Why do enzymes stop working when they get too hot

Enzymes are proteins whose function depends on a precise folding pattern held together by hydrogen bonds, ionic interactions, and hydrophobic forces. Raising the temperature increases molecular motion, which adds kinetic energy that can overcome those weak bonds. When the temperature exceeds the enzyme's stability range, the bonds rupture, the protein unfolds, and the active site is distorted or destroyed, preventing substrate binding.

Denaturation in Numbers

Consider catalase from bovine liver, which has an optimal activity at 37 °C and a measured rate of 120 mL O₂ per minute under standard conditions. Using the Q₁₀ rule, a 10 °C rise to 47 °C would predict a rate increase of about 2‑fold, but experimental data show the rate actually falls to 30 mL/min because the enzyme begins to denature. At 55 °C the activity drops to 5 mL/min, a 96% loss, illustrating that beyond a threshold the kinetic boost is outweighed by structural collapse.

Key factors that influence thermal denaturation:

• Strength of intramolecular hydrogen bonds
• Presence of disulfide bridges
• pH of the surrounding solution
• Ionic strength and presence of stabilizing salts

How to test enzyme activity at different temperatures:

1. 1Prepare identical reaction mixtures containing substrate and enzyme.
2. 2Incubate each tube at a set temperature (e.g., 25 °C, 37 °C, 45 °C, 55 °C) for a fixed time.
3. 3Measure product formation (e.g., absorbance, gas volume) and plot activity versus temperature.

Typical optimal and denaturation temperatures for common enzymes:

Denaturation is often irreversible; once the protein has unfolded, it cannot spontaneously refold into its functional conformation without assistance. Some enzymes can refold if the temperature is lowered quickly, but most lose activity permanently. Understanding the thermal limits is crucial for designing experiments, storing reagents, and interpreting results when temperature fluctuates.