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Pb2+ and Hg2+ binding to alpha-lactalbumin
D B Veprintsev1, E A Permyakov, L P Kalinichenko
1Institute for Biological Instrumentation, Russian Academy of Sciences, Pushchino, Moscow, Russia.
Biochemistry and Molecular Biology International
|August 1, 1996
Summary
This study reveals how lead (Pb2+) and mercury (Hg2+) ions interact with human alpha-lactalbumin. Both heavy metals bind to specific sites, altering protein structure and stability, and potentially causing aggregation.
Area of Science:
- Biochemistry
- Protein-metal interactions
- Spectroscopy
Background:
- Human alpha-lactalbumin is a key milk protein with essential biological functions.
- Understanding its interactions with metal ions is crucial for assessing potential toxicity and biological effects.
- Heavy metals like lead (Pb2+) and mercury (Hg2+) are environmental contaminants known to affect protein structure and function.
Purpose of the Study:
- To investigate the binding mechanisms of lead (Pb2+) and mercury (Hg2+) ions to human alpha-lactalbumin.
- To characterize the binding sites and affinities of these metal ions.
- To evaluate the impact of Pb2+ and Hg2+ binding on the protein's thermal stability and aggregation propensity.
Main Methods:
- Intrinsic protein fluorescence spectroscopy was employed to monitor metal ion binding.
- Binding constants (Kass) were determined for Pb2+ and Hg2+ at various sites on alpha-lactalbumin.
- Changes in protein conformation and thermal stability were assessed upon metal ion interaction.
Main Results:
- Lead ions bind to the strong Ca2+ site (Kass ≈ 2 x 10^6 M-1) and the strong Zn2+ site (Kass ≈ 10^5 M-1) of alpha-lactalbumin, inducing spectral changes similar to Ca2+ binding.
- Pb2+ also binds to secondary sites (Kass ≈ 10^4 M-1), likely involving histidine residues, leading to significant protein aggregation.
- Mercury ions bind to primary Zn2+ sites (Kass ≈ (1-4) x 10^4 M-1) and secondary histidine-containing sites, with binding stoichiometry dependent on the protein's conformational state.
- Both Pb2+ and Hg2+ binding decrease the thermal stability of Ca2+-loaded alpha-lactalbumin and can cause pronounced protein aggregation.
Conclusions:
- Human alpha-lactalbumin exhibits specific binding affinities for both Pb2+ and Hg2+ ions at distinct sites.
- The interaction with these heavy metals alters the protein's structural integrity, reducing thermal stability and promoting aggregation.
- These findings highlight the potential for alpha-lactalbumin to interact with and sequester toxic heavy metals, with implications for metal detoxification and protein stability studies.