Protein denaturation is a change in a protein’s three-dimensional shape that causes it to lose its normal biological function. In IB Biology, both SL and HL students must understand that changes in temperature or pH can cause denaturation.
The Reasoning and Mechanism
A protein’s function depends on its specific three-dimensional conformation. For example, an enzyme has an active site whose shape and chemical properties allow a particular substrate to bind by induced fit.
Temperature or pH changes can disrupt the interactions that maintain this conformation. The protein unfolds or changes shape, altering the active site so that the substrate is no longer able to bind effectively. The rate of the enzyme-catalysed reaction therefore decreases or stops.
Factor | Mechanism of denaturation |
|---|---|
High temperature | Increased molecular motion disrupts interactions maintaining the protein’s conformation. |
Extreme pH | Changes in hydrogen ion concentration alter charges within the protein, disrupting interactions and changing its conformation. |
For HL students, the tertiary structure can be stabilized by hydrogen bonds, ionic bonds, disulfide covalent bonds and hydrophobic interactions. Temperature and pH do not necessarily affect all these interactions equally.
Denaturation usually does not break the peptide bonds of the primary structure. Therefore, the amino acid sequence generally remains unchanged even though the protein loses its functional shape. Denaturation may be reversible in some proteins, but it is often irreversible, for example, when egg-white proteins solidify during heating.
A common misconception is that denaturation means the protein has been completely broken down into amino acids. This is incorrect: breaking peptide bonds requires hydrolysis, whereas denaturation primarily alters higher levels of protein structure.