is a well-known virulence factor produced by Pseudomonas aeruginosa, notably during cystic fibrosis lung infections. It is toxic to both eukaryotic and bacterial cells and several mechanisms, including the induction of oxidative stress, have been postulated. However, the mechanism of PYO toxicity under the physiological conditions of oxygen limitation that are encountered by P. aeruginosa and by target organisms in vivo remains unclear. In this study, wild-type and mutant strains of the yeast Saccharomyces cerevisiae were used as an effective eukaryotic model to determine the toxicity of PYO (100–500 μmol/L) under key growth conditions. Under respiro-fermentative conditions (with glucose as substrate), WT strains and certain H₂O₂-hypersensitive strains showed a low-toxic response to PYO. Under respiratory conditions (with glycerol as substrate) all the strains tested were significantly more sensitive to PYO. Four antioxidants were tested but only N-acetylcysteine was capable of partially counteracting PYO toxicity. PYO did not appear to affect short-term respiratory O₂ uptake, but it did seem to interfere with cyanide-poisoned mitochondria through a complex III-dependent mechanism. Therefore, a combination of oxidative stress and respiration disturbance could partly explain aerobic PYO toxicity. Surprisingly, the toxic effects of PYO were more significant under anaerobic conditions. More pronounced effects were observed in several strains including a ‘petite’ strain lacking mitochondrial DNA, strains with increased or decreased levels of ABC transporters, and strains deficient in DNA damage repair. Therefore, even though PYO is toxic for actively respiring cells, O₂ may indirectly protect the cells from the higher anaerobic-linked toxicity of PYO. The increased sensitivity to PYO under anaerobic conditions is not unique to S. cerevisiae and was also observed in another yeast, Candida albicans.

The energetic state of mitochondria modulates complex III biogenesis through the ATP-dependent activity of Bcs1

Our understanding of the mechanisms involved in mitochondrial biogenesis has continuously expanded during the last decades, yet little is known about how they are modulated to optimize the functioning of mitochondria. Here, we show that mutations in the ATP binding domain of Bcs1, a chaperone involved in the assembly of complex III, can be rescued by mutations that decrease the ATP hydrolytic activity of the ATP synthase. Our results reveal a Bcs1-mediated control loop in which the biogenesis of complex III is modulated by the energy-transducing activity of mitochondria. Although ATP is well known as a regulator of a number of cellular activities, we show here that ATP can be also used to modulate the biogenesis of an enzyme by controlling a specific chaperone involved in its assembly. Our study further highlights the intramitochondrial adenine nucleotide pool as a potential target for the treatment of Bcs1-based disorders.

The mitochondrial genome impacts respiration but not fermentation in interspecific Saccharomyces hybrids

In eukaryotes, mitochondrial DNA (mtDNA) has high rate of nucleotide substitution leading to different mitochondrial haplotypes called mitotypes. However, the impact of mitochondrial genetic variant on phenotypic variation has been poorly considered in microorganisms because mtDNA encodes very few genes compared to nuclear DNA, and also because mitochondrial inheritance is not uniparental. Here we propose original material to unravel mitotype impact on phenotype: we produced interspecific hybrids between S. cerevisiae and S. uvarum species, using fully homozygous diploid parental strains. For two different interspecific crosses involving different parental strains, we recovered 10 independent hybrids per cross, and allowed mtDNA fixation after around 80 generations. We developed PCR-based markers for the rapid discrimination of S. cerevisiae and S. uvarum mitochondrial DNA. For both crosses, we were able to isolate fully isogenic hybrids at the nuclear level, yet possessing either S. cerevisiae mtDNA (Sc-mtDNA) or S. uvarum mtDNA (Su-mtDNA). Under fermentative conditions, the mitotype has no phenotypic impact on fermentation kinetics and products, which was expected since mtDNA are not necessary for fermentative metabolism. Alternatively, under respiratory conditions, hybrids with Sc-mtDNA have higher population growth performance, associated with higher respiratory rate. Indeed, far from the hypothesis that mtDNA variation is neutral, our work shows that mitochondrial polymorphism can have a strong impact on fitness components and hence on the evolutionary fate of the yeast populations. We hypothesize that under fermentative conditions, hybrids may fix stochastically one or the other mt-DNA, while respiratory environments may increase the probability to fix Sc-mtDNA.

Optical microwell array for large scale studies of single mitochondria metabolic responses

Microsystems based on microwell arrays have been widely used for studies on single living cells. In this work, we focused on the subcellular level in order to monitor biological responses directly on individual organelles. Consequently, we developed microwell arrays for the entrapment and fluorescence microscopy of single isolated organelles, mitochondria herein. Highly dense arrays of 3-μm mean diameter wells were obtained by wet chemical etching of optical fiber bundles. Favorable conditions for the stable entrapment of individual mitochondria within a majority of microwells were found. Owing to NADH auto-fluorescence, the metabolic status of each mitochondrion was analyzed at resting state (Stage 1), then following the addition of a respiratory substrate (Stage 2), ethanol herein, and of a respiratory inhibitor (Stage 3), antimycin A. Mean levels of mitochondrial NADH were increased by 29% and 35% under Stages 2 and 3, respectively. We showed that mitochondrial ability to generate higher levels of NADH (i.e., its metabolic performance) is not correlated either to the initial energetic state or to the respective size of each mitochondrion. This study demonstrates that microwell arrays allow metabolic studies on populations of isolated mitochondria with a single organelle resolution.

Monitoring metabolic responses of single mitochondria within poly(dimethylsiloxane) wells: study of their endogenous reduced nicotinamide adenine dinucleotide evolution

It is now demonstrated that mitochondria individually function differently because of specific energetic needs in cell compartments but also because of the genetic heterogeneity within the mitochondrial pool-network of a cell. Consequently, understanding mitochondrial functioning at the single organelle level is of high interest for biomedical research, therefore being a target for analyticians. In this context, we developed easy-to-build platforms of milli- to microwells for fluorescence microscopy of single isolated mitochondria. Poly(dimethylsiloxane) (PDMS) was determined to be an excellent material for mitochondrial deposition and observation of their NADH content. Because of NADH autofluorescence, the metabolic status of each mitochondrion was analyzed following addition of a respiratory substrate (stage 2), ethanol herein, and a respiratory inhibitor (stage 3), Antimycin A. Mean levels of mitochondrial NADH were increased by 32% and 62% under stages 2 and 3, respectively. Statistical studies of NADH value distributions evidenced different types of responses, at least three, to ethanol and Antimycin A within the mitochondrial population. In addition, we showed that mitochondrial ability to generate high levels of NADH, that is its metabolic performance, is not correlated either to the initial energetic state or to the respective size of each mitochondrion.

Mitochondrial DNA mutations provoke dominant inhibition of mitochondrial inner membrane fusion

Mitochondria are highly dynamic organelles that continuously move, fuse and divide. Mitochondrial dynamics modulate overall mitochondrial morphology and are essential for the proper function, maintenance and transmission of mitochondria and mitochondrial DNA (mtDNA). We have investigated mitochondrial fusion in yeast cells with severe defects in oxidative phosphorylation (OXPHOS) due to removal or various specific mutations of mtDNA. We find that, under fermentative conditions, OXPHOS deficient cells maintain normal levels of cellular ATP and ADP but display a reduced mitochondrial inner membrane potential. We demonstrate that, despite metabolic compensation by glycolysis, OXPHOS defects are associated to a selective inhibition of inner but not outer membrane fusion. Fusion inhibition was dominant and hampered the fusion of mutant mitochondria with wild-type mitochondria. Inhibition of inner membrane fusion was not systematically associated to changes of mitochondrial distribution and morphology, nor to changes in the isoform pattern of Mgm1, the major fusion factor of the inner membrane. However, inhibition of inner membrane fusion correlated with specific alterations of mitochondrial ultrastructure, notably with the presence of aligned and unfused inner membranes that are connected to two mitochondrial boundaries. The fusion inhibition observed upon deletion of OXPHOS related genes or upon removal of the entire mtDNA was similar to that observed upon introduction of point mutations in the mitochondrial ATP6 gene that are associated to neurogenic ataxia and retinitis pigmentosa (NARP) or to maternally inherited Leigh Syndrome (MILS) in humans. Our findings indicate that the consequences of mtDNA mutations may not be limited to OXPHOS defects but may also include alterations in mitochondrial fusion. Our results further imply that, in healthy cells, the dominant inhibition of fusion could mediate the exclusion of OXPHOS-deficient mitochondria from the network of functional, fusogenic mitochondria.

cAMP-induced mitochondrial compartment biogenesis: role of glutathione redox state

Cell fate and proliferation are tightly linked to the regulation of the mitochondrial energy metabolism. Hence, mitochondrial biogenesis regulation, a complex process that requires a tight coordination in the expression of the nuclear and mitochondrial genomes, has a major impact on cell fate and is of high importance. Here, we studied the molecular mechanisms involved in the regulation of mitochondrial biogenesis through a nutrient-sensing pathway, the Ras-cAMP pathway. Activation of this pathway induces a decrease in the cellular phosphate potential that alleviates the redox pressure on the mitochondrial respiratory chain. One of the cellular consequences of this modulation of cellular phosphate potential is an increase in the cellular glutathione redox state. The redox state of the glutathione disulfide-glutathione couple is a well known important indicator of the cellular redox environment, which is itself tightly linked to mitochondrial activity, mitochondria being the main cellular producer of reactive oxygen species. The master regulator of mitochondrial biogenesis in yeast (i.e. the transcriptional co-activator Hap4p) is positively regulated by the cellular glutathione redox state. Using a strain that is unable to modulate its glutathione redox state (Δglr1), we pinpoint a positive feedback loop between this redox state and the control of mitochondrial biogenesis. This is the first time that control of mitochondrial biogenesis through glutathione redox state has been shown.

Physiological uncoupling of mitochondrial oxidative phosphorylation. Studies in different yeast species

Under non-phosphorylating conditions a high proton transmembrane gradient inhibits the rate of oxygen consumption mediated by the mitochondrial respiratory chain (state IV). Slow electron transit leads to production of reactive oxygen species (ROS) capable of participating in deleterious side reactions. In order to avoid overproducing ROS, mitochondria maintain a high rate of O(2) consumption by activating different exquisitely controlled uncoupling pathways. Different yeast species possess one or more uncoupling systems that work through one of two possible mechanisms: i) Proton sinks and ii) Non-pumping redox enzymes. Proton sinks are exemplified by mitochondrial unspecific channels (MUC) and by uncoupling proteins (UCP). Saccharomyces. cerevisiae and Debaryomyces hansenii express highly regulated MUCs. Also, a UCP was described in Yarrowia lipolytica which promotes uncoupled O(2) consumption. Non-pumping alternative oxido-reductases may substitute for a pump, as in S. cerevisiae or may coexist with a complete set of pumps as in the branched respiratory chains from Y. lipolytica or D. hansenii. In addition, pumps may suffer intrinsic uncoupling (slipping). Promising models for study are unicellular parasites which can turn off their aerobic metabolism completely. The variety of energy dissipating systems in eukaryote species is probably designed to control ROS production in the different environments where each species lives.

Reactive oxygen species-mediated regulation of mitochondrial biogenesis in the yeast Saccharomyces cerevisiae

Mitochondrial biogenesis is a complex process. It necessitates the participation of both the nuclear and the mitochondrial genomes. This process is highly regulated, and mitochondrial content within a cell varies according to energy demand. In the yeast Saccharomyces cerevisiae, the cAMP pathway is involved in the regulation of mitochondrial biogenesis. An overactivation of this pathway leads to an increase in mitochondrial enzymatic content. Of the three yeast cAMP protein kinases, we have previously shown that Tpk3p is the one involved in the regulation of mitochondrial biogenesis. In this paper, we investigated the molecular mechanisms that govern this process. We show that in the absence of Tpk3p, mitochondria produce large amounts of reactive oxygen species that signal to the HAP2/3/4/5 nuclear transcription factors involved in mitochondrial biogenesis. We establish that an increase in mitochondrial reactive oxygen species production down-regulates mitochondrial biogenesis. It is the first time that a redox sensitivity of the transcription factors involved in yeast mitochondrial biogenesis is shown. Such a process could be seen as a mitochondria quality control process.

Tumor cell energy metabolism and its common features with yeast metabolism

During the last decades a considerable amount of research has been focused on cancer. A number of genetic and signaling defects have been identified. This has allowed the design and screening of a number of anti-tumor drugs for therapeutic use. One of the main challenges of anti-cancer therapy is to specifically target these drugs to malignant cells. Recently, tumor cell metabolism has been considered as a possible target for cancer therapy. It is widely accepted that tumors display an enhanced glycolytic activity and oxidative phosphorylation down-regulation (Warburg effect). Therefore, it seems reasonable that disruption of glycolysis might be a promising candidate for specific anti-cancer therapy. Nonetheless, the concept of aerobic glycolysis as the paradigm of tumor cell metabolism has been challenged, as some tumor cells use oxidative phosphorylation. Mitochondria are of special interest in cancer cell energy metabolism, as their physiology is linked to the Warburg effect. Besides, their central role in apoptosis makes these organelles a promising "dual hit target" for selectively eliminate tumor cells. Thus, it is desirable to have an easy-to-use and reliable model in order to do the screening for energy metabolism-inhibiting drugs to be used in cancer therapy. From a metabolic point of view, the fermenting yeast Saccharomyces cerevisiae and tumor cells share several features. In this paper we will review these common metabolic properties and we will discuss the possibility of using S. cerevisiae as an early screening test in the research for novel anti-tumor compounds used for the inhibition of tumor cell metabolism.

The trehalose pathway regulates mitochondrial respiratory chain content through hexokinase 2 and cAMP in Saccharomyces cerevisiae

In yeast, trehalose is synthesized by a multimeric enzymatic complex: TPS1 encodes trehalose 6-phosphate synthase, which belongs to a complex that is composed of at least three other subunits, including trehalose 6-phosphate phosphatase Tps2 and the redundant regulatory subunits Tps3 and Tsl1. The product of the TPS1 gene plays an essential role in the control of the glycolytic pathway by restricting the influx of glucose into glycolysis. In this paper, we investigated whether the trehalose synthesis pathway could be involved in the control of the other energy-generating pathway: oxidative phosphorylation. We show that the different mutants of the trehalose synthesis pathway (tps1Delta, tps2Delta, and tps1,2Delta) exhibit modulation in the amount of respiratory chains, in terms of cytochrome content and maximal respiratory activity. Furthermore, these variations in mitochondrial enzymatic content are positively linked to the intracellular concentration in cAMP that is modulated by Tps1p through hexokinase2. This is the first time that a pathway involved in sugar storage, i.e. trehalose, is shown to regulate the mitochondrial enzymatic content.

Mitochondrial oxidative phosphorylation is regulated by fructose 1,6-bisphosphate. A possible role in Crabtree effect induction?

In numerous cell types, tumoral cells, proliferating cells, bacteria, and yeast, respiration is inhibited when high concentrations of glucose are added to the culture medium. This phenomenon has been named the "Crabtree effect." We used yeast to investigate (i) the short term event(s) associated with the Crabtree effect and (ii) a putative role of hexose phosphates in the inhibition of respiration. Indeed, yeast divide into "Crabtree-positive," where the Crabtree effect occurs, and "Crabtree-negative," where it does not. In mitochondria isolated from these two categories of yeast, we found that low, physiological concentrations of glucose 6-phosphate and fructose 6-phosphate slightly (20%) stimulated the respiratory flux and that this effect was strongly antagonized by fructose 1,6-bisphosphate (F16bP). On the other hand, F16bP by itself was able to inhibit mitochondrial respiration only in mitochondria isolated from a Crabtree-positive strain. Using permeabilized spheroplasts from Crabtree-positive yeast, we have shown that the sole effect observed at physiological concentrations of hexose phosphates is an inhibition of oxidative phosphorylation by F16bP. This F16bP-mediated inhibition was also observed in isolated rat liver mitochondria, extending this process to mammalian cells. From these results and taking into account that F16bP is able to accumulate in the cell cytoplasm, we propose that F16bP regulates oxidative phosphorylation and thus participates in the establishment of the Crabtree effect.

Significant increase of glycolytic flux in Torulopsis glabrata by inhibition of oxidative phosphorylation

This study was aimed at increasing the glycolytic flux of the multivitamin-auxotrophic yeast Torulopsis glabrata by disturbing oxidative phosphorylation. We examined two different strategies to impede oxidative phosphorylation. The first strategy was disruption of the activity of the electron transfer chain (ETC), by either of two approaches. One was separately adding, at 10 mg L1, specific inhibitors of complex I (rotenone) or of the bc1 complex (antimycin A) to the culture broth of T. glabrata CCTCC M202019, which resulted in significantly decreased intracellular ATP levels (43% and 27.7%) and significantly increased rates of glucose consumption (qs) and pyruvate production (qp); another approach was breeding a respiratory-deficient mutant RD-16, in which cytochromes aa3 and b in the ETC were deleted after ethidium bromide mutagenesis, to reduce the ETC activity constitutively. The second strategy was inhibiting F0F1-ATP synthase with 0.05 mM oligomycin. Also, a neomycin-resistant mutant with 65% decreased F0F1-ATPase activity was studied. With the two strategies, the specific activity of phosphofructokinase (R2=0.9971), the average specific glucose consumption rate (R2=0.9967) and the average specific pyruvate production rate (R2=0.965) were closely correlated with the intracellular ATP level, all of them being increased at a lower intracellular ATP level.

Ypr140wp, 'the yeast tafazzin', displays a mitochondrial lysophosphatidylcholine (lyso-PC) acyltransferase activity related to triacylglycerol and mitochondrial lipid synthesis

When the yeast protein Ypr140w was expressed in Escherichia coli, a lyso-PC [lysophosphatidylcholine (1-acylglycerophosphorylcholine)] acyltransferase activity was found associated with the membranes of the bacteria. To our knowledge, this is the first identification of a protein capable of catalysing the acylation of lyso-PC molecules to form PC. Fluorescence microscopy analysis of living yeasts revealed that the fusion protein Ypr140w-green fluorescent protein is targeted to the mitochondria. Moreover, in contrast with wild-type cells, in the absence of acyl-CoA, the yeast mutant deleted for the YPR140w gene has no lyso-PC acyltransferase activity associated with the mitochondrial fraction. When yeast cells were grown in the presence of lactate, the mutant synthesized 2-fold more triacylglycerols when compared with the wild-type. Moreover, its mitochondrial membranes contained a lesser amount of PC and cardiolipin, and the fatty acid composition of these latter was greatly changed. These modifications were accompanied by a 2-fold increase in the respiration rates (states 3 and 4) of the mitochondria. The relationship between the deletion of the YPR140w gene and the lipid composition of the ypr140wDelta cells is discussed.

The YIG1 (YPL201c) encoded protein is involved in regulating anaerobic glycerol metabolism in Saccharomyces cerevisiae

Under anaerobic conditions S. cerevisiae produces glycerol to regenerate NAD(+) from the excess NADH produced in cell metabolism. We here report on the role of an uncharacterized protein, Yig1p (Ypl201cp), in anaerobic glycerol production. Yig1p was previously shown to interact in two-hybrid tests with the GPP1 and GPP2 encoded glycerol 3-phosphatase (Gpp), and we here demonstrate that strains overexpressing YIG1 show strongly decreased Gpp activity and content of the major phosphatase, Gpp1p. However, cells overexpressing YIG1 exhibited only slightly decreased GPP1 transcript levels, suggesting that Yig1p modulates expression on both transcriptional and post-transcriptional levels. In agreement with such a role, a GFP-tagged derivate of Yig1p was localized to both the cytosol and the nucleus. Deletion or overexpression of YIG1 did not, however, significantly affect growth yield or glycerol yield in anaerobic batch cultures, which is consistent with the previously proposed low flux control exerted at the Gpp level.

Loss of NAD(H) from swollen yeast mitochondria

BACKGROUND

The mitochondrial electron transport chain oxidizes matrix space NADH as part of the process of oxidative phosphorylation. Mitochondria contain shuttles for the transport of cytoplasmic NADH reducing equivalents into the mitochondrial matrix. Therefore for a long time it was believed that NAD(H) itself was not transported into mitochondria. However evidence has been obtained for the transport of NAD(H) into and out of plant and mammalian mitochondria. Since Saccharomyces cerevisiae mitochondria can directly oxidize cytoplasmic NADH, it remained questionable if mitochondrial NAD(H) transport occurs in this organism.

RESULTS

NAD(H) was lost more extensively from the matrix space of swollen than normal, condensed isolated yeast mitochondria from Saccharomyces cerevisiae. The loss of NAD(H) in swollen organelles caused a greatly decreased respiratory rate when ethanol or other matrix space NAD-linked substrates were oxidized. Adding NAD back to the medium, even in the presence of a membrane-impermeant NADH dehydrogenase inhibitor, restored the respiratory rate of swollen mitochondria oxidizing ethanol, suggesting that NAD is transported into the matrix space. NAD addition did not restore the decreased respiratory rate of swollen mitochondria oxidizing the combination of malate, glutamate, and pyruvate. Therefore the loss of matrix space metabolites is not entirely specific for NAD(H). However, during NAD(H) loss the mitochondrial levels of most other nucleotides were maintained. Either hypotonic swelling or colloid-osmotic swelling due to opening of the yeast mitochondrial unspecific channel (YMUC) in a mannitol medium resulted in decreased NAD-linked respiration. However, the loss of NAD(H) from the matrix space was not mediated by the YMUC, because YMUC inhibitors did not prevent decreased NAD-linked respiration during swelling and YMUC opening without swelling did not cause decreased NAD-linked respiration.

CONCLUSION

Loss of endogenous NAD(H) from isolated yeast mitochondria is greatly stimulated by matrix space expansion. NAD(H) loss greatly limits NAD-linked respiration in swollen mitochondria without decreasing the NAD-linked respiratory rate in normal, condensed organelles. NAD addition can totally restore the decreased respiration in swollen mitochondria. In live yeast cells mitochondrial swelling has been observed prior to mitochondrial degradation and cell death. Therefore mitochondrial swelling may stimulate NAD(H) transport to regulate metabolism during these conditions.

Engineering of a novel Saccharomyces cerevisiae wine strain with a respiratory phenotype at high external glucose concentrations

The recently described respiratory strain Saccharomyces cerevisiae KOY.TM6*P is, to our knowledge, the only reported strain of S. cerevisiae which completely redirects the flux of glucose from ethanol fermentation to respiration, even at high external glucose concentrations (27). In the KOY.TM6*P strain, portions of the genes encoding the predominant hexose transporter proteins, Hxt1 and Hxt7, were fused within the regions encoding transmembrane (TM) domain 6. The resulting chimeric gene, TM6*, encoded a chimera composed of the amino-terminal half of Hxt1 and the carboxy-terminal half of Hxt7. It was subsequently integrated into the genome of an hxt null strain. In this study, we have demonstrated the transferability of this respiratory phenotype to the V5 hxt1-7Delta strain, a derivative of a strain used in enology. We also show by using this mutant that it is not necessary to transform a complete hxt null strain with the TM6* construct to obtain a non-ethanol-producing phenotype. The resulting V5.TM6*P strain, obtained by transformation of the V5 hxt1-7Delta strain with the TM6* chimeric gene, produced only minor amounts of ethanol when cultured on external glucose concentrations as high as 5%. Despite the fact that glucose flux was reduced to 30% in the V5.TM6*P strain compared with that of its parental strain, the V5.TM6*P strain produced biomass at a specific rate as high as 85% that of the V5 wild-type strain. Even more relevant for the potential use of such a strain for the production of heterologous proteins and also of low-alcohol beverages is the observation that the biomass yield increased 50% with the mutant compared to its parental strain.

Dynamical remodeling of the transcriptome during short-term anaerobiosis in Saccharomyces cerevisiae: differential response and role of Msn2 and/or Msn4 and other factors in galactose and glucose media

In contrast to previous steady-state analyses of the O(2)-responsive transcriptome, here we examined the dynamics of the response to short-term anaerobiosis (2 generations) in both catabolite-repressed (glucose) and derepressed (galactose) cells, assessed the specific role that Msn2 and Msn4 play in mediating the response, and identified gene networks using a novel clustering approach. Upon shifting cells to anaerobic conditions in galactose medium, there was an acute ( approximately 10 min) yet transient (<45 min) induction of Msn2- and/or Msn4-regulated genes associated with the remodeling of reserve energy and catabolic pathways during the switch from mixed respiro-fermentative to strictly fermentative growth. Concomitantly, MCB- and SCB-regulated networks associated with the G(1)/S transition of the cell cycle were transiently down-regulated along with rRNA processing genes containing PAC and RRPE motifs. Remarkably, none of these gene networks were differentially expressed when cells were shifted in glucose, suggesting that a metabolically derived signal arising from the abrupt cessation of respiration, rather than O(2) deprivation per se, elicits this "stress response." By approximately 0.2 generation of anaerobiosis in both media, more chronic, heme-dependent effects were observed, including the down-regulation of Hap1-regulated networks, derepression of Rox1-regulated networks, and activation of Upc2-regulated ones. Changes in these networks result in the functional remodeling of the cell wall, sterol and sphingolipid metabolism, and dissimilatory pathways required for long-term anaerobiosis. Overall, this study reveals that the acute withdrawal of oxygen can invoke a metabolic state-dependent "stress response" but that acclimatization to oxygen deprivation is a relatively slow process involving complex changes primarily in heme-regulated gene networks.

Redox interactions between Saccharomyces cerevisiae and Saccharomyces uvarum in mixed culture under enological conditions

Wine yeast starters that contain a mixture of different industrial yeasts with various properties may soon be introduced to the market. The mechanisms underlying the interactions between the different strains in the starter during alcoholic fermentation have never been investigated. We identified and investigated some of these interactions in a mixed culture containing two yeast strains grown under enological conditions. The inoculum contained the same amount (each) of a strain of Saccharomyces cerevisiae and a natural hybrid strain of S. cerevisiae and Saccharomyces uvarum. We identified interactions that affected biomass, by-product formation, and fermentation kinetics, and compared the redox ratios of monocultures of each strain with that of the mixed culture. The redox status of the mixed culture differed from that of the two monocultures, showing that the interactions between the yeast strains involved the diffusion of metabolite(s) within the mixed culture. Since acetaldehyde is a potential effector of fermentation, we investigated the kinetics of acetaldehyde production by the different cultures. The S. cerevisiae-S. uvarum hybrid strain produced large amounts of acetaldehyde for which the S. cerevisiae strain acted as a receiving strain in the mixed culture. Since yeast response to acetaldehyde involves the same mechanisms that participate in the response to other forms of stress, the acetaldehyde exchange between the two strains could play an important role in inhibiting some yeast strains and allowing the growth of others. Such interactions could be of particular importance in understanding the ecology of the colonization of complex fermentation media by S. cerevisiae.

The yeast cAMP protein kinase Tpk3p is involved in the regulation of mitochondrial enzymatic content during growth

During aerobic cell growth, mitochondria must meet energy demand either by adjusting cellular mitochondrial content or by adjusting ATP production flux, allowing a constant growth yield. On respiratory substrate, the Ras/cAMP pathway has been shown to be involved in this process in the yeast Saccharomyces cerevisiae. We show that of the three cAMP protein kinase catalytic subunits, Tpk3p is the one specifically involved in the regulation of cellular mitochondrial content when energy demand decreases. In decreased energy demand, the Deltatpk3 mitochondrial enzymatic content decreases leading to a subsequent decrease in the cellular growth rate. Moreover, enzymatic content decreases in the Deltatpk3 isolated mitochondria, suggesting that the amount of cellular mitochondria is not affected, but rather that the mitochondria are modified. Our study points to an important decrease in the cytochrome c content in the Deltatpk3 mitochondria, which leads to a decrease in the slipping process at the level of cytochrome-c-oxidase.

Organization and regulation of the cytosolic NADH metabolism in the yeast Saccharomyces cerevisiae

Keeping a cytosolic redox balance is a prerequisite for living cells in order to maintain a metabolic activity and enable growth. During growth of Saccharomyces cerevisiae, an excess of NADH is generated in the cytosol. Aerobically, it has been shown that the external NADH dehydrogenase, Nde1p and Nde2p, as well as the glycerol-3-phosphate dehydrogenase shuttle, comprising the cytoplasmic glycerol-3-phosphate dehydrogenase, Gpdlp, and the mitochondrial glycerol-3-phosphate dehydrogenase, Gut2p, are the most important mechanisms for mitochondrial oxidation of cytosolic NADH. In this review we summarize the recent results showing (i) the contribution of each of the mechanisms involved in mitochondrial oxidation of the cytosolic NADH, under different physiological situations; (ii) the kinetic and structural properties of these metabolic pathways in order to channel NADH from cytosolic dehydrogenases to the inner mitochondrial membrane and (iii) the organization in supramolecular complexes and, the peculiar ensuing kinetic regulation of some of the enzymes (i.e. Gut2p inhibition by external NADH dehydrogenase activity) leading to a highly integrated functioning of enzymes having a similar physiological function. The cell physiological consequences of such an organized and regulated network are discussed.

In Saccharomyces cerevisiae, cations control the fate of the energy derived from oxidative metabolism through the opening and closing of the yeast mitochondrial unselective channel

The yeast mitochondrial unspecific channel (YMUC) sensitivity to inorganic (Ca2+ or Mg2+) or organic (hexyl or octyl-guanidine) cations was measured. The rate of oxygen consumption in State 3 and State 4, the transmembrane potential (deltapsi), mitochondrial swelling, and the polyethylene-glycol mediated recontraction were used to follow opening of the YMUC. Addition of 0.4 mM PO4 did not close the YMUC, although it did enhance the sensitivity to Ca2+ (I50 decreased from 50 to 0.3 mM) and Mg2+ (I50 decreased from 5 to 0.83 mM Mg2+). The Ca2+ concentration needed to close the YMUC was higher than the concentrations usually observed in the cell. Nonetheless, Mg2+, Ca2+, and PO4 exhibited additive effects. These cations did not inhibit contraction of preswollen mitochondria, suggesting that the YMUC/cation interaction was labile. Octyl-guanidine (OG-I50 7.5 microM) was the only cation which inhibited mitochondrial recontraction, probably as a result of membrane binding stabilization through its hydrophobic tail. The PO4-dependent, Ca(2+)/Mg(2+)-mediated closure of the YMUC may be a means to control the proportion of oxidative energy producing ATP or being lost as heat.

Role of the non-respiratory pathways in the utilization of molecular oxygen by Saccharomyces cerevisiae

Saccharomyces cerevisiae is a facultative anaerobe devoid of mitochondrial alternative oxidase. In this yeast, the structure and biogenesis of the respiratory chain, on the one hand, and the functional interactions of oxidative phosphorylation with the cellular energetic metabolism, on the other, are well documented. However, to our knowledge, the molecular aspects and the physiological roles of the non-respiratory pathways that utilize molecular oxygen have not yet been reviewed. In this paper, we review the various non-respiratory pathways in a global context of utilization of molecular oxygen in S. cerevisiae. The roles of these pathways are examined as a function of environmental conditions, using either physiological, biochemical or molecular data. Special attention is paid to the characterization of the so-called 'cyanide-resistant respiration' that is induced by respiratory deficiency, catabolic repression and oxygen limitation during growth. Finally, several aspects of oxygen sensing are discussed.

Non-respiratory oxygen consumption pathways in anaerobically-grown Saccharomyces cerevisiae: evidence and partial characterization

Despite the absence of an alternative mitochondrial ubiquinol oxidase, Saccharomyces cerevisiae consumes oxygen in an antimycin A- and cyanide-resistant manner. Cyanide-resistant respiration is typically used when the classical respiratory chain is impaired or absent (i.e in anaerobically-grown cells shifted to normoxia or in respiratory-deficient cells). We characterized the non-respiratory oxygen consumption pathways operating during anoxic-normoxic transitions in glucose-repressed resting cells. High-resolution oxygraphy confirmed that the cellular non-respiratory oxygen consumption pathway is sensitive to high concentrations of cyanide, azide, SHAM and TTFA, and revealed several new characteristics. First, the use of sterol biosynthesis inhibitors showed that this pathway makes a considerable contribution (about 25%) to both endogenous and glucose-dependent oxygen consumption. Anaerobically-grown glucose-repressed cells exhibited high apparent oxygen affinities (K(m) for oxygen = 0.5-1 micro M), even in mutants deficient in respiration or sterol synthesis. Exogeneously added glucose and endogenous stored carbohydrates were the only substrates that were efficient for cellular oxygen consumption (apparent K(m) for exogenous glucose = 2-3 mM). On the other hand, fluorimetric measurements of the cellular NAD(P)H pool showed that the cellular oxygen consumption (sterol biosynthesis and unknown pathways) was dependent more on the intracellular level of NADPH than of NADH. High oxygen affinity NADPH-dependent oxygen consumption systems were thought to be mainly localized in microsomal membranes, and several data indicated a significant contribution made by uncoupled p450 systems, together with still uncharacterized systems. Such activities are associated in vitro with a massive production of O(2) (.-) and, to a lower extent, H(2)O(2) and a likely concomitant production of H(2)O.

Kinetic regulation of the mitochondrial glycerol-3-phosphate dehydrogenase by the external NADH dehydrogenase in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the two most important systems for conveying excess cytosolic NADH to the mitochondrial respiratory chain are external NADH dehydrogenase (Nde1p/Nde2p) and the glycerol-3-phosphate dehydrogenase shuttle. In the latter system, NADH is oxidized to NAD+ and dihydroxyacetone phosphate is reduced to glycerol 3-phosphate by the cytosolic Gpd1p; glycerol 3-phosphate gives two electrons to the respiratory chain via mitochondrial glycerol-3-phosphate dehydrogenase (Gut2p)-regenerating dihydroxyacetone phosphate. Both Nde1p/Nde2p and Gut2p are located in the inner mitochondrial membrane with catalytic sites facing the intermembranal space. In this study, we showed kinetic interactions between these two enzymes. First, deletion of either one of the external dehydrogenases caused an increase in the efficiency of the remaining enzyme. Second, the activation of NADH dehydrogenase inhibited the Gut2p in such a manner that, at a saturating concentration of NADH, glycerol 3-phosphate is not used as respiratory substrate. This effect was not a consequence of a direct action of NADH on Gut2p activity because both NADH dehydrogenase and its substrate were needed for Gut2p inhibition. This kinetic regulation of the activity of an enzyme as a function of the rate of another having a similar physiological function may be allowed by their association into the same supramolecular complex in the inner membrane. The physiological consequences of this regulation are discussed.

Cytosolic redox metabolism in aerobic chemostat cultures of Saccharomyces cerevisiae

Cytosolic redox balance has to be maintained in order to allow an enduring cellular metabolism. In other words, NADH generated in the cytosol has to be re-oxidized back to NAD(+). Aerobically this can be done by respiratory oxidation of cytosolic NADH. However, NADH is unable to cross the mitochondrial inner membrane and mechanisms are required for conveying cytosolic NADH to the mitochondrial electron transport chain. At least two such systems have proved to be functional in S. cerevisiae, the external NADH dehydrogenase (Luttik et al., 1998; Small and McAlister-Henn, 1998) and the G3P shuttle (Larsson et al., 1998). The aim of this investigation was to study the regulation and performance of these two systems in a wild-type strain of S. cerevisiae using aerobic glucose- and nitrogen-limited chemostat cultures. The rate of cytosolic NADH formation was calculated and as expected there was a continuous increase with increasing dilution rate. However, measurements of enzyme activities and respiratory activity on isolated mitochondria revealed a diminishing capacity at elevated dilution rates for both the external NADH dehydrogenase and the G3P shuttle. This suggests that adjustment of in vivo activities of these systems to proper levels is not achieved by changes in amount of protein but rather by, for example, activation/inhibition of existing enzymes. Adenine nucleotides are well-known allosteric regulators and both the external NADH and the G3P shuttle were sensitive to inhibition by ATP. The most severe inhibition was probably on the G3P shuttle, since one of its member proteins, Gpdp, turned out to be exceptionally sensitive to ATP. The external NADH dehydrogenase is suggested as the main system employed for oxidation of cytosolic NADH. The G3P shuttle is proposed to be of some importance at low growth rates and perhaps its real significance is only expressed during starvation conditions.

Dynamics of metabolism and its interactions with gene expression during sporulation in Saccharomyces cerevisiae

The dynamics of metabolism has been shown to be involved in the triggering of events that are concurrent with sporulation of the budding yeast Saccharomyces cerevisiae. Indeed, quantitative correlations have been demonstrated between sporulation and the rate of carbon substrate or oxygen consumption, and the fluxes through gluconeogenic and glyoxylate cycle pathways. The results suggest that an imbalance between catabolic and anabolic fluxes influences the occurrence of the differentiation process. The hypothesis that the initiation of sporulation is triggered by the accumulation of an intracellular metabolite is confronted with the notion that intermediary metabolism and the expression of genes involved in sporulation interact to trigger the differentiation process. Several pieces of evidence indicate that derepression of the gluconeogenic pathway is crucial for the initiation of sporulation. One of the possible pathways through which glucose repression hampers sporulation might be the repression of gluconeogenesis as well as that of respiratory activity, in turn modulating the expression of IMEL++. The stages defined in the dynamics of sporulating cultures, namely readiness and commitment, are related to metabolic events associated with sporulation. An interpretation in terms of metabolic flux dynamics is given to the reversal of commitment occurring when the normal progression to sporulation is somehow blocked. The quantitative data are here integrated in a model attempting to simulate the dynamics of metabolic as well as cellular events during sporulation. The model is envisaged as a test of the hypothesis that an imbalance between anabolism and catabolism is involved in initiation of the sporulation process. It is proposed that such an imbalance may be a signal for differential gene expression associated with the differentiation pathway.

Dynamic in vivo (31)P nuclear magnetic resonance study of Saccharomyces cerevisiae in glucose-limited chemostat culture during the aerobic-anaerobic shift

The purpose of this work was to analyse in vivo the influence of sudden oxygen depletion on Saccharomyces cerevisiae, grown in glucose-limited chemostat culture, using a recently developed cyclone reactor coupled with (31)P NMR spectroscopy. Before, during and after the transition, intracellular and extracellular phosphorylated metabolites as well as the pHs in the different cellular compartments were monitored with a time resolution of 2.5 min. The employed integrated NMR bioreactor system allowed the defined glucose-limited continuous cultivation of yeast at a density of 75 g DW/l and a p(O(2)) of 30% air saturation. A purely oxidative metabolism was maintained at all times. In vivo (31)P NMR spectra obtained were of excellent quality and even allowed the detection of phosphoenolpyruvate (PEP). During the switch from aerobic to anaerobic conditions, a rapid, significant decrease of intracellular ATP and PEP levels was observed and the cytoplasmic pH decreased from 7.5 to 6.8. This change, which was accompanied by a transient influx of extracellular inorganic phosphate (P(i)), appeared to correlate linearly with the decrease of the ATP concentration, suggesting that the cause of the partial collapse of the plasma membrane pH gradient was a reduced availability of ATP. The complete phosphorous balance established from our measurement data showed that polyphosphate was not the source of the increased intracellular P(i). The derived intracellular P(i), ATP and ADP concentration data confirmed that the glycolytic flux at the level of glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase and enolase enzymes is mainly controlled by thermodynamic constraints.

Growth of the yeast Saccharomyces cerevisiae on a non-fermentable substrate: control of energetic yield by the amount of mitochondria

The purpose of this study was to investigate the long-term control of ATP synthesis during the course of Saccharomyces cerevisiae batch grown on lactate, a purely respiratory substrate. For this, we used a respirometric and on-line calorimetric approach to analyse the energetic balances and the control of energetic metabolism during growth. Enthalpic growth yields assessed by enthalpy balance (taking account of substrate consumption, by-product accumulation, biomass formation and heat dissipation) remained constant during the entire exponential growth. Moreover, at the same time, a parallel decrease in basal respiratory rate and enthalpy flux occurred. It is shown that the decrease in respiration corresponds to a decrease in the amount of mitochondria per cell but not to a change of steady state of oxidative phosphorylation. Taking into account the part of energy used for maintenance, it can be concluded that mitochondria by themselves are the major heat dissipative system in a fully aerobic metabolism, and that the decrease in the amount of mitochondria when growth rate decreases leads to an enthalpic growth yield constant.

Internal regulation of ATP turnover, glycolysis and oxidative phosphorylation in rat hepatocytes

Previously [Ainscow, E.K. & Brand, M.D. (1999) Eur. J. Biochem. 263, 671-685], top-down control analysis was used to describe the control pattern of energy metabolism in rat hepatocytes. The system was divided into nine reaction blocks (glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, mitochondrial proton leak, mitochondrial phosphorylation and ATP consumption) linked by five intermediates (intracellular glucose 6-phosphate, pyruvate and ATP levels, cytoplasmic NADH/NAD ratio and mitochondrial membrane potential). The kinetic responses (elasticities) of reaction blocks to intermediates were determined and used to calculate control coefficients. In the present paper, these elasticities and control coefficients are used to quantify the internal regulatory pathways within the cell. Flux control coefficients were partitioned to give partial flux control coefficients. These describe how strongly one block of reactions controls the flux through another via its effects on the concentration of a particular intermediate. Most flux control coefficients were the sum of positive and negative partial effects acting through different intermediates; these partial effects could be large compared to the final control strength. An important result was the breakdown of the way ATP consumption controlled respiration: changes in ATP level were more important than changes in mitochondrial membrane potential in stimulating oxygen consumption when ATP consumption increased. The partial internal response coefficients to changes in each intermediate were also calculated; they describe how steady state concentrations of intermediates are maintained. Increases in mitochondrial membrane potential were opposed mostly by decreased supply, whereas increases in glucose-6-phosphate, NADH/NAD and pyruvate were opposed mostly by increased consumption. Increases in ATP were opposed significantly by both decreased supply and increased consumption.

Metabolic control analysis of the bc1 complex of Saccharomyces cerevisiae: effect on cytochrome c oxidase, respiration and growth rate

A number of strains varying in steady-state level of assembled bc1 complex were used to test the conclusions from inhibitor titration experiments with isolated mitochondria that, in cells of Saccharomyces cerevisiae grown on non-fermentable carbon sources, the control coefficient of the bc1 complex on the mitochondrial respiratory capacity equals 1 and the respiratory chain consists of supermolecular respiratory units [Boumans, Grivell and Berden (1998) J. Biol. Chem. 273, 4872-4877]. In addition, the control coefficient of mitochondrial respiration on the growth rate was determined. It was found that a reduced level of bc1 complex is accompanied by an almost parallel decrease in steady-state level of cytochrome c oxidase. Since the linear relationship between level of active bc1 complex and respiratory capacity still holds, it is concluded that cytochrome c oxidase has disappeared from respiratory units that are already deficient in the bc1 complex and that the cytochrome c oxidase in a respiratory unit is destabilized when the bc1 complex is deficient. The control coefficient of the bc1 complex, and thus of mitochondrial electron-transfer capacity, on respiration of intact cells (without uncoupler added) is 0.20. Addition of uncoupler results in an increase in the coefficient to 0.36. Thus changing the respiratory state changes the distribution of control, increasing the control coefficient of electron-transfer activity as the respiratory state goes towards State 3u. Rates of growth of the strains on different carbon sources were determined and subsequently fitted to calculate control coefficients of the bc1 complex (and therefore of the respiratory capacity) on growth. Little variation was found between lactate-, ethanol- and glycerol-containing media, control coefficients being around 0.18 at pH 5. At pH 7 the control coefficient increased to 0.57, indicative of a higher dependence of the cell on ATP derived from oxidative phosphorylation. During growth on glucose-containing medium, the bc1 complex has no control on the growth rate, as indicated by the fact that all strains, including a respiratory-deficient strain, grow as fast as the wild-type. However, the presence of respiratory capacity in the wild-type does result in a higher growth yield compared with the respiratory-deficient strain, indicating that, in contrast with what is generally assumed, in S. cerevisiae the 'Pasteur effect' is not restricted to special experimental conditions.

The importance of the glycerol 3-phosphate shuttle during aerobic growth of Saccharomyces cerevisiae

Maintenance of a cytoplasmic redox balance is a necessity for sustained cellular metabolism. Glycerol formation is the only way by which Saccharomyces cerevisiae can maintain this balance under anaerobic conditions. Aerobically, on the other hand, several different redox adjustment mechanisms exist, one of these being the glycerol 3-phosphate (G3P) shuttle. We have studied the importance of this shuttle under aerobic conditions by comparing growth properties and glycerol formation of a wild-type strain with that of gut2 delta mutants, lacking the FAD-dependent glycerol 3-phosphate dehydrogenase, assuming that the consequent blocking of G3P oxidation is forcing the cells to produce glycerol from G3P. To impose different demands on the redox adjustment capability we used various carbon sources having different degrees of reduction. The results showed that the shuttle was used extensively with reduced substrate such as ethanol, whereas the more oxidized substrates lactate and pyruvate, did not provoke any activity of the shuttle. However, the absence of a functional G3P shuttle did not affect the growth rate or growth yield of the cells, not even during growth on ethanol. Presumably, there must be alternative systems for maintaining a cytoplasmic redox balance, e.g. the so-called external NADH dehydrogenase, located on the outer side of the inner mitochondrial membrane. By comparing the performance of the external NADH dehydrogenase and the G3P shuttle in isolated mitochondria, it was found that the former resulted in high respiratory rates but a comparably low P/O ratio of 1.2, whereas the shuttle gave low rates but a high P/O ratio of 1.7. Our results also demonstrated that of the two isoforms of NAD-dependent glycerol 3-phosphate dehydrogenase, only the enzyme encoded by GPD1 appeared important for the shuttle, since the enhanced glycerol production that occurs in a gut2 delta strain proved dependent on GPD1 but not on GPD2.

Glycolytic flux is conditionally correlated with ATP concentration in Saccharomyces cerevisiae: a chemostat study under carbon- or nitrogen-limiting conditions

Anaerobic and aerobic chemostat cultures of Saccharomyces cerevisiae were performed at a constant dilution rate of 0.10 h(-1). The glucose concentration was kept constant, whereas the nitrogen concentration was gradually decreasing; i.e., the conditions were changed from glucose and energy limitation to nitrogen limitation and energy excess. This experimental setup enabled the glycolytic rate to be separated from the growth rate. There was an extensive uncoupling between anabolic energy requirements and catabolic energy production when the energy source was present in excess both aerobically and anaerobically. To increase the catabolic activity even further, experiments were carried out in the presence of 5 mM acetic acid or benzoic acid. However, there was almost no effect with acetate addition, whereas both respiratory (aerobically) and fermentative activities were elevated in the presence of benzoic acid. There was a strong negative correlation between glycolytic flux and intracellular ATP content; i.e., the higher the ATP content, the lower the rate of glycolysis. No correlation could be found with the other nucleotides tested (ADP, GTP, and UTP) or with the ATP/ADP ratio. Furthermore, a higher rate of glycolysis was not accompanied by an increasing level of glycolytic enzymes. On the contrary, the glycolytic enzymes decreased with increasing flux. The most pronounced reduction was obtained for HXK2 and ENO1. There was also a correlation between the extent of carbohydrate accumulation and glycolytic flux. A high accumulation was obtained at low glycolytic rates under glucose limitation, whereas nitrogen limitation during conditions of excess carbon and energy resulted in more or less complete depletion of intracellular storage carbohydrates irrespective of anaerobic or aerobic conditions. However, there was one difference in that glycogen dominated anaerobically whereas under aerobic conditions, trehalose was the major carbohydrate accumulated. Possible mechanisms which may explain the strong correlation between glycolytic flux, storage carbohydrate accumulation, and ATP concentrations are discussed.

31P NMR magnetization transfer study of the control of ATP turnover in Saccharomyces cerevisiae

31P NMR magnetization transfer measurements have been used to measure the steady state flux between Pi and ATP in yeast cells genetically modified to overexpress an adenine nucleotide translocase isoform. An increase in Pi -> ATP flux and apparent ratio of moles of ATP synthesized/atoms of oxygen consumed (P:O ratio), when these cells were incubated with glucose, demonstrated that the reactions catalyzed by the translocase and F1F0 ATP synthase were readily reversible in vivo. However, when the same cells were incubated with ethanol alone, translocase overexpression had no effect on the measured Pi -> ATP flux or apparent P:O ratio, suggesting that the synthase was now operating irreversibly. This change was accompanied by an increase in the intracellular ADP concentration. These observations are consistent with a model proposed for the kinetic control of mitochondrial ATP synthesis, which was based on isotope exchange measurements with isolated mammalian mitochondria [LaNoue, K. F., Jeffries, F. M. H. & Radda, G. K. (1986) Biochemistry 25, 7667-7675].

The mechanism for the ATP-induced uncoupling of respiration in mitochondria of the yeast Saccharomyces cerevisiae

We have recently reported that ATP induces an uncoupling pathway in Saccharomyces cerevisiae mitochondria [Prieto, Bouillaud, Ricquier and Rial (1992) Eur. J. Biochem. 208, 487-491]. The presence of this pathway would explain the reported low efficiency of oxidative phosphorylation in S. cerevisiae, and may represent one of the postulated energy-dissipating mechanisms present in these yeasts. In this paper we demonstrate that ATP exerts its action in two steps: first, at low ATP/Pi ratios, it increases the respiratory-chain activity, probably by altering the kinetic properties of cytochrome c oxidase. Second, at higher ATP/Pi ratios, an increase in membrane permeability leads to a collapse in membrane potential. The ATP effect on cytochrome c oxidase corroborates a recent report showing that ATP interacts specifically with yeast cytochrome oxidase, stimulating its activity [Taanman and Capaldi (1993) J. Biol. Chem. 268, 18754-18761].