Modification of yeast characteristics by soy peptides: cultivation with soy peptides represses the formation of lipid bodies

Efficient mannitol production by wild-type Lactobacillus reuteri CRL 1101 is attained at constant pH using a simplified culture medium

Mannitol is a natural polyol with multiple industrial applications. In this work, mannitol production by Lactobacillus reuteri CRL 1101 was studied at free- and controlled-pH (6.0-4.8) fermentations using a simplified culture medium containing yeast and beef extracts and sugarcane molasses. The activity of mannitol 2-dehydrogenase (MDH), the enzyme responsible for mannitol synthesis, was determined. The effect of the initial biomass concentration was further studied. Mannitol production (41.5 ± 1.1 g/l), volumetric productivity (Q Mtl 1.73 ± 0.05 g/l h), and yield (Y Mtl 105 ± 11 %) were maximum at pH 5.0 after 24 h while the highest MDH activity (1.66 ± 0.09 U/mg protein) was obtained at pH 6.0. No correlation between mannitol production and MDH activity was observed when varying the culture pH. The increase (up to 2000-fold) in the initial biomass concentration did not improve mannitol formation after 24 h although a 2-fold higher amount was produced at 8 h using 1 or 2 g cell dry weight/l comparing to the control (0.001 g cell dry weight/l). Finally, mannitol isolation under optimum fermentation conditions was achieved. The mannitol production obtained in this study is the highest reported so far by a wild-type L. reuteri strain and, more interestingly, using a simplified culture medium.

The Role of Food Peptides in Lipid Metabolism during Dyslipidemia and Associated Health Conditions

Animal and human clinical studies have demonstrated the ability of dietary food proteins to modulate endogenous lipid levels during abnormal lipid metabolism (dyslipidemia). Considering the susceptibility of proteins to gastric proteolytic activities, the hypolipidemic functions of proteins are possibly due, in part, to their peptide fragments. Food-derived peptides may directly modulate abnormal lipid metabolism in cell cultures and animal models of dyslipidemia. The peptides are thought to act by perturbing intestinal absorption of dietary cholesterol and enterohepatic bile acid circulation, and by inhibiting lipogenic enzymatic activities and gene expression in hepatocytes and adipocytes. Recent evidence indicates that the hypolipidemic activities of some peptides are due to activation of hepatic lipogenic transcription factors. However, detailed molecular mechanisms and structural requirements of peptides for these activities are yet to be elucidated. As hypolipidemic peptides can be released during enzymatic food processing, future studies can explore the prospects of combating metabolic syndrome and associated complications using peptide-rich functional food and nutraceutical products.

Effects of soy protein hydrolysates on the growth and fermentation performances of brewer's yeast

Soy protein isolate was hydrolysed with Alcalase, Papain, Flavorzyme and Protemax, respectively, and further fractioned by ultrafiltration. The resulting soy protein hydrolysates (SPH) and their ultrafiltration fractions were used to examine their effects on the growth and fermentation performances of brewer's yeast. Results showed that degree of hydrolysis, molecular weight distribution and amino acid composition of SPH significantly affected the growth, viability and fermentation performance of brewer's yeast. The SPH prepared from different proteolytic enzymes exhibited distinct growth‐ and fermentation‐promoting activity for brewer's yeast. The SPH treated with Protemax for 9 h and with the molecular weight below 3 kDa showed the highest growth‐promoting activity and induced more rapidly reducing sugar consumption and higher ethanol production. The relatively lower molecular weight and the hydrophilic and electropositive amino acid residues (Lys, His, Arg and Ile) in SPH might be responsible for its functionality, promoting the growth and fermentation of brewer's yeast.

Functional implications and ubiquitin-dependent degradation of the peptide transporter Ptr2 in Saccharomyces cerevisiae

The peptide transporter Ptr2 plays a central role in di- or tripeptide import in Saccharomyces cerevisiae. Although PTR2 transcription has been extensively analyzed in terms of upregulation by the Ubr1-Cup9 circuit, the structural and functional information for this transporter is limited. Here we identified 14 amino acid residues required for peptide import through Ptr2 based on the crystallographic information of Streptococcus thermophilus peptide transporter PepTst and based on the conservation of primary sequences among the proton-dependent oligopeptide transporters (POTs). Expression of Ptr2 carrying one of the 14 mutations of which the corresponding residues of PepTst are involved in peptide recognition, salt bridge interaction, or peptide translocation failed to enable ptr2Δtrp1 cell growth in alanyl-tryptophan (Ala-Trp) medium. We observed that Ptr2 underwent rapid degradation after cycloheximide treatment (half-life, approximately 1 h), and this degradation depended on Rsp5 ubiquitin ligase. The ubiquitination of Ptr2 most likely occurs at the N-terminal lysines 16, 27, and 34. Simultaneous substitution of arginine for the three lysines fully prevented Ptr2 degradation. Ptr2 mutants of the presumed peptide-binding site (E92Q, R93K, K205R, W362L, and E480D) exhibited severe defects in peptide import and were subjected to Rsp5-dependent degradation when cells were moved to Ala-Trp medium, whereas, similar to what occurs in the wild-type Ptr2, mutant proteins of the intracellular gate were upregulated. These results suggest that Ptr2 undergoes quality control and the defects in peptide binding and the concomitant conformational change render Ptr2 subject to efficient ubiquitination and subsequent degradation.

Overexpression of membrane proteins from higher eukaryotes in yeasts

Heterologous expression and characterisation of the membrane proteins of higher eukaryotes is of paramount interest in fundamental and applied research. Due to the rather simple and well-established methods for their genetic modification and cultivation, yeast cells are attractive host systems for recombinant protein production. This review provides an overview on the remarkable progress, and discusses pitfalls, in applying various yeast host strains for high-level expression of eukaryotic membrane proteins. In contrast to the cell lines of higher eukaryotes, yeasts permit efficient library screening methods. Modified yeasts are used as high-throughput screening tools for heterologous membrane protein functions or as benchmark for analysing drug-target relationships, e.g., by using yeasts as sensors. Furthermore, yeasts are powerful hosts for revealing interactions stabilising and/or activating membrane proteins. We also discuss the stress responses of yeasts upon heterologous expression of membrane proteins. Through co-expression of chaperones and/or optimising yeast cultivation and expression strategies, yield-optimised hosts have been created for membrane protein crystallography or efficient whole-cell production of fine chemicals.

Candida zeylanoides as a new yeast model for lipid metabolism studies: effect of nitrogen sources on fatty acid accumulation

Lipid homeostasis is well-known in oleaginous yeasts, but there are few non-oleaginous yeast models apart from Saccharomyces cerevisiae. We are proposing the non-oleaginous yeast Candida zeylanoides QU 33 as model. The aim of this study was to investigate the influence of the carbon/nitrogen ratio and the type of nitrogen source upon oil accumulation by this yeast grown on shake flask cultures. The maximum biomass was obtained in yeast extract (2.39 ± 0.19 g/l), followed by peptone (2.24 ± 0.05 g/l), while the highest content of microbial oil (0.35 ± 0.01 g/l) and the maximum lipid yield (15.63%) were achieved with peptone. Oleic acid was the predominant cellular fatty acid in all culture media (>32.23%), followed by linoleic (>15.79%) and palmitic acids (>13.47%). The highest lipid yield using glucose and peptone was obtained at the C/N ratio of 200:1.

Effects of nitrogen composition on fermentation performance of brewer's yeast and the absorption of peptides with different molecular weights

Four kinds of worts with different nitrogen compositions were used to examine their effects on fermentation performance of brewer's yeast. The absorption pattern of peptides with different molecular weights (Mw) in yeast cells during wort fermentation was also investigated. Results showed that both the nitrogen composition and level had significant impacts on the yeast biomass accumulation, ethanol production, and free amino nitrogen and sugars consumption rates. Worts supplemented with wheat gluten hydrolysates increased 11.5% of the biomass, 5.9% of fermentability, and 0.6% of ethanol content and decreased 25.6% of residual sugar content during wort fermentation. Moreover, yeast cells assimilated peptides with various Mw differently during fermentation. Peptides with Mw below 1 kDa decreased quickly, and the rate of assimilation was more than 50% at the end of fermentation, while those with Mw above 10 kDa almost could not be assimilated by yeast. All these results further indicated that the level and composition of wort nitrogen had significant impacts on the growth and fermentation performances of brewer's yeast, and peptides with Mw below 1 kDa were one of preferred nitrogen sources for brewer's yeast.

Soy peptides enhance heterologous membrane protein productivity during the exponential growth phase of Saccharomyces cerevisiae

In this study, the production of eight G protein-coupled receptors by Saccharomyces cerevisiae was compared using two types of media, one of which contained soy peptides and the other free amino acids. Yeast cell growth improved in the medium with soy peptides, and the expression levels of six of the receptors increased during the exponential phase by an average of 2.3-fold as against the free amino acid-based medium. The enhancement of protein expression by soy peptides can be explained by alleviation of metabolite stress due to amino acid source depletion caused by heterologous protein expression.