The yeast Yarrowia lipolytica is able to secrete high amounts of several organic acids under conditions of growth limitation and carbon source excess. Here we report the production of citric acid (CA) in a fed-batch cultivation process on sucrose using the recombinant Y. lipolytica strain H222-S4(p67ICL1) T5, harbouring the invertase encoding ScSUC2 gene of Saccharomyces cerevisiae under the inducible XPR2 promoter control and multiple ICL1 copies (10-15). The pH-dependent expression of invertase was low at pH 5.0 and was identified as limiting factor of the CA-production bioprocess. The invertase expression was sufficiently enhanced at pH 6.0-6.8 and resulted in production of 127-140 g l(-1) CA with a yield Y (CA) of 0.75-0.82 g g(-1), whereas at pH 5.0, 87 g l (-1) with a yield Y (CA) of 0.51 g g(-1) were produced. The CA-productivity Q (CA) increased from 0.40 g l (-1) h(-1) at pH 5.0 up to 0.73 g l (-1) h(-1) at pH 6.8. Accumulation of glucose and fructose at high invertase expression level at pH 6.8 indicated a limitation of CA production by sugar uptake. The strain H222-S4(p67ICL1) T5 also exhibited a gene-dose-dependent high isocitrate lyase expression resulting in strong reduction (<5%) of isocitric acid, a by-product during CA production.

Citric acid (CA) is mainly produced in a biotechnological process using Aspergillus niger. In this process, large amounts of wastes have to be removed. Since the use of Yarrowia lipolytica for CA production is an environmental compatible alternative method, the CA production was optimized in regard to growth temperature and pH as well as substrate and product inhibition. The highest value of the maximum specific growth rate at pH 6.5 was found to be μmax = 0.192 h–1, whereas the largest amount of CA of 24.91 g/L as well as the highest selectivity of the bioprocess (89.9 % CA) and the maximum yield (0.22 gCA/gGlucose) were obtained at pH 6.0. During the growth phase, the temperature optimum was found to be in the range of 30–34 °C (μmax = 0.132 h–1). Nevertheless, the highest concentration of CA during the production phase was obtained at 30 °C (41 g/L CA, 93.1 % CA, 0.55 gCA/gglucose). In studying the substrate inhibition of the process, a clear tendency of decrease in the maximum specific growth rate was detected when the initial glucose concentration was increased from 50 g/L (μmax = 0.17 h–1) to 200 g/L (μmax = 0.055 h–1). The addition of 120 g/L CA to the culture broth at the start of the production phase reduced the production of CA from 32.1 g/L to 7.4 g/L.

Functionalized compounds, which are difficult to produce by classical chemical synthesis, are of special interest as biotechnologically available targets. They represent useful building blocks for subsequent organic syntheses, wherein they can undergo stereoselective or regioselective reactions. "White Biotechnology" (as defined by the European Chemical Industry [ http://www.europabio.org/white_biotech.htm ], as part of a sustainable "Green Chemistry,") supports new applications of chemicals produced via biotechnology. Environmental aspects of this interdisciplinary combination include: Use of renewable feedstock Optimization of biotechnological processes by means of: New "high performance" microorganisms On-line measurement of substrates and products in bioreactors Alternative product isolation, resulting in higher yields, and lower energy demand In this overview we describe biotechnologically produced pyruvic, 2-oxopentaric and 2-oxohexaric acids as promising new building blocks for synthetic chemistry. In the first part, the microbial formation of 2-oxocarboxylic acids (2-OCAs) in general, and optimization of the fermentation steps required to form pyruvic acid, 2-oxoglutaric acid, and 2-oxo-D-gluconic acid are described, highlighting the fundamental advantages in comparison to chemical syntheses. In the second part, a set of chemical formula schemes demonstrate that 2-OCAs are applicable as building blocks in the chemical synthesis of, e.g., hydrophilic triazines, spiro-connected heterocycles, benzotriazines, and pyranoic amino acids. Finally, some perspectives are discussed.

No longer just analytical: Previously, (2R,3S)‐isocitric acid (1), a component of the citric acid cycle, had not been available on a preparative scale. A new route to this acid on a kilogram scale combines a biotechnological formation through fermentation from sunflower oil with a chemical separation process. In a variety of transformations into further chiral derivatives, 1 is established as a valuable new member of the chiral pool (see scheme).

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Oxo- and hydroxy-carboxylic acids are of special interest in organic synthesis. However, their introduction by chemical reactions tends to be troublesome especially with regard to stereoselectivity. We describe herein the biotechnological preparation of selected oxo- and hydroxycarboxylic acids under "green" conditions and their use as promising new building blocks. Thereby, our biotechnological goal was the development of process fundamentals regarding the variable use of renewable raw materials, the development of a multi purpose bioreactor and application of a pilot plant with standard equipment for organic acid production to minimize the technological effort. Furthermore the development of new product isolation procedures, with the aim of direct product recovery, capture of products or single step operation, was necessary. The application of robust and approved microorganisms, also genetically modified, capable of using a wide range of substrates as well as producing a large spectrum of products, was of special importance. Microbiologically produced acids, like 2-oxo-glutaric acid and 2-oxo-D-gluconic acid, are useful educts for the chemical synthesis of hydrophilic triazines, spiro-connected heterocycles, benzotriazines, and pyranoic amino acids. The chiral intermediate of the tricarboxylic acid cycle, (2R,3S)-isocitric acid, is another promising compound. For the first time our process provides large quantities of enantiopure trimethyl (2R,3S)-isocitrate which was used in subsequent chemical transformations to provide new chiral entities for further usage in total synthesis and pharmaceutical research.Oxo- and hydroxy-carboxylic acids are of special interest in organic synthesis. However, their introduction by chemical reactions tends to be troublesome especially with regard to stereoselectivity. We describe herein the biotechnological preparation of selected oxo- and hydroxycarboxylic acids under "green" conditions and their use as promising new building blocks. Thereby, our biotechnological goal was the development of process fundamentals regarding the variable use of renewable raw materials, the development of a multi purpose bioreactor and application of a pilot plant with standard equipment for organic acid production to minimize the technological effort. Furthermore the development of new product isolation procedures, with the aim of direct product recovery, capture of products or single step operation, was necessary. The application of robust and approved microorganisms, also genetically modified, capable of using a wide range of substrates as well as producing a large spectrum of products, was of special importance. Microbiologically produced acids, like 2-oxo-glutaric acid and 2-oxo-D-gluconic acid, are useful educts for the chemical synthesis of hydrophilic triazines, spiro-connected heterocycles, benzotriazines, and pyranoic amino acids. The chiral intermediate of the tricarboxylic acid cycle, (2R,3S)-isocitric acid, is another promising compound. For the first time our process provides large qua...

A new, efficient method for the isolation of (2R,3S)-isocitric acid (ICA) from its fermentation solution was developed. It is noteworthy that this method is based on selective adsorption directly from the fermentation solution on activated carbon, followed by the release of both ICA and citric acid by means of elution with methanol and their final separation by known methods. Thereby, several disadvantages were overcome: Electrodialysis is no longer necessary to remove cations such as Na + from the fermentation solution. Also, several hitherto accompanying dyestuffs were not observed with this method. Furthermore, removal of water by distillation is expendable. Eventually, the new crude product is of a quality that also avoids the use of a tedious slide vane rotary vacuum pump distillation of the trimethyl esters of both acids, which hitherto was the basis for the separation of ICA. In summary, the new method distinctly spares energy as well as time.

The yeast Yarrowia lipolytica is one of the most intensively studied "non-conventional" yeast species. Its ability to secrete various organic acids, like pyruvic (PA), citric, isocitric, and alpha-ketoglutaric (KGA) acid, in large amounts is of interest for biotechnological applications. We have studied the effect of the alpha-ketoglutarate dehydrogenase (KGDH) complex on the production process of KGA. Being well studied in Saccharomyces cerevisiae this enzyme complex consists of three subunits: alpha-ketoglutarate dehydrogenase, dihydrolipoyl transsuccinylase, and lipoamide dehydrogenase. Here we report the effect of overexpression of these subunits encoding genes and resulting increase of specific KGDH activity on organic acid production under several conditions of growth limitation and an excess of carbon source in Y. lipolytica. The constructed strain containing multiple copies of all three KGDH genes showed a reduced production of KGA and an elevated production of PA under conditions of KGA production. However, an increased activity of the KGDH complex had no influence on organic acid production under citric acid production conditions.