Enzymatic synthesis of lipid A molecules with four amide-linked acyl chains: LpxA acyltransferases selective for an analog of UDP-N-acetylglucosamine in which an amine replaces the 3″-hydroxyl group

LpxA of Escherichia coli catalyzes the acylation of the glucosamine 3-OH group of UDP-GlcNAc, using R-3-hydroxymyristoyl-acyl carrier protein (ACP) as the donor substrate. We now demonstrate that LpxA in cell extracts of Mesorhizobium loti and Leptospira interrogans, which synthesize lipid A molecules containing 2,3-diamino-2,3-dideoxy-D-glucopyranose (GlcN3N) units in place of glucosamine, do not acylate UDP-GlcNAc. Instead, these LpxA acyltransferases require a UDP-Glc-NAc derivative (designated UDP 2-acetamido-3-amino-2,3-dideoxy-α-D-glucopyranose or UDP-GlcNAc3N), characterized in the preceding paper (Sweet, C. R., Ribeiro, A. A., and Raetz, C. R. H. (2004) J. Biol. Chem. 279, 25400-25410), in which an amine replaces the glucosamine 3-OH group. L. interrogans LpxA furthermore displays absolute selectivity for 3-hydroxylauroyl-ACP as the donor, whereas M. loti LpxA functions almost equally well with 10-, 12-, and 14-carbon 3-hydroxyacyl-ACPs. The substrate selectivity of L. interrogans LpxA is consistent with the structure of L. interrogans lipid A. The mechanism of L. interrogans LpxA appears to be similar to that of E. coli LpxA, given that the essential His ¹²⁵ residue of E. coli LpxA is conserved and is also required for acyltransferase activity in L. interrogans. Acidithiobacillus ferrooxidans (an organism that makes lipid A molecules containing both GlcN and GlcN3N) has an ortholog of LpxA that is selective for UDP-GlcNAc3N, but the enzyme also catalyzes the acylation of UDP-GlcNAc at a slow rate. E. coli LpxA acylates UDP-GlcNAc and UDP-GlcNAc3N at comparable rates in vitro. However, UDP-GlcNAc3N is not synthesized in vivo, because E. coli lacks gnnA and gnnB. When the latter are supplied together with A. ferrooxidans lpxA, E. coli incorporates a significant amount of GlcN3N into its lipid A.