Structure and reactivity of LpxD, the N-acyltransferase of lipid A biosynthesis

The external layer of the Gram-negative bacterial outer membrane is primarily composed of a protective, selectively permeable LPS. The biosynthesis of LPS relies on UDP-3-O-acyl-glucosamine N-acyltransferase (LpxD), which transfers 3-hydroxy-arachidic acid from acyl carrier protein to the 2' amine of UDP-3-O-myristoyl glucosamine in Chlamydia trachomatis. Our crystallographic study reveals that LpxD is a homotrimer, each subunit of which is constructed from a novel combination of an N-terminal uridine-binding domain, a core lipid-binding domain, and a C-terminal helical extension. Highly conserved residues dominate nucleotide binding. Phe-43 and Tyr-49 form p-stacking interactions with uracil, and Asn-46 and His-284 form hydrogen bonds with the phosphate groups. These interactions place the glucosamine moiety at the catalytic center formed by two adjacent subunits. Here His-247 and His-284 contribute to a mechanism involving nucleophilic attack by the amine of one substrate on the carbonyl carbon of an acyl carrier protein thioester conjugate. Serendipitously, our study reveals a fatty acid (FA) binding groove near the catalytic center. MS elucidated the presence of a FA mixture binding to LpxD, with palmitic acid the most prevalent. The placement of UDP-N-acetylglucosamine and the FA provides details of N-acyltransferase ligand interactions and allows for a description of structure and reactivity at an early stage of LPS assembly.