Purification and characterization of glycerol-3-phosphate dehydrogenase (NAD+) in the salt-tolerant yeast Debaryomyces hansenii

Morphological response to salinity, temperature, and pH changes by marine fungus Epicoccum nigrum

Epicoccum nigrum (strain LQRA39-P) was isolated from sediments collected in Chilean Patagonian fjords using microscopy and molecular techniques. We analyzed adaptive responses of cell wall morphology to salinity, temperature, and pH in order to explain the ability of E. nigrum to co-inhabit both marine and freshwater environments. For this purpose, E. nigrum was cultured in a series of media with variations in salinity (freshwater and seawater), pH (acidic, neutral, and basic), and temperature (5 to 25 °C). Changes were observed through transmission electron microscopy. A direct correlation between increased salinity and cell wall thickening (> 0.2 μm) was observed, along with a significant relationship between pH and the presence of extracellular polymeric substances (EPS) on the outside of the cell wall. The observed morphological changes could confirm that an ubiquitous fungus such as E. nigrum requires adaptive responses to co-inhabit freshwater, marine, and terrestrial substrates.

Differential response of the catalase, superoxide dismutase and glycerol-3-phosphate dehydrogenase to different environmental stresses in Debaryomyces nepalensis NCYC 3413

The effect of salt, pH, and temperature stress on the cellular level of antioxidant enzymes, catalase and superoxide dismutase (SOD) and glycerol-3-phosphate dehydrogenase (G3PDH) was studied in Debaryomyces nepalensis NCYC 3413, a halotolerant yeast. The catalase activity increased in different phases, while SOD and G3PDH activities declined in late stationary phase. A significant increase in SOD activity was observed under different stress as compared to control. Salt and temperature stress enhanced the catalase activity where as it was suppressed by pH stress. G3PDH level increased with salt stress, however, no significant change was observed under pH and temperature stress. The observations recorded in this investigation suggested that D. nepalensis has an efficient protective mechanism of antioxidant enzymes and G3PDH against salt, pH, and temperature stresses.

Expression of theGPD1 andGPP2 orthologues and glycerol retention during growth ofDebaryomyces hansenii at high NaCl concentrations

The highly NaCl-tolerant yeast Debaryomyces hansenii produces and obtains high levels of intracellular glycerol as a compatible solute when grown at high NaCl concentrations. The effect of high NaCl concentrations (4%, 8% and 12% w/v) on the glycerol production and the levels of intra- and extracellular glycerol was determined for two D. hansenii strains with different NaCl tolerance and compared to one strain of the moderately NaCl-tolerant yeast Saccharomyces cerevisiae. Initially, high NaCl tolerance seems to be determined by enhanced glycerol production, due to an increased expression of DhGPD1 and DhGPP2 (AL436338) in D. hansenii and GPD1 and GPP2 in S. cerevisiae; however, the ability to obtain high levels of intracellular glycerol seems to be more important. The two D. hansenii strains had higher levels of intracellular glycerol than the S. cerevisiae strain and were able to obtain high levels of intracellular glycerol, even at very high NaCl concentrations, indicating the presence of, for example, a type of closing channel, as previously described for other yeast species.

Mechanisms underlying the halotolerant way of

The yeast Debaryomyces hansenii is usually found in salty environments such as the sea and salted food. It is capable of accumulating sodium without being intoxicated even when potassium is present at low concentration in the environment. In addition, sodium improves growth and protects D. hansenii in the presence of additional stress factors such as high temperature and extreme pH. An array of advantageous factors, as compared with Saccharomyces cerevisiae, is putatively involved in the increased halotolerance of D. hansenii: glycerol, the main compatible solute, is kept inside the cell by an active glycerol-Na+ symporter; potassium uptake is not inhibited by sodium; sodium protein targets in D. hansenii seem to be more resistant. The whole genome of D. hansenii has been sequenced and is now available at http://cbi.labri.fr/Genolevures/ and, so far, no genes specifically responsible for the halotolerant behaviour of D. hansenii have been found.

Salt-dependent expression of ammonium assimilation genes in the halotolerant yeast, Debaryomyces hansenii

Debaryomyces hansenii is adapted to grow in saline environments, accumulating high intracellular Na(+) concentrations. Determination of the DhGDH1-encoded NADP-glutamate dehydrogenase enzymatic activity showed that it increased in a saline environment. Thus, it was proposed that, in order to overcome Na(+) inhibition of enzyme activity, this organism possessed salt-dependent mechanisms which resulted in increased activity of enzymes pertaining to the central metabolic pathways. However, the nature of the mechanisms involved in augmented enzyme activity were not analyzed. To address this matter, we studied the expression of DhGDH1 and DhGLN1 encoding glutamine synthetase, which constitute the central metabolic circuit involved in ammonium assimilation. It was found that: (1) expression of DhGDH1 is increased when D. hansenii is grown in the presence of high NaCl concentrations, while that of DhGLN1 is reduced, (2) DhGDH1 expression in Saccharomyces cerevisiae takes place in a GLN3- and HAP2,3-dependent manner and (3) salt-dependent DhGDH1 and DhGLN1 expression involves mechanisms which are limited to D. hansenii and are not present in S. cerevisiae. Thus, salt-dependent regulation of the genes involved in central metabolic pathways could form part of a strategy leading to the ability to grow under hypersaline conditions.

Glycerol production by microbial fermentation: a review

Microbial production of glycerol has been known for 150 years, and glycerol was produced commercially during World War I. Glycerol production by microbial synthesis subsequently declined since it was unable to compete with chemical synthesis from petrochemical feedstocks due to the low glycerol yields and the difficulty with extraction and purification of glycerol from broth. As the cost of propylene has increased and its availability has decreased especially in developing countries and as glycerol has become an attractive feedstock for production of various chemicals, glycerol production by fermentation has become more attractive as an alternative route. Substantial overproduction of glycerol by yeast from monosaccharides can be obtained by: (1) forming a complex between acetaldehyde and bisulfite ions thereby retarding ethanol production and restoring the redox balance through glycerol synthesis; (2) growing yeast cultures at pH values near 7 or above; or (3) using osmotolerant yeasts. In recent years, significant improvements have been made in the glycerol production using osmotolerant yeasts on a commercial scale in China. The most outstanding achievements include: (1) isolation of novel osmotolerant yeast strains producing up to 130 g/L glycerol with yields up to 63% and the productivities up to 32 g/(L day); (2) glycerol yields, productivities and concentrations in broth up to 58%, 30 g/(L day) and 110-120 g/L, respectively, in an optimized aerobic fermentation process have been attained on a commercial scale; and (3) a carrier distillation technique with a glycerol distillation efficiency greater than 90% has been developed. As glycerol metabolism has become better understood in yeasts, opportunities will arise to construct novel glycerol overproducing microorganisms by metabolic engineering.

Metabolic control analysis of glycerol synthesis in Saccharomyces cerevisiae

Glycerol, a major by-product of ethanol fermentation by Saccharomyces cerevisiae, is of significant importance to the wine, beer, and ethanol production industries. To gain a clearer understanding of and to quantify the extent to which parameters of the pathway affect glycerol flux in S. cerevisiae, a kinetic model of the glycerol synthesis pathway has been constructed. Kinetic parameters were collected from published values. Maximal enzyme activities and intracellular effector concentrations were determined experimentally. The model was validated by comparing experimental results on the rate of glycerol production to the rate calculated by the model. Values calculated by the model agreed well with those measured in independent experiments. The model also mimics the changes in the rate of glycerol synthesis at different phases of growth. Metabolic control analysis values calculated by the model indicate that the NAD(+)-dependent glycerol 3-phosphate dehydrogenase-catalyzed reaction has a flux control coefficient (C(J)v1) of approximately 0.85 and exercises the majority of the control of flux through the pathway. Response coefficients of parameter metabolites indicate that flux through the pathway is most responsive to dihydroxyacetone phosphate concentration (R(J)DHAP= 0.48 to 0.69), followed by ATP concentration (R(J)ATP = -0.21 to -0.50). Interestingly, the pathway responds weakly to NADH concentration (R(J)NADH = 0.03 to 0.08). The model indicates that the best strategy to increase flux through the pathway is not to increase enzyme activity, substrate concentration, or coenzyme concentration alone but to increase all of these parameters in conjunction with each other.

Metabolic flux response to salt-induced stress in the halotolerant yeast Debaryomyces hansenii

The toxic effect of NaCl and KCl on growth of the marine yeast Debaryomyces hansenii on glucose or glycerol was studied. Above a threshold value, both salts reduced the specific growth rate, specific glucose and glycerol respiration rates and specific glucose fermentation rate, as well as biomass yields. The exponential inhibition constant, k, and minimum toxic concentration, Cmin were similar for all physiological parameters assayed. The effect of either salt on the specific activity of several glycolytic enzymes showed a similar inhibition pattern, although at much lower salt concentrations compared with the physiological parameters. In agreement with published results on glycerol phosphate dehydrogenase stimulation by salt, we present evidence that a general glycolytic flux deviation could occur naturally during salt stress, due to the intrinsic sensitivity of the glycolytic enzymes to intracellular ion concentrations.

Fast purification and kinetic studies of the glycerol-3-phosphate dehydrogenase from the yeast Saccharomyces cerevisiae

The glycerol-3-phosphate dehydrogenase has been purified from Saccharomyces cerevisiae 140-fold to electrophoretic homogeneity by a simple procedure involving affinity and ion exchange chromatography. The purified enzyme was most active at pH 6.8 and 51 degrees C. Its molecular mass was determined to be 45000 +/- 2000 Da by SDS-polyacrylamide gel electrophoresis. At physiological pH values the thermodynamic equilibrium constant was determined to be 3.5 x 10(-3) (M-1). Product inhibition as well as competitive inhibition patterns were found which clearly indicate that the kinetic mechanism of the glycerol-3-phosphate dehydrogenase is random bi-bi with the formation of dead-end complexes. In vivo concentrations of selected metabolites and kinetic expression for G3P-DH were used to explain regulatory properties of this enzyme under conditions of short-term glucose effect in Saccharomyces cerevisiae.

Purification and characterization of glycerol-3-phosphate dehydrogenase of Saccharomyces cerevisiae

The NAD-dependent glycerol-3-phosphate dehydrogenase (glycerol-3-phosphate:NAD+ oxidoreductase; EC 1.1.1.8; G3P DHG) was purified 178-fold to homogeneity from Saccharomyces cerevisiae strain H44-3D by affinity- and ion-exchange chromatography. SDS-PAGE indicated that the enzyme had a molecular mass of approximately 42,000 (+/- 1,000) whereas a molecular mass of 68,000 was observed using gel filtration, implying that the enzyme may exist as a dimer. The pH optimum for the reduction of dihydroxyacetone phosphate (DHAP) was 7.6 and the enzyme had a pI of 7.4. NADPH will not substitute for NADH as coenzyme in the reduction of DHAP. The oxidation of glycerol-3-phosphate (G3P) occurs at 3% of the rate of DHAP reduction at pH 7.0. Apparent Km values obtained were 0.023 and 0.54 mM for NADH and DHAP, respectively. NAD, fructose-1,6-bisphosphate (FBP), ATP and ADP inhibited G3P DHG activity. Ki values obtained for NAD with NADH as variable substrate and FBP with DHAP as variable substrate were 0.93 and 4.8 mM, respectively.