Lesson 5 of 6 / Biological molecules
Why temperature and pH alter proteins
Why can a protein lose function without its chain being cut?
In this lesson: Explain environmental effects on protein shape and interactions.
About 5 min
The key ideaDenaturation changes higher-level structure and function; it usually leaves the primary peptide sequence intact.
Explore the idea
Shape can change without cutting the chain
Folding depends on interactions. Temperature or pH can disrupt them; reversing a condition does not guarantee that a real protein refolds.
Explanation
Increasing temperature raises molecular motion. At sufficiently high temperatures, interactions stabilising a protein's specific folded conformation are disrupted. Binding sites may no longer have the required shape and chemistry. Denaturation is a structural change, not simply slower movement or an enzyme being used up.
Changing pH changes protonation and charge on suitable R groups. This can disrupt ionic interactions and alter hydrogen bonding, changing tertiary structure or a binding site's chemistry. Proteins have different functional pH ranges because their structures and environments differ.
Ordinary heat or pH denaturation does not necessarily hydrolyse peptide bonds. The primary sequence may remain intact while secondary, tertiary or quaternary structure changes. Extreme chemical treatment can also break covalent bonds, but that is a separate claim requiring evidence.
Cooling does not always restore function because unfolded proteins can aggregate or fail to refold correctly. Conversely, low temperature often reduces an enzyme's reaction rate without denaturation. Distinguish reversible kinetic effects from loss of functional conformation.
Step by step
- 1
Identify the condition
Separate low temperature from excessive heating.
- 2
Name the interaction
Link pH to charge and ionic interactions.
- 3
Link shape to function
Explain why the altered binding site works less effectively.
Worked example
Work through the evidence
An enzyme heated to 80 C remains inactive after cooling. Its polypeptide length is unchanged. Explain.
One way to explain it
Heat disrupted its functional folded structure; aggregation or failed refolding can prevent recovery. The unchanged chain length is consistent with intact peptide bonds despite denaturation.
Why this answer works
- Function depends on conformation, not just sequence length.
- Irreversibility is supported by the failure to recover after cooling.
Is this true? "Cold always denatures enzymes."
Moderate cooling usually lowers collision frequency and rate without permanently disrupting the protein.