| During polyketide biosynthesis, acyl carrier proteins (ACPs) perform the central role of transferring polyketide intermediates between active sites of a polyketide synthase. The 4′-phosphopantetheine prosthetic group of a holo-ACP is a long and flexible arm that can reach into different active sites and provide a terminal sulfhydryl group for the attachment of acyl groups through a thioester linkage. We have determined the solution structure and characterized backbone dynamics of the holo-form of the frenolicin acyl carrier protein (fren holo-ACP) by Nuclear Magnetic Resonance. The solution structure is composed of three major α-helices packed in a bundle with two additional short helices in intervening loops; one of the short helices slowly exchanges between two conformations. Although the three-helix bundle fold is conserved among acyl carrier proteins involved in fatty acid synthases and polyketide synthases, a detailed comparison revealed that ACPs from polyketide biosynthetic pathways are more related to each other in tertiary fold than to their homologs from fatty acid biosynthetic pathways.;The acyl transferase(AT) is responsible for transferring extender units, usually malonyl or methylmalonyl groups, from CoA to the 4′-phosphopantetheine arm of the holo-ACP. The extender unit is then delivered to the active site of ketosynthase, the enzyme that catalyzes the decarboxylative condensation reaction for chain elongation. The malonyl-CoA:ACP transacylase (MAT) is the primary determinant of extender unit specificity in aromatic polyketide biosynthesis, and has strict specificity for malonyl-CoA. In contrast, both the MAT and the ketosynthase appear to be tolerant toward most heterologous acyl carrier proteins, although quantitative differences among alternative ACPs have been observed. If the molecular recognition features are well understood for MAT:ACP interactions, ACP could be engineered to recognize MATS (either natural MATs or engineered MATS) with different substrate specificity and introduce structural diversity into the polyketide backbone. We have identified the nucleophile in wildtype and mutant MATs and the MAT binding site of holo-ACP was identified by chemical shift perturbation mapping during MAT titration. Together with the recently solved X-ray crystal structure of the MAT, these findings were used to map the protein-protein interface between the two proteins. |
