Protein–Protein Complex Stability Controls Substrate Scope in a β‐Ketoacyl‐ACP Reductase Specific for Medium Chains

Assembly‐line enzymes carry out multistep synthesis of important metabolites by using acyl carrier proteins (ACPs) to shuttle intermediates along defined sequences of active sites. Despite longstanding interest in reprogramming these systems for metabolic engineering and biosynthetic chemistry, the mechanisms underlying their reaction order remain poorly understood and difficult to control. Here we describe a β‐ketoacyl‐ACP reductase from Pseudomonas putida ( Pp FabG4) with an unusual selectivity for medium chains and use it to explore the molecular basis of substrate specificity in enzymes that pull intermediates from fatty acid synthesis, a common route to specialized products. X‐ray crystallography shows no obvious barriers to short‐chain binding. Molecular simulations and supporting mutational analyses indicate that substrate preference arises instead from a weak enzyme–ACP interaction that is stabilized by medium acyl chains but not by short chains. Indeed, mutations that strengthen this interaction for Pp FabG4 or weaken it for Ec FabG, an Escherichia coli β‐ketoacyl‐ACP reductase with a broad substrate specificity, can enhance or reduce activity on short‐chain substrates by over 100‐fold. Our findings show how the stability of enzyme‐ACP interactions can control substrate scope in promiscuous enzymes and guide the exchange of intermediates between (and within) assembly‐line systems.