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.