Improved aromatic alcohol production by strengthening the shikimate pathway in Saccharomyces cerevisiae

Biotechnological 2-Phenylethanol Production: Recent Developments

2-Phenylethanol (2-PE) is a key flavor compound with a rose-like scent, used in the cosmetics, perfume, home care and food industries. This aroma compound can be obtained naturally from various flowers, however chemical synthesis is the most used route to meet market demand. The increasing interest in natural products has led to the development of more environmentally friendly alternatives for 2-PE production through biotechnological approaches. The most efficient approach involves the biotransformation of L-phenylalanine into 2-PE via the Ehrlich pathway, a process observed in different microorganisms such as yeasts and bacteria. 2-PE produced by this way can be considered as natural. However, due to the toxicity of the aroma to the producing microorganism, low production yields are typically obtained, motivating efforts to develop production processes that can overcome this bottleneck, enhance 2-PE yields and reduce the production costs. This review presents and discusses the latest advances in the bioproduction of 2-PE through microbial fermentation, in terms of producing strains, the optimization of cultivation processes, strategies to mitigate product toxicity, and the use of low value feedstocks. Novel applications for 2-PE are also highlighted.

Metabolic engineering of the oleaginous yeast Yarrowia lipolytica for 2-phenylethanol overproduction

The rose fragrance molecule 2-phenylethanol (2-PE) has huge market demand in the cosmetics, food and pharmaceutical industries. However, current 2-PE synthesis methods do not meet the efficiency market requirement. In this study, CRISPR-Cas9-related metabolic engineering strategies were applied to Yarrowia lipolytica for the de novo biosynthesis of 2-PE. Initially, overexpressing exogenous feedback-resistant EcAROGfbr and EcPheAfbr increased 2-PE production to 276.3 mg/L. Subsequently, the ylARO10 and ylPAR4 from endogenous genes were enhanced with the multi-copies to increase the titer to 605 mg/L. Knockout of ylTYR1 and enhancement of shikimate pathway by removing the precursor metabolic bottleneck and overexpressing the genes ylTKT, ylARO1, and ylPHA2 resulted in a significant increase of the 2-PE titer to 2.4 g/L at 84 h, with the yield of 0.06 g/gglu, which is the highest yield for de novo synthesis in yeast. This study provides a valuable precedent for the efficient biosynthesis of shikimate pathway derivatives.

Comprehensive investigations of 2-phenylethanol production by the filamentous fungus Annulohypoxylon stygium

2-Phenylethanol (2-PE) is an aromatic compound with a rose-like fragrance that is widely used in food and other industries. Yeasts have been implicated in the biosynthesis of 2-PE; however, few studies have reported the involvement of filamentous fungi. In this study, 2-PE was detected in Annulohypoxylon stygium mycelia grown in both potato dextrose broth (PDB) and sawdust medium. Among the 27 A. stygium strains investigated in this study, the strain "Jinjiling" (strain S20) showed the highest production of 2-PE. Under optimal culture conditions, the concentration of 2-PE was 2.33 g/L. Each of the key genes in Saccharomyces cerevisiae shikimate and Ehrlich pathways was found to have homologous genes in A. stygium. Upon the addition of L-phenylalanine to the medium, there was an upregulation of all key genes in the Ehrlich pathway of A. stygium, which was consistent with that of S. cerevisiae. A. stygium as an associated fungus provides nutrition for the growth of Tremella fuciformis and most spent composts of T. fuciformis contain pure A. stygium mycelium. Our study on the high-efficiency biosynthesis of 2-PE in A. stygium offers a sustainable solution by utilizing the spent compost of T. fuciformis and provides an alternative option for the production of natural 2-PE. KEY POINTS: • Annulohypoxylon stygium can produce high concentration of 2-phenylethanol. • The pathways of 2-PE biosynthesis in Annulohypoxylon stygium were analyzed. • Spent compost of Tremella fuciformis is a potential source for 2-phenylethanol.

Metabolic engineering of Saccharomyces cerevisiae for the synthesis of valuable chemicals

In the twenty first century, biotechnology offers great opportunities and solutions to climate change mitigation, energy and food security and resource efficiency. The use of metabolic engineering to modify microorganisms for producing industrially significant chemicals is developing and becoming a trend. As a famous, generally recognized as a safe (GRAS) model microorganism, Saccharomyces cerevisiae is widely used due to its excellent operational convenience and high fermentation efficiency. This review summarizes recent advancements in the field of using metabolic engineering strategies to construct engineered S. cerevisiae over the past ten years. Five different types of compounds are classified by their metabolites, and the modified metabolic pathways and strategies are summarized and discussed independently. This review may provide guidance for future metabolic engineering efforts toward such compounds and analogues. Additionally, the limitations of S. cerevisiae as a cell factory and its future trends are comprehensively discussed.

Enzymatic properties and inhibition tolerance analysis of key enzymes in β-phenylethanol anabolic pathway of Saccharomyces cerevisiae HJ

Huangjiu is known for its unique aroma, primarily attributed to its high concentration of β-phenylethanol (ranging from 40 to 130 mg/L). Phenylalanine aminotransferase Aro9p and phenylpyruvate decarboxylase Aro10p are key enzymes in the β-phenylethanol synthetic pathway of Saccharomyces cerevisiae HJ. This study examined the enzymatic properties of these two enzymes derived from S. cerevisiae HJ and S288C. After substrate docking, Aro9pHJ (-24.05 kJ/mol) and Aro10pHJ (-14.33 kJ/mol) exhibited lower binding free energies compared to Aro9pS288C (-21.93 kJ/mol) and Aro10pS288C (-12.84 kJ/mol). ARO9 and ARO10 genes were heterologously expressed in E. coli BL21. Aro9p, which was purified via affinity chromatography, showed inhibition by l-phenylalanine (L-PHE), but the reaction rate Vmax(Aro9pHJ: 23.89 μmol·(min∙g)-1) > Aro9pS288C: 21.3 μmol·(min∙g)-1) and inhibition constant Ki values (Aro9pHJ: 0.28 mol L-1>Aro9pS288C 0.26 mol L-1) indicated that Aro9p from S. cerevisiae HJ was more tolerant to substrate stress during Huangjiu fermentation. In the presence of the same substrate phenylpyruvate (PPY), Aro10pHJ exhibited a stronger affinity than Aro10pS288C. Furthermore, Aro9pHJ and Aro10pHJ were slightly more tolerant to the final metabolites β-phenylethanol and ethanol, respectively, compared to those from S288C. The study suggests that the mutations in Aro9pHJ and Aro10pHJ may contribute to the increased β-phenylethanol concentration in Huangjiu. This is the first study investigating enzyme tolerance mechanisms in terms of substrate and product, providing a theoretical basis for the regulation of the β-phenylethanol metabolic pathway.

Influence of spontaneous, "pied de cuve" and commercial dry yeast fermentation strategies on wine molecular composition and sensory properties

While most producers in recent decades have relied on commercial yeasts (ADY) as their primary choice given their reliability and reproducibility, the fear of standardising the taste and properties of wine has led to the employment of alternative strategies that involve autochthonous yeasts such as pied de cuve (PdC) and spontaneous fermentation (SF). However, the impact of different fermentation strategies on wine has been a subject of debate and speculation. Consequently, this study describes, for the first time, the differences between the three kinds of fermentation at the metabolomic, chemical, and sensory levels in two wines: Chardonnay and Pinot Noir. The results showed how the yeast chosen significantly impacted the molecular composition of the wines, as revealed by metabolomic analysis that identified biomarkers with varying chemical compositions according to the fermentation modality. Notably, higher numbers of lipid markers were found for SF and PdC than ADY, which contained more peptides. Key molecules from the metabolic amino acid pathway, which are addressed in this article, showed evidence of such variations. In addition, the analysis of volatile aromatic compounds revealed an increase in groups of compounds specific to each fermentation. The sensorial analysis of Chardonnay wine showed a more qualitative sensory outcome (Higher fruit intensity) for ADY and SF compared to PdC. Our finding challenges the common speculation among wine producers that autochthonous yeast fermentations may offer greater complexity and uniqueness in comparison to commercial yeast fermentations.

First report on Vitamin B9 production including quantitative analysis of its vitamers in the yeast Scheffersomyces stipitis

Background

The demand for naturally derived products is continuously growing. Nutraceuticals such as pre- and post-biotics, antioxidants and vitamins are prominent examples in this scenario, but many of them are mainly produced by chemical synthesis. The global folate market is expected to register a CAGR of 5.3% from 2019 to 2024 and reach USD 1.02 billion by the end of 2024. Vitamin B9, commonly known as folate, is an essential micronutrient for humans. Acting as a cofactor in one-carbon transfer reactions, it is involved in many biochemical pathways, among which the synthesis of nucleotides and amino acids. In addition to plants, many microorganisms can naturally produce it, and this can pave the way for establishing production processes. In this work, we explored the use of Scheffersomyces stipitis for the production of natural vitamin B9 by microbial fermentation as a sustainable alternative to chemical synthesis.

Results

Glucose and xylose are the main sugars released during the pretreatment and hydrolysis processes of several residual lignocellulosic biomasses (such as corn stover, wheat straw or bagasse). We optimized the growth conditions in minimal medium formulated with these sugars and investigated the key role of oxygenation and nitrogen source on folate production. Vitamin B9 production was first assessed in shake flasks and then in bioreactor, obtaining a folate production up to 3.7 ± 0.07 mg/L, which to date is the highest found in literature when considering wild type microorganisms. Moreover, the production of folate was almost entirely shifted toward reduced vitamers, which are those metabolically active for humans.

Conclusions

For the first time, the non-Saccharomyces yeast S. stipitis was used to produce folate. The results confirm its potential as a microbial cell factory for folate production, which can be also improved both by genetic engineering strategies and by fine-tuning the fermentation conditions and nutrient requirements.

De Novo Biosynthesis of Methyl Cinnamate in Engineered Escherichia coli

Methyl cinnamate with a fruity balsamic odor is an important fragrance ingredient in perfumes and cosmetics. Chemical processes are currently the only means of producing methyl cinnamate. But consumers prefer natural flavors. Therefore, it is necessary to design and develop microbial cell factories for the production of methyl cinnamate. In this study, we established for the first time a biosynthetic pathway in engineered Escherichia coli for production of methyl cinnamate from glucose. We further increased the methyl cinnamate production to 302 mg/L by increasing the availability of the metabolic precursors. Finally, the titer was increased to 458 mg/L in a two-phase culture system.