bi048551f_si_001.pdf (111.23 kB)
Crystal Structure of Escherichia coli ArnA (PmrI) Decarboxylase Domain. A Key Enzyme for Lipid A Modification with 4-Amino-4-deoxy-l-arabinose and Polymyxin Resistance†,‡
journal contribution
posted on 2004-10-26, 00:00 authored by Petia Z. Gatzeva-Topalova, Andrew P. May, Marcelo C. SousaGram-negative bacteria including Escherichia coli, Salmonella typhimurium, and Pseudomonas
aeruginosa can modify the structure of lipid A in their outer membrane with 4-amino-4-deoxy-l-arabinose
(Ara4N). Such modification results in resistance to cationic antimicrobial peptides of the innate immune
system and antibiotics such as polymyxin. ArnA is a key enzyme in the lipid A modification pathway,
and its deletion abolishes both the Ara4N-lipid A modification and polymyxin resistance. ArnA is a
bifunctional enzyme. It can catalyze (i) the NAD+-dependent decarboxylation of UDP-glucuronic acid to
UDP-4-keto-arabinose and (ii) the N-10-formyltetrahydrofolate-dependent formylation of UDP-4-amino-4-deoxy-l-arabinose. We show that the NAD+-dependent decarboxylating activity is contained in the
360 amino acid C-terminal domain of ArnA. This domain is separable from the N-terminal fragment, and
its activity is identical to that of the full-length enzyme. The crystal structure of the ArnA decarboxylase
domain from E. coli is presented here. The structure confirms that the enzyme belongs to the short-chain
dehydrogenase/reductase (SDR) family. On the basis of sequence and structure comparisons of the ArnA
decarboxylase domain with other members of the short-chain dehydrogenase/reductase (SDR) family, we
propose a binding model for NAD+ and UDP-glucuronic acid and the involvement of residues T432, Y463,
K467, R619, and S433 in the mechanism of NAD+-dependent oxidation of the 4‘ ‘-OH of the UDP-glucuronic
acid and decarboxylation of the UDP-4-keto-glucuronic acid intermediate.