This page reproduces content from Synthetic Biology for Organic Syntheses, in Green and Sustainable Medicinal Chemistry: Methods, Tools and Strategies for the 21st Century Pharmaceutical Industry, The Royal Society of Chemistry, 2016, ch. 14, pp. 165-179..,
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The biggest challenges to the production of fine chemicals on large scale include environmental pollution, toxic waste, and the increasing cost of prevention and clean-up of environmental waste. Compared with chemical synthesis, biosynthetic approaches can offer several advantages; enzymes generally operate under biologically relevant conditions such as ambient temperatures and pressures, aqueous media and neutral pHs. This enables an array of diverse enzymes to work concurrently in the same environment e.g. a cell, as such it becomes possible to assemble a multistep synthetic pathway for the in vivo production of a complex molecule from cheap and abundant materials. This is in great contrast to traditional chemical multistep syntheses which would require purification, recovery and further processing of intermediates en route to the final product. 
These characteristics of biotransformation facilitate “one-pot synthesis”, which is a challenging approach in chemotransfomations. The use of biosynthesis would thus circumvent the need for energy intensive and harsh reaction conditions, toxic or caustic chemicals and would enable the reactions to be carried out in greener solvents. Bioderived chemicals would also limit the greenhouse gas emissions and the cost through use of cheaper simpler starting materials such as CO2,CO, hydrogen and salts.
Although the repertoire of products made by naturally occurring organisms is broad, it is still predefined by evolution as the organisms have adapted their biochemistries for survival in particular environments. Synthetic biology can be used to modify the biochemistries of these organisms and tailor their metabolic pathways towards the production of pharmaceutically relevant products or intermediates from simple molecules.