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Conformational studies on murein-lipoprotein from the outer membrane of Escherichia coli
Abstract:
Conformational studies on an isolated integral membrane protein are reported. Lipoprotein of Escherichia coli outer membrane was released from murein by treatment with either lysozyme or trypsin. The isolated lysozyme-released lipoprotein (lipoprotein I) contained 2 or 3 muropeptides covalently linked at the C-terminal end, while the trypsin-released lipoprotein (lipoprotein II) was free of muropeptides and lacked the C-terminal peptide Tyr-Arg-Lys. Circular dichroism spectra of the two preparations were essentially identical, and they show an alpha-helix content of about 80%. According to calculations based on the Chou-Fasman rules for proteins of known sequence, lipoprotein is 64% alpha-helix and 15% beta-structure. Infrared spectroscopy qualitatively supports these values. The conformation was stable in the pH range of 5 - 12. Danaturation of lipoprotein by heat, 8 M urea, or sodium dodecylsulphate was a fully reversible, cooperative process. The thermal denaturation of lipoprotein occurs in two steps with transition points at 79.4 degrees C for lipoprotein I and at 85.1 degrees C for lipoprotein II. Lioprotein markedly changes conformation at dodecylsulphate concentrations where micelle formation sets in. The unusual behaviour of the lipoprotein convormation in sodium dodecylsulphate is discussed in relation to the lipoprotein conformation and aggregation within the membrane.
Insights
This study investigated the structure of Escherichia coli outer membrane lipoprotein. Conformational analysis revealed a high alpha-helix content, stable across a wide pH range and reversible denaturation.
Area of Science:
- Structural biology
- Biochemistry
- Microbiology
Background:
- Integral membrane proteins play crucial roles in cellular processes.
- Escherichia coli outer membrane lipoprotein is a key component of bacterial cell envelopes.
- Understanding lipoprotein structure is essential for deciphering its function.
Purpose of the Study:
- To determine the secondary structure and conformational stability of isolated Escherichia coli outer membrane lipoprotein.
- To compare the conformations of lipoprotein preparations with and without associated muropeptides.
- To investigate the effects of denaturation agents on lipoprotein structure.
Main Methods:
- Isolation of lipoprotein from Escherichia coli outer membrane using lysozyme or trypsin.
- Circular dichroism spectroscopy to assess alpha-helix content.
- Infrared spectroscopy to qualitatively support structural findings.
- Analysis of thermal and chemical denaturation using various agents (heat, urea, sodium dodecyl sulphate).
Main Results:
- Isolated lipoprotein exhibits approximately 80% alpha-helix content, confirmed by circular dichroism and infrared spectroscopy.
- Lipoprotein conformation is stable in the pH range of 5-12.
- Denaturation by heat, urea, or sodium dodecyl sulphate is a reversible, cooperative process.
- Thermal denaturation occurs in two steps, with distinct transition points for lysozyme- and trypsin-released preparations.
- Significant conformational changes occur in sodium dodecyl sulphate at concentrations inducing micelle formation.
Conclusions:
- Escherichia coli outer membrane lipoprotein possesses a predominantly alpha-helical structure.
- The protein's conformation is remarkably stable under physiological conditions.
- Lipoprotein's behavior in sodium dodecyl sulphate suggests an interplay between its conformation, aggregation, and membrane environment.
- The presence or absence of muropeptides does not significantly alter the overall secondary structure.