Interactions between lipids and bacterial reaction centers determined by protein crystallography

Area of Science:

• Structural biology
• Biochemistry
• Membrane protein research

Background:

• The reaction center of Rhodobacter sphaeroides is a crucial complex for photosynthesis.
• Understanding the structural environment of its cofactors is key to elucidating electron transfer mechanisms.

Purpose of the Study:

• To resolve the structure of the Rhodobacter sphaeroides reaction center at high resolution.
• To identify and characterize lipid molecules associated with the reaction center.
• To investigate the role of these lipids in cofactor energetics and electron transfer.

Main Methods:

• X-ray diffraction to determine protein structure at 2.55-A resolution.
• Laser mass spectroscopy to confirm lipid identities.
• Analysis of lipid-protein interactions and proximity to cofactors.

Main Results:

• Three lipid molecules (cardiolipin, phosphatidylcholine, and glucosylgalactosyl diacylglycerol) were identified on the reaction center surface.
• These lipids are situated in the hydrophobic region, interacting with aromatic amino acid residues.
• Phosphatidylcholine and the glycolipid are in close proximity to electron transfer cofactors, potentially influencing their energetics and function.
• The lipid arrangement suggests a bilayer-like "inner shell" around the membrane protein.

Conclusions:

• The identified lipids, particularly phosphatidylcholine and the glycolipid, play a significant role in modulating the electronic properties of reaction center cofactors.
• These lipids may be critical for the asymmetry of electron transfer, a fundamental aspect of photosynthetic energy conversion.
• The findings highlight the importance of the lipid-protein interface in membrane protein function and stability.