Fatty acids (FAs) are essential building blocks for all the domains of life, of which bacterial de novo synthesis, called type II FA synthesis (FAS II), is energetically expensive. The recycling of exogenous FAs (eFAs) partially relieves the FAS II demand and, therefore, compromises the efficacy of FAS II-directed antimicrobials. The versatile acyl-acyl carrier protein (ACP) synthetase, AasS, enables bacterial channeling of diverse eFA nutrients through holo-ACP, an activated form of ACP. However, the molecular mechanism for AasS catalysis is not fully understood. Here we report a series of cryo-electron microscopy structures of AasS from the bioluminescent bacterium Vibrio harveyi to provide insights into the catalytic cycle. AasS forms a ring-shaped hexamer, with each protomer folding into two distinct domains. Biochemical and structural analysis suggests that AasS accommodates distinct eFA substrates and the conserved W230 residue has a gating role. Adenosine triphosphate and Mg