Читать книгу Biomolecular Engineering Solutions for Renewable Specialty Chemicals - Группа авторов - Страница 57

Bioethanol (C₂H₅OH) is the most common liquid biofuel. It is used as an additive to gasoline as 5–10% blend. It burns clean and help in reduction of greenhouse gases and is being commercially produced in Brazil, Canada, and United States for transportation (Sirajunnisa and Surendhiran, 2016). Initially, bioethanol was produced from biorefinery of sugar‐rich agricultural biomass (Rastogi et al., 2018). This bioethanol production was limited due to issues with food competition. Photosynthetic biomass production and getting the biomass converted into ethanol is of great interest and is done easily by cyanobacteria. Some strains can naturally convert carbon source to ethanol and some have to be engineered with key enzymes for ethanol production. Simply ethanol production requires two key enzymes pyruvate decarboxylase which converts pyruvate to acetaldehyde and alcohol dehydrogenase converting acetaldehyde to ethanol.

Pyruvate decarboxylase (pdc) enzyme from Zymomonas mobilis is well characterized and used to engineer several cyanobacterial strains such as Synechocystis sp. PCC6803. Pdc and alcohol dehydrogenase from Z. mobilis were integrated into the genome of PCC6803 using double homologous recombination. The genes were under the control of strong light inducible psbAII promoter (Dexter and Fu, 2009). With incorporation of pyruvate decarboxylase, overexpression of endogenous alcohol dehydrogenase and hampering the biosynthetic pathway of poly‐β‐hydroxybutyrate, PCC6803 gives an ethanol efficiency of 5.50 g/l (Gao et al., 2012). Synechococcus sp. PCC7002 is more tolerant to higher temperature as compared with PCC6803. Glycogen synthesis was blocked in PCC7002 by introducing two glycogen synthase genes. This lead to hamper the growth of the cyanobacteria. This was accommodated by incorporating double copies of ethanolgenic pathways giving 2.2 g/l ethanol in 10 days (Wang et al., 2020). Pdc and Adh with E. coli lac and CI‐PL temperature inducible promoter increase ethanol productivity in Synechococcus sp. PCC 7942 (Dexter et al., 2015). It is said that increased NADPH increases biomass and ethanol yield. This was confirmed by endogenous expression of glucose‐6‐phosphate dehydrogenase (zwf) gene in PCC6803. zwf gene of pentose phosphate pathway increases NADPH production and hence biomass and ethanol production (Choi and Park, 2016).