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Efficient production of L-lactic acid from xylose by Pichia stipitis.

ABSTRACT: Microbial conversion of renewable raw materials to useful products is an important objective in industrial biotechnology. Pichia stipitis, a yeast that naturally ferments xylose, was genetically engineered for l-(+)-lactate production. We constructed a P. stipitis strain that expressed the l-lactate dehydrogenase (LDH) from Lactobacillus helveticus under the control of the P. stipitis fermentative ADH1 promoter. Xylose, glucose, or a mixture of the two sugars was used as the carbon source for lactate production. The constructed P. stipitis strain produced a higher level of lactate and a higher yield on xylose than on glucose. Lactate accumulated as the main product in xylose-containing medium, with 58 g/liter lactate produced from 100 g/liter xylose. Relatively efficient lactate production also occurred on glucose medium, with 41 g/liter lactate produced from 94 g/liter glucose. In the presence of both sugars, xylose and glucose were consumed simultaneously and converted predominantly to lactate. Lactate was produced at the expense of ethanol, whose production decreased to approximately 15 to 30% of the wild-type level on xylose-containing medium and to 70 to 80% of the wild-type level on glucose-containing medium. Thus, LDH competed efficiently with the ethanol pathway for pyruvate, even though the pathway from pyruvate to ethanol was intact. Our results show, for the first time, that lactate production from xylose by a yeast species is feasible and efficient. This is encouraging for further development of yeast-based bioprocesses to produce lactate from lignocellulosic raw material.

SUBMITTER: Ilmen M

PROVIDER: S-EPMC1797125 | biostudies-literature | 2007 Jan

REPOSITORIES: biostudies-literature

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Efficient production of L-lactic acid from xylose by Pichia stipitis.

Ilmén Marja M Koivuranta Kari K Ruohonen Laura L Suominen Pirkko P Penttilä Merja M

Applied and environmental microbiology 20061027 1

Microbial conversion of renewable raw materials to useful products is an important objective in industrial biotechnology. Pichia stipitis, a yeast that naturally ferments xylose, was genetically engineered for l-(+)-lactate production. We constructed a P. stipitis strain that expressed the l-lactate dehydrogenase (LDH) from Lactobacillus helveticus under the control of the P. stipitis fermentative ADH1 promoter. Xylose, glucose, or a mixture of the two sugars was used as the carbon source for la ...[more]

PMID: 17071782

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Project description:BACKGROUND: The NAD(P)H-dependent Pichia stipitis xylose reductase (PsXR) is one of the key enzymes for xylose fermentation, and has been cloned into the commonly used ethanol-producing yeast Saccharomyces cerevisiae. In order to eliminate the redox imbalance resulting from the preference of this enzyme toward NADPH, efforts have been made to alter the coenzyme specificity of PsXR by site-directed mutagenesis, with limited success. Given the industrial importance of PsXR, it is of interest to investigate further ways to create mutants of PsXR that prefers NADH rather than NADPH, by the alternative directed evolution approach. RESULTS: Based on a homology model of PsXR, six residues were predicted to interact with the adenine ribose of NAD(P)H in PsXR and altered using a semi-rational mutagenesis approach (CASTing). Three rounds of saturation mutagenesis were carried to randomize these residues, and a microplate-based assay was applied in the screening. A best mutant 2-2C12, which carried four mutations K270S, N272P, S271G and R276F, was obtained. The mutant showed a preference toward NADH over NADPH by a factor of about 13-fold, or an improvement of about 42-fold, as measured by the ratio of the specificity constant kcat/Kmcoenzyme. Compared with the wild-type, the kcatNADH for the best mutant was only slightly lower, while the kcatNADPH decreased by a factor of about 10. Furthermore, the specific activity of 2-2C12 in the presence of NADH was 20.6 U.mg-1, which is highest among PsXR mutants reported. CONCLUSION: A seemingly simplistic and yet very effective mutagenesis approach, CASTing, was applied successfully to alter the NAD(P)H preference for Pichia stipitis xylose reductase, an important enzyme for xylose-fermenting yeast. The observed change in the NAD(P)H preference for this enzyme seems to have resulted from the altered active site that is more unfavorable for NADPH than NADH in terms of both Km and kcat. There are potentials for application of our PsXR in constructing a more balanced XR-XDH pathway in recombinant xylose-fermenting S. cerevisiae strains.

Project description:BackgroundBioplastics, like polylactic acid (PLA), are renewable alternatives for petroleum-based plastics. Lactic acid, the monomer of PLA, has traditionally been produced biotechnologically with bacteria. With genetic engineering, yeast have the potential to replace bacteria in biotechnological lactic acid production, with the benefits of being acid tolerant and having simple nutritional requirements. Lactate dehydrogenase genes have been introduced to various yeast to demonstrate this potential. Importantly, an industrial lactic acid producing process utilising yeast has already been implemented. Utilisation of D-xylose in addition to D-glucose in production of biochemicals such as lactic acid by microbial fermentation would be beneficial, as it would allow lignocellulosic raw materials to be utilised in the production processes.ResultsThe yeast Candida sonorensis, which naturally metabolises D-xylose, was genetically modified to produce L-lactic acid from D-xylose by integrating the gene encoding L-lactic acid dehydrogenase (ldhL) from Lactobacillus helveticus into its genome. In microaerobic, CaCO3-buffered conditions a C. sonorensis ldhL transformant having two copies of the ldhL gene produced 31 g l-1 lactic acid from 50 g l-1 D-xylose free of ethanol.Anaerobic production of lactic acid from D-xylose was assessed after introducing an alternative pathway of D-xylose metabolism, i.e. by adding a xylose isomerase encoded by XYLA from Piromyces sp. alone or together with the xylulokinase encoding gene XKS1 from Saccharomyces cerevisiae. Strains were further modified by deletion of the endogenous xylose reductase encoding gene, alone or together with the xylitol dehydrogenase encoding gene. Strains of C. sonorensis expressing xylose isomerase produced L-lactic acid from D-xylose in anaerobic conditions. The highest anaerobic L-lactic acid production (8.5 g l-1) was observed in strains in which both the xylose reductase and xylitol dehydrogenase encoding genes had been deleted and the xylulokinase encoding gene from S. cerevisiae was overexpressed.ConclusionsIntegration of two copies of the ldhL gene in C. sonorensis was sufficient to obtain good L-lactic acid production from D-xylose. Under anaerobic conditions, the ldhL strain with exogenous xylose isomerase and xylulokinase genes expressed and the endogenous xylose reductase and xylitol dehydrogenase genes deleted had the highest L- lactic acid production.

Dataset Information

Efficient production of L-lactic acid from xylose by Pichia stipitis.

Publications

Efficient production of L-lactic acid from xylose by Pichia stipitis.

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