Copper Reductase Activity and Free Radical Chemistry by Cataract-Associated Human Lens γ-Crystallins

Area of Science:

• Biochemistry
• Ophthalmology
• Protein Chemistry

Background:

• Cataracts result from light-scattering protein aggregates in the human eye lens.
• Metal ions, particularly copper, are implicated in cataract formation due to protein aggregation.
• Human lens γ-crystallins are susceptible to metal-induced aggregation.

Purpose of the Study:

• To investigate the interaction of copper ions (Cu2+) with major human lens γ-crystallins (γD, γC, γS).
• To elucidate the molecular mechanisms underlying copper-induced γ-crystallin aggregation.
• To explore the redox activity and binding sites of copper in γ-crystallins.

Main Methods:

• Solution turbidimetry, SDS-PAGE, circular dichroism, differential scanning calorimetry.
• Isothermal titration calorimetry (ITC) for binding affinity.
• Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) for copper binding and redox state.

Main Results:

• Cu2+ induced non-amyloid aggregation in γD-, γC-, and γS-crystallins.
• Aggregation mechanisms include loss of β-sheet structure, decreased stability, metal-bridging, and disulfide dimer formation.
• Distinct Cu2+ binding affinities and two binding sites per protein were identified.
• Cu2+ was reduced to Cu+ by γ-crystallins, forming protein-based free radicals (likely Tyr-based in γD-crystallin).

Conclusions:

• Copper-induced aggregation of γ-crystallins involves complex structural and redox changes.
• γ-crystallins possess Cu2+ reductase activity, generating reactive free radicals.
• These findings highlight the role of copper and γ-crystallins in lens homeostasis and cataract pathogenesis.