Prior knowledge - Proteins - Physical changes
3 important questions on Prior knowledge - Proteins - Physical changes
What are the effects of denaturation of proteins?
- Decreased solubility --> as more hydrophobic groups are exposed to water
- Aggregation of proteins --> hydrophobic groups of proteins interact with each other. Aggregates inhibit the refolding.
- Increased accessibility of peptide bonds to proteolytic enzymes, increased degradability
- Loss of biological activity --> biological activity depends on precise conformation of protein, so change in conformation results in a lower biological activity. Also called, enzyme activity
- Increased reactivity --> due to unfolding reactive groups become exposed. Exposure of thiol group (-SH), reshuffling can take place.
How is the solubility of proteins related to the pH and what interactions are involved?
- pH closed to pI --> aggregates (by hydrophobic groups) --> solubility decreases
- pH far from pI --> electrostatic repulsion --> soluble
- Hydrophobic amino acid residues --> solubility lower
- Hydrophilic amino acid residues --> solubility higher
The solubility of proteins is the balance between repulsive electrostatic interactions and attractive hydrophobic interactions between proteins.
How is the solubility of proteins affected by salt?
- Low salt concentration (10-20 mM) --> can increase the solubility, at pI --> salting in effect, charged groups are solvated --> interaction with solvent increased
- Medium salt concentration (10 mM - 500 mM) --> DLVO theory --> Na+ will screen the charged groups, less repulsion
- Very high salt concentration (~5%) --> solubility decreases --> salting out effect --> purification of proteins by adding ammonium sulphate to a mixture of soluble proteins, after which proteins start to precipitate.
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