Enzymes depend on precise 3D shape to function. Temperature, pH, and certain chemicals can disrupt the weak interactions that maintain that shape, reducing reaction rate or stopping catalysis entirely.

Core idea: denaturation stops catalysis

Enzymes are proteins whose function depends on structure at multiple levels (especially tertiary structure). When environmental conditions disrupt stabilising interactions, the enzyme’s shape changes and catalytic activity falls.

Denaturation matters because even small shape changes can misalign key amino acid side chains required for binding and catalysis, preventing formation of a functional enzyme–substrate complex.

What structures are affected?

Denaturing conditions commonly disrupt:

• Hydrogen bonds (important in secondary and tertiary structure)

• Ionic bonds (salt bridges) between charged side chains

• Hydrophobic interactions that stabilise the protein core

• Sometimes disulfide bonds (covalent) under harsh chemical conditions

Temperature and enzyme denaturation

Temperature affects enzymes in two competing ways:

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page_url: https://biomedfoundation.com/knowledge-base/03-05-factors-affecting-enzyme-action-temperature/

image_identifier: “The effects of temperature on enzyme activity” three-panel diagram (linked as ‘Diagram: Image’ on the page)

This multi-panel schematic explains why enzymes show an optimum temperature: catalytic rate tends to increase as molecules move faster, while the fraction of properly folded (active) enzyme decreases once denaturation begins. The observable activity curve is the combined outcome of these opposing trends, producing a peak where catalysis is fast but the protein is still mostly folded.
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• Moderate warming increases molecular motion, increasing collisions and often increasing activity.

• Excessive heat destabilises folding, causing thermal denaturation and a rapid activity drop.

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page_url: https://www.savemyexams.com/o-level/biology/cie/23/revision-notes/5-enzymes/5-2-effects-of-temperature-and-ph/enzymes-temperature-and-ph/

image_identifier: Graph showing the effect of temperature on rate of enzyme activity (served via Save My Exams CDN; displayed at width ~3840 px)

This graph summarizes the classic temperature dependence of enzyme activity: reaction rate rises with temperature as kinetic energy and collision frequency increase, then falls steeply once the protein begins to denature. The labeled “optimum temperature” marks the condition where catalytic activity is maximal before unfolding disrupts the active site and eliminates function.
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Why high temperature denatures enzymes

At high temperature, increased vibration and movement can overwhelm the weak interactions that maintain folding. As the protein unfolds:

• The active conformation is lost

• Substrate binding becomes less effective

• Catalytic residues may no longer be positioned correctly to stabilise the transition state

Key features of temperature–activity behavior

• Enzymes often show an optimum temperature where activity is highest under those conditions.

• Beyond the optimum, activity typically decreases sharply because denaturation can be fast and extensive.

• Different enzymes have different optima based on adaptation (e.g., enzymes from thermophiles are more heat-stable due to more stabilising interactions).

DEFINITION

Optimum temperature: The temperature at which a specific enzyme shows maximal catalytic activity under a defined set of conditions.

pH and enzyme denaturation

pH influences the protonation state (charge) of amino acid side chains. Because folding and catalysis depend on charge-based interactions, changing pH can reduce activity and, at extremes, denature the enzyme.

DEFINITION

pH: A measure of hydrogen ion concentration, defined as $pH = -\log_{10}[H^+]$.

How pH changes disrupt enzymes

pH affects enzymes by:

• Altering charges on acidic/basic side chains (e.g., Asp, Glu, Lys, Arg, His)

• Disrupting ionic bonds that help stabilise tertiary structure

• Changing charge patterns in catalytic residues, which can prevent acid–base catalysis

• Reducing substrate binding if complementary charges are lost

Typical pH–activity pattern

• Many enzymes display an optimum pH near where key residues have the correct charges.

• At very low or high pH, unfolding becomes more likely, so activity may drop due to both poorer catalysis and denaturation.

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page_url: https://www.savemyexams.com/o-level/biology/cie/23/revision-notes/5-enzymes/5-2-effects-of-temperature-and-ph/enzymes-temperature-and-ph/

image_identifier: Graph showing effect of pH on rate of activity for an enzyme from duodenum (served via Save My Exams CDN; displayed at width ~3840 px)

This figure shows how enzyme activity typically peaks at an optimum pH and decreases as pH becomes more acidic or more basic. The decline reflects changes in amino-acid side-chain protonation that disrupt ionic interactions and the chemistry of catalytic residues, and at extreme pH values the enzyme can become denatured and lose activity entirely.
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Chemical conditions that denature enzymes

In addition to temperature and pH, several chemical environments can disrupt protein structure and stop catalysis:

• Detergents can disrupt hydrophobic interactions by solubilising nonpolar regions.

• Organic solvents can destabilise the hydrophobic core and alter folding.

• Urea/guanidinium can interfere with hydrogen bonding and promote unfolding.

• Heavy metals (e.g., Hg²⁺, Pb²⁺) can bind to side chains (notably sulfur-containing groups), distorting structure.

• High salt or extreme ionic strength can interfere with ionic interactions and protein solubility.

Functional consequence in cells

When enzymes denature:

• Reaction rates decrease or stop because the functional shape is lost.

• Metabolic pathways can fail if key enzymes lose activity, especially under stressful environmental conditions.