Biofuels pathways

Bond-Watts BB, BeUerose RJ, Chang MCY. (2011). Enzyme mechanism as a kinetic control element for designing synthetic biofuel pathways. Nat Chem Biol, 7, 222-227. 

Most LCAs are performed only xmtil Step 2, since impact assessment and interpretation involve many more qualitative assumptions. In this case, LCA are called life cycle inventories (LCIs). This latter is a tool required to estimate the direct and indirect inputs of each step of a biofuel pathway. The results are the use of resources (eg, energy consumption) and the environmental emissions (eg, CO2, sulfur oxides, nitrogen oxides). LCIs permit the assessment of impact categories, such as climate change, photooxidant formation, acidification, eutrophication, ecotoxicity and human toxicity, and the depletion of biotic and abiotic resources. These factors of the LCI will be converted into environmental damages. Various indicators can be derived from these mechanisms at intermediate levels (midpoints) or damage levels (endpoints) after normalization, often weighting approaches. 

The key question is how to measure biofuel sustainability in such a complex system with a diversity of feedstock, a large number of biofuel pathways, and variations on specific interests of the stakeholders. The answer lies within the establishment of environmental and other indicators, which enable the assessing of the sustainability of different types of bioenergy systems. The indicators should, however, apply to both large installations and local sites, and also should be useful to diverse stakeholders (McBride et al., 2011). 

Fig. 5.6 C02-savings by use of biofuels made from energy crops according to Schmitz (2003), Quirin et al. (2004), CONCAWE (2006), Hill (2007) and BMELV (2007b). Conversion pathways 1 Straight oil —> Drive 2 Biodiesel —> Drive 3 Grain —> Heat 4 Ethanol —> Drive 5 Ethanol —> Heat Power 6 Ethanol —> Drive 7 Bales —> Heat Power 8 BtL —> Drive 9 Methanol —> Drive 10 Ethanol —> Drive 11 Biogas —> Drive 12 Biogas —> Heat Power 13 Chips — Heat 14 Chips —> Heat Power 15 BtL —> Drive...

Microbial biofuel cells were the earliest biofuel cell technology to be developed, as an alternative to conventional fuel cell technology. The concept and performance of several microbial biofuel cells have been summarized in recent review chapters." The most fuel-efficient way of utilizing complex fuels, such as carbohydrates, is by using microbial biofuel cells where the oxidation process involves a cascade of enzyme-catalyzed reactions. The two classical methods of operating the microbial fuel cells are (1) utilization of the electroactive metabolite produced by the fermentation of the fuel substrate " and (2) use of redox mediators to shuttle electrons from the metabolic pathway of the microorganism to the electrodes. ... 

Justin Stege (Diversa Corporation) discussed the molecular evolution of enzymes for particular pathways, with a focus on the modification of oil composition. Oleochemical applications for such enzymes include applications as biocatalysts for fatty acid modifications. In a program to integrate production and processing, such enzymes can be used to modify the fatty acid content of vegetable oils in planta. Results show that expressing such new enzymes in oilseed crops has resulted in altered oil composition, and that the features may be used to better design plant-based oils for use as biofuels and as improved renewable feedstocks. 

Another biofuel candidate that can be derived from LA is 2-methyl tetrahydro-furan (2-MTHF), which is obtained via the intermediate yVl in a reaction pathway that includes both dehydration and hydrogenation steps [99, 100]. In a comparable reaction, itaconic acid, a platform chemical produced industrially by fermentation of carbohydrates such as glucose, can be converted into 3-methyl tetrahydrofuran [100]. The hydrogen required to produce yVl may be obtained from formic acid, via transfer... 

Metabolic pathway engineering [125] is used to optimise the production of the required product based on the amount of substrate (usually biomass-derived) consumed. A so-called biobased economy is envisaged in which commodity chemicals (including biofuels), specialty chemicals such as vitamins, flavors and fragrances and industrial monomers will be produced in biorefineries (see Chapter 8 for a more detailed discussion). 

For 3.8 billion years, enzyme evolution has occurred primarily in microbes exposed to novel environmental conditions. However, in the last two decades, new methods have been developed for laboratory evolution of enzymes for production of chemicals, pharmaceuticals, and biofuels. Directed evolution has been widely used to improve thermostability and alter substrate specificity. Current efforts aim to improve the catalytic abilities of evolved enzymes, which are usually considerably poorer than those of naturally occurring enzymes, and to evolve novel pathways using promiscuous activities of existing enzymes. These efforts will provide new insights into the adaptation of protein scaffolds for new functions that will both help us to understand the evolutionary history of modern enzymes and provide the basis for a wide range of applications in biotechnology. 

Atsumi, S., Hanai, T, and Liao, J.C. (2008) Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels. Nature, 451,... 

Fatty acid synthesis has been engineered as well. Two pathways to very long chain polyunsaturated fatty acids were realized in P. pastoris to demonstrate their feasibility for future reengineering in oilseed crops [163]. Fatty acids and their esters are also interesting potential biofuels. Fatty acid esters with branched chain alcohols are potential low-viscosity biodiesels, and were successfully synthesized in Escherichia coli and P. pastoris by metabolic engineering [164]. 

---