Table of Contents
Temperature strongly affects how enzymes work. Because enzymes are proteins with a specific 3D structure, any change in temperature changes both how fast molecules move and how stable that 3D structure is. In this chapter the focus is on how and why reaction rates change with temperature, not on the general function of enzymes (treated elsewhere).
Collision rate and kinetic energy
For enzyme-catalyzed reactions in living cells, the enzyme and substrate must collide correctly for a reaction to occur. Temperature influences this by changing the kinetic energy of the molecules:
- At higher temperatures (within limits), molecules move faster.
- Faster motion means more collisions per second between enzyme and substrate.
- More collisions increase the probability of forming the enzyme–substrate complex, so the reaction rate rises.
On a graph of reaction rate (vertical axis) versus temperature (horizontal axis), this appears as an initial rising part of the curve: as temperature increases from low values, the reaction becomes faster.
Q₁₀ – the temperature coefficient
A common way to describe how sensitive a biological reaction is to temperature is the Q₁₀ value.
Definition:
- $Q_{10}$ is the factor by which the reaction rate changes when the temperature is increased by $10^\circ\text{C}$.
Mathematically:
$$
Q_{10} = \frac{v_{T+10}}{v_T}
$$
where:
- $v_T$ = reaction rate at temperature $T$
- $v_{T+10}$ = reaction rate at temperature $T + 10^\circ\text{C}$
For many enzyme-controlled biological processes in a moderate temperature range (for example $0{-}30^\circ\text{C}$), $Q_{10}$ is often around 2. That means:
- When the temperature increases by $10^\circ\text{C}$, the reaction rate roughly doubles.
- When the temperature decreases by $10^\circ\text{C}$, the reaction rate roughly halves.
This rule is an empirical approximation and holds only across a limited temperature range where the enzyme is still structurally intact.
The temperature optimum
Every enzyme has a temperature optimum:
- This is the temperature at which the enzyme’s reaction rate is maximal under given conditions (substrate concentration, pH, etc.).
- On the reaction rate vs. temperature curve, this is the peak.
The temperature optimum reflects a balance between two opposing temperature effects:
- Positive effect: increasing temperature speeds molecular motion and increases collision frequency (reaction rate goes up).
- Negative effect: at higher temperatures, the enzyme’s 3D structure becomes unstable and begins to denature (reaction rate goes down).
Different organisms and different enzymes have different optima:
- Enzymes of humans and many other mammals often have optima around $35{-}40^\circ\text{C}$.
- Enzymes of cold-adapted organisms (e.g., polar fish) tend to have lower optima.
- Enzymes of heat-adapted organisms (e.g., thermophilic bacteria from hot springs) often have much higher optima, sometimes above $70^\circ\text{C}$.
The optimum is not a universal constant but depends on the organism’s habitat temperature and on the enzyme’s structural adaptation.
Denaturation at high temperatures
The functional shape of an enzyme depends on many non-covalent interactions (hydrogen bonds, ionic interactions, hydrophobic interactions). These interactions are relatively weak and sensitive to thermal motion.
When temperature becomes too high:
- Increased molecular motion disrupts these interactions.
- The enzyme unfolds or changes shape; this is called denaturation.
- The active site loses its specific shape and can no longer bind the substrate correctly.
- The reaction rate rapidly falls despite high kinetic energy.
Key points about thermal denaturation:
- Above a certain temperature (often slightly above the optimum), small further increases in temperature can cause a sharp, often irreversible drop in activity.
- In many enzymes, denaturation is irreversible: once unfolded, they do not spontaneously refold into the correct functional structure under normal conditions.
- Loss of enzyme activity at high temperature can lead to cell damage or death, because essential reactions no longer proceed fast enough.
On a reaction rate vs. temperature curve, denaturation causes a steep decline after the optimum.
Reduced activity at low temperatures
At low temperatures, enzymes are structurally stable, but reactions still slow down. The main reasons:
- Molecules move slowly, resulting in fewer collisions per unit time.
- Even when enzyme and substrate collide, they may not have enough energy to reach the transition state.
Consequences:
- Reaction rates decrease gradually as temperature drops.
- Enzymes are usually not denatured at low temperatures; they are just less active.
- If temperature is raised again (without freezing damage), the enzyme’s activity usually returns to its previous level.
This explains:
- Why metabolism slows in cold environments.
- Why cooling is widely used to preserve food: lower temperatures slow down both enzymatic activity of microorganisms and chemical spoilage reactions.
Shape of the temperature–activity curve
Putting the effects together gives a characteristic curve:
- Low temperatures
- Enzymes intact, but molecular motion slow.
- Reaction rate low but increases with temperature.
- Moderate temperatures
- Enzymes still stable.
- Higher kinetic energy and more collisions.
- Reaction rate rises roughly exponentially with temperature (often well described by a roughly constant $Q_{10}$).
- Temperature optimum
- Best compromise between high kinetic energy and preserved enzyme structure.
- Maximal reaction rate.
- Above the optimum
- Enzyme’s structure becomes increasingly unstable.
- Denaturation accelerates, active sites are destroyed.
- Reaction rate drops steeply, often to nearly zero.
Graphically, this is an asymmetric bell-shaped curve: a relatively smooth rise followed by a sharper fall.
Adaptations of enzymes to environmental temperature
Enzymes can be adapted to the typical temperature of an organism’s environment. This adaptation mainly involves changes in protein stability and flexibility:
- Cold-adapted (psychrophilic) enzymes
- Function efficiently at low temperatures.
- Tend to be more flexible, allowing them to change shape and bind substrate even when thermal motion is low.
- As a result, they are often less stable at high temperatures and denature more easily.
- Heat-adapted (thermophilic) enzymes
- Remain stable and active at very high temperatures.
- Often have stronger internal interactions (more ionic bonds, hydrogen bonds, hydrophobic packing), making them structurally rigid.
- They are less active at low temperatures, because their rigidity requires more thermal energy for conformational changes during catalysis.
Such adaptations shift both the temperature optimum and the shape of the temperature–activity curve.
Biological and practical implications
Because enzyme activity is temperature-dependent, many physiological and practical processes rely on careful temperature control:
- Body temperature regulation in warm-blooded animals keeps enzymes near their optimum.
- Fever slightly raises body temperature, which can temporarily speed some immune reactions, but very high fever risks enzyme denaturation.
- Cold-blooded animals experience metabolic rates that change strongly with environmental temperature, reflecting the temperature dependence of their enzymes.
- Laboratory and industrial enzyme uses (e.g., in biotechnology, food processing, PCR) require optimal temperature settings to achieve high reaction rates without denaturation.
- Food preservation uses cooling or freezing to slow enzymatic and microbial activity, extending shelf life.
Understanding how temperature influences enzyme activity thus connects molecular behavior (enzyme structure and kinetics) with whole-organism physiology and many applications in everyday life and technology.