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Kurs: MCAT > Rozdział 2

Lekcja 1: Biological sciences practice passage questions

Preventing lipopolysaccharide formation in cystic fibrosis patients


Young patients diagnosed with cystic fibrosis are at high risk of developing debilitating pulmonary infections caused by the bacteria Pseudomonas aeruginosa. In these patients, P. aeruginosa secretes a mucus substance called alginate and overproduces a virulent lipopolysaccharide (LPS), both of which contribute to chronic pulmonary infections. The basic chemical structure of an LPS molecule is drawn in Figure 1.

Figure 1
A precursor of LPS is glucose-1-phosphate, which can be created when the enzyme phosphoglucomutase (PGM) rearranges glucose-6-phosphate. A precursor of alginate is mannose-1-phosphate, which can be created when the enzyme phosphomannomutase (PMM) rearranges mannose-6-phosphate. In P. aeruginosa both the conversion of glucose and mannose are performed by a single bifunctional enzyme identified as PGM/PMM. Since both alginate and LPS cause the infection, researchers are actively looking for mechanisms to block the PGM/PMM enzyme and thus stop their production.
The blocking of PGM/PMM has other benefits too. For example, when cells break down glycogen, the major product is glucose-1-phosphate. Large supplies of glucose-1-phosphate can be converted by PGM into glucose-6-phosphate, which is a beginning molecule for both the glycolysis and pentose phosphate pathways. If the PGM enzyme is blocked, glucose-1-phosphate will not be able to participate in these metabolic pathways and the cells will not generate the critical resources needed for survival.
Experiment 1
The enzyme kinetics of PGM/PMM were investigated and the results are detailed in Table 1.
SubstrateKm (μM)Vmax (μM•mol1•mg1)
Table 1: Kinetics of the bifunctional PGM/PMM enzyme.
Experiment 2
Through a series of column gel filtrations, the PGM/PMM molecule was extracted and then processed through SDS PAGE. Figure 2 shows the SDS PAGE gel of the processed enzyme. Lane 1 is a standard protein ladder, lane 2 is PGM/PMM prior to filtration, and lanes 3 to 5 are progressive purifications of the enzyme. Lane 5 is the final purification of PGM/PMM.

Figure 2
Sources: Ye, R. W., Zielinski, N. A., and Chakrabarty, A. M. (1994) Purification and characterization of phosphomannomutase/phosphoglucomutase from Pseudomonas aeruginosa involved in biosynthesis of both alginate and lipopolysaccharide, J Bacteriol 176, 4851-4857.
Where in the cell would LPS likely be found?
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