Catalyzes the activation of long-chain fatty acids as acyl-adenylates (acyl-AMP), which are then transferred to the multifunctional polyketide synthase (PKS) type III for further chain extension (Probable). Involved in the biosynthesis of sulfolipid 1 (SL-1) (PubMed:17389997, PubMed:17768256). ATP + hexadecanoate + holo-[(hydroxy)phthioceranic acid synthase] = AMP + diphosphate + hexadecanoyl-[(hydroxy)phthioceranic acid synthase] ATP + holo-[(hydroxy)phthioceranic acid synthase] + octadecanoate = AMP + diphosphate + octadecanoyl-[(hydroxy)phthioceranic acid synthase] ATP + dodecanoate + H(+) = diphosphate + dodecanoyl-AMP ATP + H(+) + hexadecanoate = diphosphate + hexadecanoyl-AMP Lipid metabolism; fatty acid biosynthesis. Disruption of fadD23 increases binding affinity to human macrophage-like cell THP-1 and shows a lack of sulfolipid 1 production as compared to wild-type. Belongs to the ATP-dependent AMP-binding enzyme family.