Determination of phosphorylation sites in lipooligosaccharides from Pseudoalteromonas haloplanktis TAC 125 grown at 15 degrees C and 25 degrees C by nano-electrospray ionization quadrupole time-of-flight tandem mass spectrometry

Abstract

Lipooligosaccharides (LOSs) are macromolecules present on the external cellular membrane of Gram-negative bacteria, structurally made of two distinct regions, lipid A and Core. By varying their growth temperature, bacteria such as psychrophiles change the phosphorylation distribution of the LOSs produced. The level of phosphorylation and the phosphate group positions in LOSs produced by the extremophile psychrophilic bacterium Pseudoalteromonas haloplanktis TAC 125, grown at 15 degrees C and 25 degrees C, were investigated by nano-electrospray ionization quadrupole time-of-flight mass spectrometry (nanoESI-QTOF-MS) and tandem mass spectrometry (MS/MS). The samples, obtained by phenol/chloroform/petroleum ether (PCP) extraction of dried cells, were treated with hydrazine at 37 degrees C in order to reduce the heterogeneity by removal of the ester-linked fatty acid moieties. The molecular ion distributions in these LOS fractions were investigated in negative ion mode. Based on these data it was postulated that the sample grown at 25 degrees C contained four phosphate groups while that at 15 degrees C contained three. In order to determine phosphorylation sites in sugar chains, the samples were submitted to low collision energy MS/MS for sequencing. In the sample with three phosphates, one was found to be linked to the tetrasaccharide Core region, more precisely to position C-4 of the Kdo unit. The two remaining phosphate groups were both linked to the 2-acylamide-2-deoxy-D-glucopyranose of the lipid A moiety, and two possible distributions could be postulated on the basis of the fragmentation pattern obtained; in the first case both phosphate groups are linked as a pyrophosphate moiety to position C-1 of the proximal glucosamine (reducing residue), while in the second case one phosphate is linked to position C-1 of the proximal glucosamine and the other to position C-4' of the distal glucosamine (non-reducing residue). This distribution was also found in the lipid A moiety of the tetraphosphorylated sample grown at 25 degrees C, which bears two phosphate groups on the Core region, one on position C-4 of the Kdo and the other on position C-7 or C-8 of the same residue. The phosphate locations were derived from the intra-ring cleavage ions of sugar moieties in the LOSs obtained by an optimized CID procedure using negative ion QTOF-MS/MS.