Glucose alone does not completely hydrate bacteriorhodopsin in glucose-embedded purple membrane

Area of Science:

• Structural Biology
• Biophysics
• Electron Microscopy

Background:

• Glucose embedding is a common technique for preparing biological macromolecules for high-resolution electron microscopy.
• Maintaining native structure and function is critical for accurate structural determination.
• The M-state intermediate in the bacteriorhodopsin (bR) photocycle is important for understanding its function.

Purpose of the Study:

• To investigate the optimal conditions for glucose embedding of bacteriorhodopsin (bR) to preserve its native photocycle.
• To determine the effect of humidity on the structural integrity and functional properties of glucose-embedded bR.
• To assess whether glucose-embedded samples retain their native hydrated structure under various humidity conditions.

Main Methods:

• Preparation of glucose-embedded bacteriorhodopsin (bR) samples.
• Exposure of samples to varying relative humidity levels.
• Assessment of bR photocycle activity and structural integrity using electron microscopy and diffraction.

Main Results:

• Glucose-embedded bR prepared at ambient humidity lost its native photocycle capability.
• Equilibration at 81% relative humidity restored the native photocycle properties of bR.
• Higher relative humidities caused glucose to dissolve, potentially making samples too thick for electron microscopy.
• It cannot be assumed that glucose-embedded macromolecules retain their native structure even with high-resolution diffraction.

Conclusions:

• Humidity control is critical for glucose embedding techniques in electron microscopy.
• Equilibration at 81% relative humidity is a recommended precaution for glucose-embedded samples to preserve native structure and function.
• Caution is advised when interpreting data from glucose-embedded biological specimens, as native hydration may not be fully retained.