An Alternative Route for UDP-Diacylglucosamine Hydrolysis in Bacterial Lipid A Biosynthesis

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Abstract
The outer leaflet of the outer membranes of Gram-negative bacteria is composed primarily of lipid A, the hydrophobic anchor of lipopolysaccharide. Like Escherichia coli, most Gram-negative bacteria encode one copy of each of the nine genes required for lipid A biosynthesis. An important exception exists in the case of the fourth enzyme, LpxH, a peripheral membrane protein that hydrolyzes UDP-2,3-diacylglucosamine to form 2,3-diacylglucosamine 1-phosphate and UMP by catalyzing the attack of water at the α-P atom. Many Gram-negative organisms, including all α-proteobacteria and diverse environmental isolates, lack LpxH. Here, we report a distinct UDP-2,3-diacylglucosamine pyrophosphatase, designated LpxI, which has no sequence similarity to LpxH but generates the same products by a different route. LpxI was identified because its structural gene is located between lpxA and lpxB in Caulobacter crescentus. The lpxI gene rescues the conditional lethality of lpxH-deficient E. coli. Lysates of E. coli in which C. crescentus LpxI (CcLpxI) is overexpressed display high levels of UDP-2,3-diacylglucosamine pyrophosphatase activity. CcLpxI was purified to >90% homogeneity. CcLpxI is stimulated by divalent cations and is inhibited by EDTA. Unlike E. coli LpxH, CcLpxI is not inhibited by an increase in the concentration of detergent, and its pH dependency is different. When the CcLpxI reaction is conducted in the presence of H218O, the 18O is incorporated exclusively into the 2,3-diacylglucosamine 1-phosphate product, as judged by mass spectrometry, demonstrating that CcLpxI catalyzes the attack of water on the β-P atom of UDP-2,3-diacylglucosamine.

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