Catalyzes the esterification, concomitant with transport, of exogenous fatty acids into metabolically active CoA thioesters for subsequent degradation or incorporation into phospholipids. Is maximally active on C6:0, C8:0 and C12:0 fatty acids, while has a low activity on C14-C18 chain length fatty acids (PubMed:15213221, PubMed:19477415). Is involved in the anaerobic beta-oxidative degradation of fatty acids, which allows anaerobic growth of E.coli on fatty acids as a sole carbon and energy source in the presence of nitrate or fumarate as a terminal electron acceptor (PubMed:12535077). Can functionally replace FadD under anaerobic conditions (PubMed:12535077). ATP + CoA + hexanoate = AMP + diphosphate + hexanoyl-CoA ATP + CoA + octanoate = AMP + diphosphate + octanoyl-CoA ATP + CoA + dodecanoate = AMP + diphosphate + dodecanoyl-CoA Optimum pH is 7.7-8.2. Lipid metabolism; fatty acid beta-oxidation. Homodimer. Partially membrane-associated. Expression independent of FadR (PubMed:12535077). FadK is not expressed under aerobic growth conditions, the levels of anaerobic expression vary with the terminal electron acceptor, with more expression during growth on fumarate than on nitrate (at protein level) (PubMed:15213221). Cells lacking this gene grow normally on aerobic oleate plates, but grow relatively poorly on oleate plus nitrate plates under anaerobic conditions. The double fadD fadK deletion mutant fails to grow on fatty acids under either aerobic or anaerobic conditions, although fadD mutants grow on fatty acids under anaerobic conditions. Probably starts on Met-1; overexpressed protein starting at Met-19 has lower activity, forms aggregates during reaction and is unstable in storage at -80 degrees Celsius (PubMed:15213221). The enzymatic mechanism is a two-step reaction that proceeds via the intermediate formation of an acyl-adenylate (acyl-AMP) intermediate. Belongs to the ATP-dependent AMP-binding enzyme family. Truncated N-terminus.