Type I fatty acid synthase (FAS) trapped in the octanoyl-bound state
De novo fatty acid biosynthesis in humans is accomplished by a multidomain protein, the type I fatty acid synthase (FAS). Although ubiquitously expressed in all tissues, fatty acid synthesis is not essential in normal healthy cells due to sufficient supply with fatty acids by the diet. However, FAS is overexpressed in cancer cells and correlates with tumor malignancy, which makes FAS an attractive selective therapeutic target in tumorigenesis. Herein, we present a crystal structure of the condensing part of murine FAS, highly homologous to human FAS, with octanoyl moieties covalently bound to the transferase (MAT) and the condensation (KS) domain. The MAT domain binds the octanoyl moiety in a novel (unique) conformation, which reflects the pronounced conformational dynamics of the substrate binding site responsible for the MAT substrate promiscuity. In contrast, the KS binding pocket just subtly adapts to the octanoyl moiety upon substrate binding. Besides the rigid domain structure, we found a positive cooperative effect in the substrate binding of the KS domain by a comprehensive enzyme kinetic study. These structural and mechanistic findings contribute significantly to our understanding of the mode of action of FAS and may guide future rational inhibitor designs.\n\nHighlightsO_LIThe X-ray structure of the KS-MAT didomain of murine type I FAS is presented in an octanoyl-bound state.\nC_LIO_LIMultiple conformations of the MAT domain and a dynamic active site pocket explain substrate promiscuity.\nC_LIO_LIThe rigid domain structure and minor structural changes upon acylation are in line with the strict substrate specificity of the KS domain.\nC_LIO_LIEnzyme kinetics reveals cooperativity in the KS-mediated transacylation step.\nC_LI