Exploring the Multiligand Binding Specificity of Saposin B Reveals Two Binding Sites

Area of Science:

• Lysosomal storage diseases
• Protein-lipid interactions
• Biochemistry

Background:

• Saposin B (SapB) is essential for degrading sulfatides in lysosomes.
• SapB deficiency causes metachromatic leukodystrophy, a fatal neurodegenerative disorder.
• SapB's structure suggests potential for binding diverse substrates.

Purpose of the Study:

• To investigate the substrate binding capabilities of Saposin B.
• To explore the binding affinity and mechanism of SapB with various ligands.
• To determine if SapB possesses multiple ligand-binding sites.

Main Methods:

• Fluorescence spectroscopy was used to measure binding affinities (K_D).
• Computational docking studies predicted ligand interactions with SapB.
• Ligand binding was analyzed to understand the thermodynamic driving forces.

Main Results:

• Saposin B binds a broad range of ligands with affinities from micromolar to nanomolar.
• Entropy is identified as the primary thermodynamic driver for SapB-ligand binding.
• SapB possesses two distinct binding sites, accommodating up to two ligands sequentially.

Conclusions:

• Saposin B exhibits a wider substrate specificity than previously recognized.
• The dual-binding site mechanism offers new insights into SapB's function in lysosomal degradation.
• Findings may inform therapeutic strategies for SapB-related metabolic disorders.