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Dissecting ArnA – a required enzyme in the polymyxin resistance pathway
Author(s) -
GatzevaTopalova Petia Zvezdanova,
Sousa Marcelo C.
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.5.a904-c
Subject(s) - chemistry , decarboxylation , enzyme , biochemistry , lipid a , active site , stereochemistry , bacteria , biology , catalysis , genetics
ArnA is a key enzyme, catalyzing two distinct reactions in the pathway leading to modification of lipid A with 4‐amino‐4‐deoxy‐L‐arabinose (Ara4N). Ara4N‐lipid A modification of the bacterial outer membrane results in microbial resistance to key components of the innate immune system, and antibiotics, such as polymyxin. A clear understanding of the ArnA structure and mechanism is crucial for the design of selective inhibitors of the pathway. Such inhibitors may prove particularly useful in treating chronic bacterial infections. The crystal structures of the unliganded individual domains, as well as the structures of the liganded (with ATP/UDP‐glucuronic acid (UGA) and UGA alone) full‐length ArnA have been solved. Based on structural and kinetic data we propose that the dehydrogenation reaction catalyzed by the C‐terminal domain of ArnA follows an ordered mechanism in which UGA is the first substrate to bind, inducing a conformational change that opens the active site for NAD + . Sequence comparisons of the ArnA dehydrogenase domain with enzymes annotated as UDP‐xylose synthases allowed the identification of catalytic residues playing role in the decarboxylation of the UDP‐4‐keto glucuronic acid intermediate. Based on this information a novel metal‐independent decarboxylation mechanism is proposed. The results of this study have important implications for the design of selective inhibitors of ArnA. This research was supported by a NIH grant to M.C.S (AI060841‐01) and a NIH training grant to P.Z.G.‐T. (GM65103).