Abstract
Lipid A is the essential component of endotoxin (Gram-negative lipopolysaccharide), a potent immunostimulatory compound. As the outer surface of the outer membrane, the details of lipid A structure are crucial not only to bacterial pathogenesis but also to membrane integrity. This work characterizes the structure of lipid A in two psychrophiles, Psychromonas marina and Psychrobacter cryohalolentis, and also two mesophiles to which they are related using MALDI-TOF MS and fatty acid methyl ester (FAME) GC-MS. P. marina lipid A is strikingly similar to that of Escherichia coli in organization and total acyl size, but incorporates an unusual doubly unsaturated tetradecadienoyl acyl residue. P. cryohalolentis also shows structural organization similar to a closely related mesophile, Acinetobacter baumannii, however it has generally shorter acyl constituents and shows many acyl variants differing by single methylene (-CH2-) units, a characteristic it shares with the one previously reported psychrotolerant lipid A structure. This work is the first detailed structural characterization of lipid A from an obligate psychrophile and the second from a psychrotolerant species. It reveals distinctive structural features of psychrophilic lipid A in comparison to that of related mesophiles which suggest constitutive adaptations to maintain outer membrane fluidity in cold environments.
Highlights
Bacterial lipopolysaccharide (LPS) is an essential component of the cellular envelope in Gram-negative bacteria such as Escherichia coli
The predominant, hexa-acyl bis-phosphate form of P. marina lipid A strongly resembles that of the related mesophile E. coli, with the critical difference of a distinctive polyunsaturated tetradecadienoyl acyl chain
P. marina mirrors the alteration of E. coli lipid A during the metabolic survival response of cold-shock, in which an unsaturated hexadecenoyl acyl chain is used in place of a saturated dodecanoyl one [16]
Summary
Bacterial lipopolysaccharide (LPS) is an essential component of the cellular envelope in Gram-negative bacteria such as Escherichia coli. It is a molecule of fundamental importance in medicine; LPS was first identified as endotoxin due to its activation of the mammalian innate immune system and its role in septic shock [1]. The discovery that LPS is a potent bioactive material led to its intensive characterization as a class of structurally related molecules, built around an essential disaccharide glycolipid known as lipid A [2]. Condensation of two of these units to disaccharide monophosphate with four acyl chains (by LpxH and LpxB), and subsequent phosphorylation to form tetra-acyl lipid
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