Crystallographic studies on complexes between retinoids and plasma retinol-binding protein

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biophysics

Background:

• Retinol-binding protein (RBP) transports retinol (vitamin A) in the bloodstream.
• Understanding RBP interactions with retinol analogs is crucial for drug development and nutrient transport studies.

Purpose of the Study:

• To determine the 3D structures of bovine RBP complexed with three distinct retinol analogs: fenretinide, all-trans retinoic acid, and axerophthene.
• To investigate how variations in retinoid end groups affect RBP structure and binding interactions.

Main Methods:

• X-ray crystallography was used to determine the high-resolution (1.8-1.9 Å) structures of RBP-retinoid complexes.
• Structural comparison with the native retinol-RBP complex was performed using root mean square deviation (RMSD) analysis.

Main Results:

• Retinoid analogs bound within the RBP beta-barrel cavity with conformations similar to retinol, showing minimal impact on overall protein structure.
• Fenretinide and retinoic acid binding induced limited conformational changes in the flexible loop at the RBP entrance, accommodating their end groups.
• Axerophthene binding, lacking a hydroxyl group, did not cause significant loop conformational changes.
• These structural findings suggest potential mechanisms for reduced affinity of certain retinoids to RBP and transthyretin.

Conclusions:

• Intact hydroxyl end groups are not strictly required for retinoid binding to RBP.
• Specific retinoid end groups can induce subtle RBP conformational adjustments, influencing binding dynamics.
• Observed structural changes correlate with potentially altered affinities for RBP and transthyretin, impacting retinoid transport and interactions.