Human S100A5 binds Ca²⁺ and Cu²⁺ independently.

Area of Science:

• Biochemistry
• Molecular Biology
• Protein-metal interactions

Background:

• S100A5 is a calcium-binding protein with poorly understood biological roles, potentially involving inflammation and olfactory signaling.
• Previous studies suggested an antagonistic relationship between Ca²⁺ and Cu²⁺ binding in S100A5.

Purpose of the Study:

• To investigate the interplay between Ca²⁺ and Cu²⁺ binding by S100A5.
• To clarify the biochemical basis of S100A5's interaction with these metal ions.

Main Methods:

• Isothermal titration calorimetry (ITC) to quantify metal ion binding.
• Circular dichroism (CD) spectroscopy to assess structural changes.
• Analytical ultracentrifugation (AUC) to analyze protein oligomerization.

Main Results:

• Human S100A5 binds Ca²⁺ and Cu²⁺ simultaneously and independently.
• Wildtype S100A5 exhibits aggregation in the presence of both ions; a Cys-free mutant prevents this.
• Cysteine mutations do not affect Ca²⁺ or Cu²⁺ binding affinity or induce antagonism.

Conclusions:

• S100A5 binds Ca²⁺ and Cu²⁺ independently, challenging prior antagonism findings attributed to aggregation.
• The independent binding suggests separate biological functions for Ca²⁺ and Cu²⁺ interactions with S100A5.
• Accurate biochemical understanding is crucial for developing informed biological hypotheses regarding S100A5.