Discovery and Development of 1,2-Propanediol - Essay Example

?Discovery and Development of 2-Propanediol (Propylene Glycol) Introduction Innumerable products of industrial microbiology have been developed andproduced through various biotechnological approaches such as metabolic pathway engineering and metabolic control engineering of microorganisms. One such product is 1,2-propanediol (propylene glycol), a chemical feedstock produced using the metabolic pathway engineering approach. 1,2-Propanediol is a colorless and clear liquid that has a high boiling point1. It is very hygroscopic in pure form. Industrially, it is a very useful product. It is used in the manufacture of unsaturated polyester resins and is used as solvent for surfactants, paints and cleaning agents1. It is also used as antifreeze for de-icing aircrafts at airports as it lowers the freezing point of water. Recent applications include use as a humectant and a preservative. It is used in cosmetics, pharmaceutical preparations and also in electronic cigars2. The structure of 1,2-Propanediol comprises of three carbons with a stereogenic center at the central carbon3. It exists in two racemic forms: S and R as shown in figure 1 below. Figure 1: Chemical structure (S and R form) of 1,2-propanediol 3 1,2-Propanediol as well as 1,3-Propanediol can be manufactured though bioengineered microorganisms. By turning on and off some genes, it is possible to make the microorganism overproduce a specific metabolic product. As shown in figure 2, the intermediate metabolism of Escherichia coli can be shifted to produce 1,2 and 1,3-propanediol using enzymes such as aldose reductase from rat lens, E. coli glycerol dehydrogenase and Klebsiella pneumoniae glycerol dehydrogenase and 1,3-propanedioloxidoreductase4. There are various biochemical pathways through which 1,2-propanediol can be produced. Figure 2: Metabolic pathway engineering of E. coli to produce 1,2 and 1,3-propanediol using enzymes marked in green 4 The discovery The production of 1,2-propanediol through sugar fermentation has long been reported in bacteria5. Many subsequent studies have reported the production of this glycol in other bacteria and yeasts. Production of 1,2-propanediol by wild-type E. coli from common sugars is not known. As back as 1981, 1,2-propanediol was found to be produced through a direct route from deoxy sugars by E.coli6. It was shown that E. coli can grow on the L-fucose and L-rhamnose deoxy sugars as sole carbon and energy sources resulting in the production of propanediol. The first instance of metabolic engineering to produce 1,2-propanediol was when Altaras and Cameron metabolically bioengineered the 1,2-propanediol pathway in E. coli7. In 1999, they reported that E. coli that overexpressed methylglyoxal synthase gene produced 1,2-propanediol. Expression of methylglyoxal synthase or glycerol dehydrogenase led to anaerobic production of around 0.25 g of 1,2-propanediol per liter. The yield was found to be higher both the enzymes were coexpressed7. The development After their first report of the production of 1,2-propanediol from metabolically engineering E. coli, Altaras and Cameron again in 2000 published another report. This time they reported enhanced production of 1,2-propanediol8. They investigated three methods. First method involved the elimination of lactate byproduct. The second method involved the construction of a complete pathway from dihydroxyacetone phosphate – an intermediate of the glycolytic cycle. The third method involved bioprocessing improvements through fed-batch fermentation using the best bioengineered strains. They were able to successfully produced a final yield of 0.19 g of 1,2-propanediol per gram of consumed glucose. Many later studies have investigated the production of 1,2-propanediol by metabolically engineered bacteria. Berrios-Rivera, San and Bennett studied the effects of various cofactor manipulations on the production of 1,2-propanediol9. They used sugars that were similar to glucose and that can be fed into glycolysis for pyruvate production. The sugars used had different oxidation states to enable the manipulation of NADH/NAD+ ratio under anaerobic conditions. This was done because NADH is required for the synthesis of 1,2-propanediol. They found that overexpression of glycerol dehydrogenase enzyme from E. coli and methylglyoxal synthase enzyme from Clostridium acetobutylicum in bioengineered E. coli resulted in the production of 1,2-propanediol. However, they found that using a reduced carbon source was not effective in increasing the yield. Instead, the incorporation of a mutation in ldh involved in the methylglyoxal pathway was found to improve the yield. Thus, it was ascertained that the production of 1,2-propanediol is not limited by NADH cofactor but by the pathways concerning methylglyoxal. More recently, Clomburg and Gonzalez investigated the metabolism of glycerol by E. coli to produce 1,2-propanediol10. They engineered a functional 1,2-propanediol pathway by the overexpression of genes required in 1,2-propanediol synthesis from dihydroxyacetone phosphate and the manipulation of glycerol utilization pathway. It was seen that by manipulating the glycerol utilization pathway by replacing PEP-dependent dihydroxyacetone kinase (DHAK) of native E. coli with ATP-dependent dihydroxyacetone kinase from Citrobacter freundii, the availability of dihydroxyacetone phosphate can be increased, thereby resulting in a higher yield of 1,2-propanediol. Ethanol was identified as the required co-product during the fermentative pathway, which increases both the titer as well as the yield of 1,2-propanediol. Clomburg and Gonzalez successfully combined these metabolic manipulations and produced a metabolically engineered E. coli strain that could produce 5.6?g 1,2-propanediol per liter at 21.3% weight by weight yield. Using crude glycerol also a substantial amount of 1,2-propanediol was produced. While many subsequent as well as earlier studies researched the production of 1,2-propanediol by bacteria from glycerol, Jung et al investigated the production by yeast Saccharomyces cerevisiae11. Glycerol is in a reduced state and is a cheap substrate that can be used as a carbon source. However, its utilization rate by Saccharomyces cerevisiae is very low. Jung et al used glycerol as the main source of carbon for yeast. The yeast strains were metabolically engineered to overexpress glycerol dissimilation pathway genes such as glycerol dehydrogenase (gdh), glycerol kinase (GUT1), glycerol transporter gene (GUP1) and glycerol 3-phosphate dehydrogenase (GUT2). These strains were able to significantly utilize glycerol. This was further improved by introducing 1,2-propanediol pathway genes such as gldA (glycerol dehydrogenase) and mgs (methylglyoxal synthase) from E. coli11. By using a combination of these pathways, they successfully increased the growth rate by 77% and glycerol utilization by 141%. They were also successful in achieving an yield of 2.19 g 1,2- propanediol per liter. Safety and environmental concerns Although 1,2-propanediol is an extremely useful product, it does pose a number of hazards and safety concerns, both to organism health and to environmental health. Exposure to the product can occur through workplace exposure in a product manufacturing facility, or through the use of consumer products containing 1,2-propanediol12. Exposure may also occur by environmental releases of the product or through industrial spills. The oral toxicity of the product is low12. It is thus considered to be safe for used as a food additive. From 1942 onwards, it is included as a safe product for pharmaceutical products in New and Non-Official Remedies12. It has also been approved for use in dental products. Prolonged contact to the product is non irritating to the skin and eye. However, it may produce mild transient conjunctivitis12. Eye irritation may occur upon exposure to mists of the product. Inhalation of its vapors is known to produce no significant hazard, although it may be irritating for some individuals. There is no evidence showing that it is a carcinogen or is genotoxic12. 1,2-propanediol is however listed as an immunotoxicant, respiratory toxicant, neurotoxicant, and skin and sense organ toxicant (EDF)13. Chronic exposure in large doses may result in lactic acidosis, stupor, hypoglycemia, seizures, and central nervous system depression13. 1,2-propanediol is not known to pose serious environmental concerns. It is easily biodegraded in soil or water through both aerobic and anaerobic mechanisms. During degradation in surface waters, it is known to exert high levels of BOD (biological oxygen demand), which can adversely affect aquatic life by depleting the amount of dissolved oxygen14. The oxygen depletion potential of 1,2-propanediol is several times higher than raw sewage14. Figure 3: Many organisms degrade 1,2-propanediol. The pathway for degradation by Salmonella enterica is shown in the figure above15 Potential problems While 1,2-propanediol is considered safe in small doses, both health-wise and environmentally, it is important to note that it may pose yet unknown health and environmental concerns. As it is constantly used in cosmetics, industrial solvents, and innumerable consumer goods, potential problems cannot be undermined. Industrial discharges should also be regularly monitored to ensure safety for marine environment. Potential for improvement The developments in bioengineering processes of microorganisms for the production of 1,2-propanediol have resulted in very high yields of the product. However, there are limitations. While many direct fermentation routes have been developed, the production of 1,2-propanediol is limited3. This is most probably because the sugars used are expensive and most organisms used for the production are poorly characterized3. High cost of raw materials, low reactor productivity and reaction rate and low product titer compared to chemical manufacturing processes are the major limitations of producing 1,2-propanediol from microorganisms3. By focusing on the elimination of competing pathways and by discovering cheaper raw materials, the onus of production of 1,2-propanediol can completely shift from chemical production to eco-friendly biological production of the product. References 1. Kemi Swedish Chemicals Agency (2003) Information on substances.Apps.kemi.se. http://apps.kemi.se/flodessok/floden/kemamne_eng/1,2-propandiol_eng.htm 2. Schripp, T., et al. (2013) Does e-cigarette consumption cause passive vaping. Indoor Air. 23, 25-31. doi: 10.1111/j.1600-0668.2012.00792.x 3. Saxena, R. K., et al. (2010) Microbial production and applications of 1,2-propanediol. Indian J. Microbiol. 50, 2–11. doi: 10.1007/s12088-010-0017-x 4. Prescott, L., et al. (2002) Prescott?Harley?Klein:Microbiology, McGraw Hill. 5. Badia, J., et al. (1885) Fermentation mechanism of fucose and rhamnose in Salmonella typhium and Klebsiella pneumoniae. J. Bacteriol. 161, 435–437. PMCID: PMC3450292. 6. Boronat, A., et al. (1981) Metabolism of L-fucose and L-rhamnose in Escherichia coli: differences in induction of propanediol oxidoreductase. J. Bacteriol. 147, 181–185. PMCID: PMC216023 7. Altaras, N. E., et al. (1999) Metabolic engineering of a 1,2-propanediol pathway in E. coli. Appl. Environ. Microbiol. 65, 1180–1185. PMCID: PMC91161 8. Altaras, N. E., et al. (2000) Enhanced production of (R)-1,2-propanediol by metabolically engineered Escherichia coli. Biotechnol. Prog. 16, 940-6. PMID: 11101319 [PubMed - indexed for MEDLINE] 9. Berrios-Rivera, S. J., et al. (2003) The effect of carbon sources and lactate dehydrogenase deletion on 1,2-propanediol production in Escherichia coli. J. Ind. Microbiol. Biot. 30, 34–40. PMID: 12545384 10. Clomburg, J. M., et al. (2010) Metabolic engineering of Escherichia coli for the production of 1,2-propanediol from glycerol. Biotechnol. Bioeng. 108, 867-879. doi: 10.1002/bit.22993 11. Jung, J. Y., et al. (2011) Production of 1,2-propanediol from glycerol in Saccharomyces cerevisiae. J. Microbiol. Biotechnol. 21, 846–853. PMID: 21876375 12. Dow (2013) Product Safety Assessment (PSA): Propylene Glycol. Dow. http://www.dow.com/productsafety/finder/prog.htm#HealthInfo 13. Thorp, N. (2011) 1,2- Propanediol. Toxipedia. http://toxipedia.org/display/toxipedia/1%2C2-+Propanediol 14. The Winterizer (2013) Environmental impact of propylene glycol. The Winterizer. http://www.thewinterizer.com/index.php?option=com_content&view=article&id=5&Itemid=2 15. Sinha, S., et al. (2012) The PduM Protein Is a Structural Component of the Microcompartments Involved in Coenzyme B12-Dependent 1,2-Propanediol Degradation by Salmonella enterica. J. Bacteriol. 194, 1912–1918. doi: 10.1128/?JB.06529-11 Read More