Structural changes in a secretory phospholipase A2 induced by membrane binding: a clue to interfacial activation?

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Biophysics

Background:

• Phospholipase A2 (PLA2) activation mechanisms upon binding to phospholipid assemblies remain unclear.
• Previous studies using X-ray crystallography and NMR showed minimal or small conformational changes in PLA2 with substrate analogs or micelles.
• The structural state of PLA2 when bound to phospholipid bilayers is unknown.

Purpose of the Study:

• To investigate the structural changes of PLA2 upon binding to phospholipid bilayers.
• To understand the molecular basis of PLA2 activation at membrane interfaces.

Main Methods:

• Fourier-transform infrared (FTIR) spectroscopy was used to analyze the secondary structure of PLA2.
• The study focused on the amide I absorbance band, which is sensitive to protein secondary structure, particularly alpha-helices.
• PLA2 was studied in solution and when bound to lipid bilayers.

Main Results:

• FTIR spectroscopy revealed a splitting in the alpha-helical amide I absorbance band of PLA2 upon binding to lipid bilayers.
• A distinct higher frequency component, specific to membrane-bound PLA2, was observed.
• This component is attributed to increased flexibility within the alpha-helical regions of the enzyme.

Conclusions:

• Binding to phospholipid bilayers induces significant structural changes in PLA2, specifically increasing the flexibility of its alpha-helices.
• The formation of these flexible helices upon membrane interaction is a key factor likely contributing to PLA2 activation.
• This finding provides new insights into the mechanism of enzyme activation at membrane surfaces.