Biological production of adipic acid from renewable substrates: Current and future methods

Adipic acid has been widely used for producing thermoplastic polyurethane resins, nylon 6-6, adhesives, synthetic lubricants and plasticizers, and the global market for adipic acid is estimated at approximate $6.3 billion per year. Currently, 2.85 million tons of adipic acid are produced from petrochemical sources, mostly by oxidation of KA oil (cyclohexanol and cyclohexanone) catalyzed by nitric acid. The chemical synthesis of adipic acid is of serious consequences such as greenhouse gases and heavy pollution to the environment, etc. The researchers are struggling to establish more reliable, renewable and affordable adipic acid synthesis methods, and among which, biological synthesis of adipic acid is attracting the most attention. There are two biological routes for adipate synthesis: (1) biological accumulations of precursors of adipic acid such as d-glucaric acid and cis,cis-muconic acid, which can be further catalyzed to adipate; (2) synthesis of adipate directly from carbon source. With the emergence of metabolic engineering and synthetic biology, biological synthesis of adipic acid from renewable substrates is now feasible. Here, we review the new approaches and technologies to produce adipic acid biologically, with the direct synthesis of adipic acid from the renewable substrates of special interest.