Temperature and Enzyme Function

Section 1 of 7Why shape matters

Enzymes are proteins

• Enzymes are proteins An enzyme is a long chain of amino acids folded into a precise three-dimensional shape,
  • which is held in place by bonds between parts of the chain.

The active site

• The active site The folding creates the active site.
  • The substrate can only bind and react if its shape is complementary to this site.
• Anything that changes the shape of the enzyme therefore changes how well it works.

Section 2 of 7Heating towards the optimum

1Temperature increases

• Temperature increases Enzyme and substrate molecules gain kinetic energy.

2Faster movement

• Faster movement The molecules move around more quickly in the solution.

3More collisions

• More collisions Substrate molecules hit active sites more often and with more energy, so more successful collisions occur each second.

4Rate rises

• Rate rises More enzyme-substrate complexes form and the rate of reaction increases, up to the optimum temperature.

Section 3 of 7Above the optimum: denaturation

Denaturation

Bonds break

• Bonds break At high temperatures the enzyme molecule vibrates so strongly that the bonds holding its shape in place begin to break.

Shape lost

• Shape lost As the protein unfolds, the shape of the active site changes.
  • The substrate is no longer complementary to it and cannot bind.

Activity stops

• Activity stops The enzyme can no longer catalyse the reaction. This is called denaturation.

Irreversible

• Irreversible Denaturation is permanent. Cooling the enzyme again does not restore its shape or its activity.

Section 4 of 7Reading the temperature graph

Region of the graph	What is happening	Explanation
Low temperature, rate low but rising	Slow reaction	Little kinetic energy, so few successful collisions between enzyme and substrate
Rising steeply towards the peak	Rate increasing	More kinetic energy, so more frequent and more energetic collisions
Peak of the curve	Optimum temperature	Fastest rate; about 37 °C for human enzymes
Falling steeply after the peak	Rate decreasing	Bonds break and the active site changes shape as enzymes denature
Zero rate at high temperature	No activity	All enzymes have been denatured

Section 5 of 7Low temperatures

Slowed, not destroyed

• Slowed, not destroyed Cold does not denature enzymes.
  • At low temperatures the molecules simply have less kinetic energy and the reaction is slow.
• If the temperature is raised again the enzymes work faster, so a refrigerator slows food spoilage without permanently harming the enzymes.

Optimum temperature

• Optimum temperature Each enzyme has its own optimum.
  • Enzymes in the human body usually work best at around 37 °C, whereas those in organisms from hot springs can have a much higher optimum.

Section 6 of 7Key terms

Term	Meaning
Optimum temperature	The temperature at which an enzyme works fastest.
Denaturation	A permanent change in the shape of an enzyme, so that its active site no longer fits the substrate.
Kinetic energy	The energy a particle has because of its movement.
Active site	The part of an enzyme where the substrate binds.
Rate of reaction	How quickly the substrate is used up or the product is formed.

Section 7 of 7Exam technique

1

Saying the enzyme has 'died' or been 'killed' by heat. The correct word is denatured, because enzymes are not living.

2

Claiming cold temperatures denature enzymes. They only slow the reaction down, and the effect is reversible.

3

Explaining the fall in rate after the optimum with 'the molecules move too fast'. The cause is the active site changing shape, so the substrate can no longer fit.

4

Forgetting to use the word 'complementary' when explaining why the substrate no longer fits a denatured enzyme.