Binding sites for cholesterol on Ca(2+)-ATPase studied by using a cholesterol-containing phospholipid

Area of Science:

• Biochemistry
• Membrane Biophysics
• Lipid Chemistry

Background:

• The Ca(2+)-ATPase enzyme is crucial for muscle contraction, residing in the sarcoplasmic reticulum membrane.
• Phospholipids and cholesterol are key components of biological membranes, influencing protein function.
• Understanding lipid-protein interactions is vital for elucidating enzyme mechanisms in their native environment.

Purpose of the Study:

• To synthesize novel phosphatidylcholines with an integrated cholesterol moiety.
• To investigate the binding characteristics of these cholesterol-containing phospholipids to the Ca(2+)-ATPase.
• To determine the functional consequences of these interactions on enzyme activity and calcium binding.

Main Methods:

• Synthesis of phosphatidylcholines with cholesterol at the glycerol backbone's 2-position.
• Fluorescence quenching assays to assess binding affinity at the lipid-protein interface.
• Enzyme activity assays and Ca(2+) binding stoichiometry measurements of reconstituted Ca(2+)-ATPase.

Main Results:

• Cholesterol-containing phosphatidylcholines bind to the Ca(2+)-ATPase interface with moderate affinity.
• Enzyme activity and Ca(2+) binding stoichiometry are altered by these modified phospholipids, varying with fatty acyl chain length.
• Tryptophan fluorescence changes upon Ca(2+) removal are modulated by cholesterol-linked phospholipids, indicating altered protein conformation.

Conclusions:

• Cholesterol covalently linked to phospholipids can interact with the Ca(2+)-ATPase at the lipid-protein interface.
• These interactions influence the enzyme's functional properties, including catalytic activity and calcium ion binding.
• The findings provide insights into the role of membrane lipid composition in regulating ion pump function.