Tolerance of immobilized baker’s yeast in organic solvents

A Novel Baker’s Yeast-Mediated Microwave-Induced Reduction of Racemic 3-Keto-2-Azetidinones: Facile Entry to Optically Active Hydroxy β-Lactam Derivatives

Objective:

Microwave technology, together with enzymatic catalysis is a nature-friendly chemical synthesis method with low wastage of solvent and good yield of the products.

Methods:

Enzymes from various microorganisms can be used in the biochemical processes of a wide range of compounds assisted by microwave irradiation

Results:

In this work, the microwave-induced reaction of α-keto β-lactams by Baker's yeast in organic solvent was conducted to afford optically active cis and trans-α-hydroxy-β-lactams for the first time.

Conclusion:

These hydroxy compounds are the precursors of numerous natural products of medicinal significances.

Encapsulation enhances protoplast fusant stability

A barrier to cost‐efficient biomanufacturing is the instability of engineered genetic elements, such as plasmids. Instability can also manifest at the whole‐genome level, when fungal dikaryons revert to parental species due to nuclear segregation during cell division. Here, we show that by encapsulating Saccharomyces cerevisiae‐Pichia stipitis dikaryons in an alginate matrix, we can limit cell division and preserve their expanded metabolic capabilities. As a proxy to cellulosic ethanol production, we tested the capacity of such cells to carry out ethanologenic fermentation of glucose and xylose, examining substrate use, ploidy, and cell viability in relation to planktonic fusants, as well as in relation to planktonic and encapsulated cell cultures consisting of mixtures of these species. Glucose and xylose consumption and ethanol production by encapsulated dikaryons were greater than planktonic controls. Simultaneous co‐fermentation did not occur; rather the order and kinetics of glucose and xylose catabolism by encapsulated dikaryons were similar to cultures where the two species were encapsulated together. Over repeated cycles of fed‐batch culture, encapsulated S. cerevisiae‐P. stipitis fusants exhibited a dramatic increase in genomic stability, relative to planktonic fusants. Encapsulation also increased the stability of antibiotic‐resistance plasmids used to mark each species and preserved a fixed ratio of S. cerevisiae to P. stipitis cells in mixed cultures. Our data demonstrate how encapsulating cells in an extracellular matrix restricts cell division and, thereby, preserves the stability and biological activity of entities ranging from genomes to plasmids to mixed populations, each of which can be essential to cost‐efficient biomanufacturing.

Highly enantioselective resolution of racemic 1-phenyl-1,2-ethanediol to (S)-1-phenyl-1,2-ethanediol by Kurthia gibsonii SC0312 in a biphasic system

The asymmetric resolution of racemic 1-phenyl-1,2-ethanediol (PED) to (S)-PED by Kurthia gibsonii SC0312 (K. gibsonii SC0312) was conducted in a biphasic system comprised of an organic solvent and aqueous phosphate buffer. The impacts of organic solvents on the whole cell catalytic activity, metabolic activity, membrane integrity, and material distribution were first evaluated. The results showed that all organic solvents, except for dibutyl phthalate, showed a detrimental effect on the metabolic activity of the cells, especially for those with low log P values. All organic solvents were capable of changing the membrane permeability and membrane integrity of the cells. Moreover, some organic solvents showed a good extraction of the oxidation product. Finally, a high yield of 47.7 % of (S)-PED was obtained by the asymmetric resolution of racemic PED using K. gibsonii SC0312 in a biphasic system under the optimal conditions: racemic PED 120 mM, temperature 35 °C, reaction time 6 h, 180 rpm, and a volume ratio of dibutyl phthalate to aqueous phosphate buffer of 1:1. The optical purity of (S)-PED increased from 51.3 % to >99 %. This work described an efficient approach to improve reaction efficiency, and constructed a highly effective biphasic reaction system for the fabrication of (S)-PED via K. gibsonii SC0312.

Matrices (re)loaded: Durability, viability, and fermentative capacity of yeast encapsulated in beads of different composition during long-term fed-batch culture

Encapsulated microbes have been used for decades to produce commodities ranging from methyl ketone to beer. Encapsulated cells undergo limited replication, which enables them to more efficiently convert substrate to product than planktonic cells and which contributes to their stress resistance. To determine how encapsulated yeast supports long-term, repeated fed-batch ethanologenic fermentation, and whether different matrices influence that process, fermentation and indicators of matrix durability and cell viability were monitored in high-dextrose, fed-batch culture over 7 weeks. At most timepoints, ethanol yield (g/g) in encapsulated cultures exceeded that in planktonic cultures. And frequently, ethanol yield differed among the four matrices tested: sodium alginate crosslinked with Ca2+ and chitosan, sodium alginate crosslinked with Ca2+ , Protanal alginate crosslinked with Ca2+ and chitosan, Protanal alginate crosslinked with Ca2+ , with the last of these consistently demonstrating the highest values. Young's modulus and viscosity were higher for matrices crosslinked with chitosan over the first week; thereafter values for both parameters declined and were indistinguishable among treatments. Encapsulated cells exhibited greater heat shock tolerance at 50°C than planktonic cells in either stationary or exponential phase, with similar thermotolerance observed across all four matrix types. Altogether, these data demonstrate the feasibility of re-using encapsulated yeast to convert dextrose to ethanol over at least 7 weeks.

An engineered fatty acid synthase combined with a carboxylic acid reductase enables de novo production of 1-octanol in Saccharomyces cerevisiae

BACKGROUND

The ideal biofuel should not only be a regenerative fuel from renewable feedstocks, but should also be compatible with the existing fuel distribution infrastructure and with normal car engines. As the so-called drop-in biofuel, the fatty alcohol 1-octanol has been described as a valuable substitute for diesel and jet fuels and has already been produced fermentatively from sugars in small amounts with engineered bacteria via reduction of thioesterase-mediated premature release of octanoic acid from fatty acid synthase or via a reversal of the β-oxidation pathway.

RESULTS

The previously engineered short-chain acyl-CoA producing yeast Fas1R1834K/Fas2 fatty acid synthase variant was expressed together with carboxylic acid reductase from Mycobacterium marinum and phosphopantetheinyl transferase Sfp from Bacillus subtilis in a Saccharomyces cerevisiae Δfas1 Δfas2 Δfaa2 mutant strain. With the involvement of endogenous thioesterases, alcohol dehydrogenases, and aldehyde reductases, the synthesized octanoyl-CoA was converted to 1-octanol up to a titer of 26.0 mg L-1 in a 72-h fermentation. The additional accumulation of 90 mg L-1 octanoic acid in the medium indicated a bottleneck in 1-octanol production. When octanoic acid was supplied externally to the yeast cells, it could be efficiently converted to 1-octanol indicating that re-uptake of octanoic acid across the plasma membrane is not limiting. Additional overexpression of aldehyde reductase Ahr from Escherichia coli nearly completely prevented accumulation of octanoic acid and increased 1-octanol titers up to 49.5 mg L-1. However, in growth tests concentrations even lower than 50.0 mg L-1 turned out to be inhibitory to yeast growth. In situ extraction in a two-phase fermentation with dodecane as second phase did not improve growth, indicating that 1-octanol acts inhibitive before secretion. Furthermore, 1-octanol production was even reduced, which results from extraction of the intermediate octanoic acid to the organic phase, preventing its re-uptake.

CONCLUSIONS

By providing chain length control via an engineered octanoyl-CoA producing fatty acid synthase, we were able to specifically produce 1-octanol with S. cerevisiae. Before metabolic engineering can be used to further increase product titers and yields, strategies must be developed that cope with the toxic effects of 1-octanol on the yeast cells.

Current progress in high cell density yeast bioprocesses for bioethanol production

High capital costs and low reaction rates are major challenges for establishment of fermentation-based production systems in the bioeconomy. Using high cell density cultures is an efficient way to increase the volumetric productivity of fermentation processes, thereby enabling faster and more robust processes and use of smaller reactors. In this review, we summarize recent progress in the application of high cell density yeast bioprocesses for first and second generation bioethanol production. High biomass concentrations obtained by retention of yeast cells in the reactor enables easier cell reuse, simplified product recovery and higher dilution rates in continuous processes. High local cell density cultures, in the form of encapsulated or strongly flocculating yeast, furthermore obtain increased tolerance to convertible fermentation inhibitors and utilize glucose and other sugars simultaneously, thereby overcoming two additional hurdles for second generation bioethanol production. These effects are caused by local concentration gradients due to diffusion limitations and conversion of inhibitors and sugars by the cells, which lead to low local concentrations of inhibitors and glucose. Quorum sensing may also contribute to the increased stress tolerance. Recent developments indicate that high cell density methodology, with emphasis on high local cell density, offers significant advantages for sustainable second generation bioethanol production.

Physiological tests for yeast brewery cells immobilized on modified chamotte carrier

In this study yeast cell physiological activity was assessed on the basis of the in situ activity of two important enzymes, succinate dehydrogenase and pyruvate decarboxylase. FUN1 dye bioconversion and cellular ATP content were also taken as important indicators of yeast cell activity. The study was conducted on six brewing yeast strains, which were either free cells or immobilized on a chamotte carrier. The experimental data obtained indicate clearly that, in most cases, the immobilized cells showed lower enzyme activity than free cells from analogous cultures. Pyruvate decarboxylase activity in immobilized cells was higher than in planktonic cell populations only in the case of the Saccharomyces pastorianus 680 strain. However, in a comparative assessment of the fermentation process, conducted with the use of free and immobilized cells, much more favorable dynamics and carbon dioxide productivity were observed in immobilized cells, especially in the case of brewing lager yeast strains. This may explain the higher total cell density per volume unit of the fermented medium and the improved resistance of immobilized cells to environmental changes.

Effect of linoleic acids on the release of β-carotene from carotenoid-producing Saccharomyces cerevisiae into sunflower oil

In situ extraction is important for highly productive and cost-efficient processes in industrial biotechnology, but it is difficult to establish for intracellularly accumulating carotenoids like β-carotene. In this study, the organic solvent used in aqueous-organic two-phase media exerted a strong effect on the release of β-carotene from recombinant yeast cells. The carotenoid-synthesizing Saccharomyces cerevisiae strain YB/I/E was cultivated in two-liquid-phase media with 20% dodecane or 20% sunflower oil. Up to 0.6 µg/ml β-carotene was released into sunflower oil, but less than 0.1 µg/ml into dodecane, although biocompatibility and solubility of β-carotene is appropriate for both solvents. Addition of linoleic acid, the main component of sunflower oil, to the dodecane phase increased the amount of β-carotene released, indicating that linoleic acid is the component responsible for the β-carotene release into sunflower oil. These findings demonstrate that the effect of the organic solvent should be taken into consideration for further research on in situ extraction of carotenoids.

Highly enantioselective reduction of 2-hydroxy-1-phenylethanone to enantiopure (R)-phenyl-1,2-ethanediol using Saccharomyces cerevisiae of remarkable reaction stability

Saccharomyces cerevisiae JUC15 was successfully obtained by target reaction-oriented screening, which reduced 2-hydroxy-1-phenylethanone (HPE) to (R)-phenyl-1,2-ethanediol ((R)-PED) of excellent enantiomeric excess (e.e. >99.9%). There was no significant decrease in the yield and optical purity of (R)-PED when the free cells were reused for 40 repeated cycles at 2gL(-1) substrate concentration. The strain used cheap sucrose for cofactor regeneration and worked over a considerably wider range of pH (4-9). The optimum substrate concentration was 8gL(-1), which was higher than any other biocatalysts reported so far. Interesting, when HPE concentration reached 20gL(-1) in reaction system, where 43.2% of the substrate was present in suspended solid form, the reaction still gave enantiopure (R)-PED in 71% yield. Last but not least, the product e.e. kept above 99.9% in all examined conditions. These results suggest the potential of this strain for the industrial production of optically active (R)-PED.

Enhancing the biocatalytic potential of carbonyl reductase of Candida viswanathii using aqueous-organic solvent system

In the present work, the toxic effect of various solvents with different Log P values was studied on the whole cells of Candida viswanathii. Experiments showed that the lower concentrations of some solvent increased both the activity retention and enzyme activity as compared to the control while this was not the case with higher concentrations of the same solvents. The model compound taken in the present study was 1-acetophenone. The percentage conversion improved from 76 to 94%. Addition of 2-propanol increased the substrate tolerance, giving the conversion of 90% compared to 9% in control at a substrate concentration of 70 mM in 1h. The operational stability increased at higher temperatures with the addition of 2-propanol in the reaction mixture with good conversion (90%) and enantiomeric excess (>99%) at 45 degrees C and 50 degrees C. The effect was also found to be prominent in other tested substrates. In order to further stabilize the cells for long term use in higher concentration of organic solvents, the cells were further immobilized, and were found to have higher activity retention than that of free cells.

Continuous Beer Fermentation Using Immobilized Yeast Cell Bioreactor Systems

Traditional beer fermentation and maturation processes use open fermentation and lager tanks. Although these vessels had previously been considered indispensable, during the past decades they were in many breweries replaced by large production units (cylindroconical tanks). These have proved to be successful, both providing operating advantages and ensuring the quality of the final beer. Another promising contemporary technology, namely, continuous beer fermentation using immobilized brewing yeast, by contrast, has found only a limited number of industrial applications. Continuous fermentation systems based on immobilized cell technology, albeit initially successful, were condemned to failure for several reasons. These include engineering problems (excess biomass and problems with CO(2) removal, optimization of operating conditions, clogging and channeling of the reactor), unbalanced beer flavor (altered cell physiology, cell aging), and unrealized cost advantages (carrier price, complex and unstable operation). However, recent development in reactor design and understanding of immobilized cell physiology, together with application of novel carrier materials, could provide a new stimulus to both research and application of this promising technology.