Acyl carrier protein (ACP) plays a central role in fatty acid biosynthesis. However, the molecular machinery that mediates its function is not yet fully understood. Therefore, structural studies were carried out on the acyl-ACP intermediates of Plasmodium falciparum using NMR as a spectroscopic probe. Chemical shift perturbation studies put forth a new picture of the interaction of ACP molecule with the acyl chain, namely, the hydrophobic core can protect up to 12 carbon units, and additional carbons protrude out from the top of the hydrophobic cavity. The latter hypothesis stems from chemical shift changes observed in Cα and Cβ of Ser-37 in tetradecanoyl-ACP. 13C,15N-Double-filtered nuclear Overhauser effect (NOE) spectroscopy experiments further substantiate the concept; in octanoyl (C8)- and dodecanoyl (C12)-ACP, a long range NOE is observed within the phosphopantetheine arm, suggesting an arch-like conformation. This NOE is nearly invisible in tetradecanoyl (C14)-ACP, indicating a change in conformation of the prosthetic group. Furthermore, the present study provides insights into the molecular mechanism of ACP expansion, as revealed from a unique side chain-to-backbone hydrogen bond between two fairly conserved residues, Ile-55 HN and Glu-48 O. The backbone amide of Ile-55 HN reports a pKα value for the carboxylate, ∼1.9 pH units higher than model compound value, suggesting strong electrostatic repulsion between helix II and helix III. Charge-charge repulsion between the helices in combination with thrust from inside due to acyl chain would energetically favor the separation of the two helices. Helix III has fewer structural restraints and, hence, undergoes major conformational change without altering the overall-fold of P. falciparum ACP. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.