Related Experiment Videos
Sulfate-binding protein dislikes protonated oxyacids. A molecular explanation
1Howard Hughes Medical Institute, Department of Biochemistry, Baylor College of Medicine, Houston, TX 77030.
Journal of Molecular Biology
|December 5, 1988
Summary
The sulfate-binding protein tightly binds fully ionized oxyacids. Its binding site has specific hydrogen bond acceptors, crucial for high affinity, while protonated ligands significantly reduce binding energy.
Area of Science:
- Biochemistry
- Structural Biology
- Protein-ligand interactions
Background:
- Sulfate-binding protein (SBP) is crucial for sulfate uptake in bacteria.
- Understanding SBP's binding mechanism provides insights into protein-ligand interactions and anion recognition.
- The influence of pH on binding affinities of various oxyacids to SBP is not fully elucidated.
Purpose of the Study:
- To investigate the effect of pH on the binding affinities of four tetrahedral oxyacids to SBP.
- To elucidate the structural and chemical factors governing the selective binding of oxyanions to SBP.
- To determine the binding characteristics of sulfate, selenate, chromate, and phosphate to SBP.
Main Methods:
- Equilibrium dissociation constants (Kd) were measured for sulfate, selenate, chromate, and phosphate binding to SBP across a pH range of 5 to 8.1.
- Structural analysis of the SBP-sulfate complex was utilized to interpret binding data.
- Protonation states and net charges of the oxyacids were considered in relation to binding affinities.
Main Results:
- Sulfate and selenate binding affinities were pH-independent between pH 5 and 8.1.
- Chromate binding affinity showed pH dependence, correlating with chromic acid's pK2.
- Phosphate binding affinity was significantly lower (five orders of magnitude) than sulfate binding.
Conclusions:
- SBP's binding site is optimized for tetrahedral, fully ionized oxyacid dianions.
- The pH dependence of chromate binding and low affinity of phosphate are attributed to insufficient hydrogen bond acceptors in the SBP binding site.
- Ligands with donatable protons experience a reduced binding energy of approximately 7 kcal/mol, highlighting the importance of ligand ionization state for binding affinity.