Copper-induced spectroscopic and structural changes in short peptides derived from azurin

Area of Science:

• Biochemistry
• Biophysical Chemistry
• Metalloprotein Chemistry

Background:

• Metalloprotein active sites can be mimicked by designing peptides that bind metal ions.
• Understanding metal-ligand binding and structural changes is crucial for protein functions like metal uptake, transport, and electron transfer.
• Azurin is a model copper-binding electron transfer protein.

Purpose of the Study:

• To investigate the copper-binding associated spectroscopic and structural properties of peptide loops from the azurin copper-binding site.
• To determine the stoichiometry and structural consequences of copper binding to these peptides.

Main Methods:

• Circular dichroism spectroscopy to study structural changes (β-turn, helical features).
• UV-Visible spectroscopy to monitor electronic transitions and absorption maxima.
• Mass spectrometry to identify copper-binding residues and observe dimerization.
• Fluorescence quenching studies to confirm copper-binding induced changes.

Main Results:

• Peptides formed a β-turn upon binding Cu²⁺ at 1:1 stoichiometry, with electronic transitions at 340 nm and 540 nm.
• At 2:1 Cu²⁺:peptide stoichiometry, peptides adopted a helical structure with shifted absorption maxima (~360 nm and ~580 nm).
• Mass spectrometry confirmed copper binding to cysteine, histidine, and methionine, and revealed disulfide-linked dimerization at 2:1 stoichiometry.
• Fluorescence quenching indicated bi-phasic copper-binding induced changes in the peptides.

Conclusions:

• Peptide loops from azurin's copper-binding site can mimic metalloprotein active sites.
• Copper binding induces significant, stoichiometry-dependent structural changes (β-turn to helix).
• The observed dimerization via disulfide linkage at higher stoichiometry mirrors denatured azurin behavior.