Structural Basis for Acyl‐substrate Delivery and Product Release by ACP in the Biosynthesis of Lipid A

Abstract

Acyl‐carrier‐protein (ACP) is the principal transporter of fatty acids, coordinating acyl‐transfer among a vast network of diverse enzymes and biochemical processes. ACP association with protein partners is thought to be exceedingly transient. This paradigm has posed challenges for understanding the molecular basis for acyl‐delivery and dissociation. During biosynthesis of the lipid A component of lipopolysaccharides, ACP shuttles acyl‐intermediates thioester‐linked to its prosthetic 4′‐phosphopantetheine arm among four acyltransferases, including LpxD. LpxD is responsible for transferring a single β‐OH‐C14 acyl‐chain delivered by ACP to uridine diphosphate‐acyl‐glucosamine. We determined the crystal structures of three modified forms of Escherichia coli ACP engaging LpxD, which represent stalled intermediates along the reaction coordinate. The structures reveal the intricate interactions at the interface that optimally position ACP for acyl‐delivery and that directly contact the prosthetic group. Conformational differences among the stalled ACPs provide the molecular basis for the association‐dissociation process. An unanticipated conformational shift between 4′‐phosphopantetheine groups within the LpxD catalytic chamber exposes an unprecedented role of ACP in facilitating product release.