Temperature and pH
Introduced| English | Meaning |
|---|---|
| optimum/ˈɒptɪməm/ | The condition or value at which a process performs best under the stated criteria. |
| denaturing/ˈdenətʃərɪŋ/ | denaturing |
| denatured/ˈdenətʃəd/ | Describes an enzyme whose active site has lost its shape, so it no longer works. |
Why can warming speed a reaction and then stop it?
- An enzyme reaction may speed up as it warms, then slow sharply at higher temperatures. Two effects compete: faster molecular movement and a change in enzyme shape.
- Read the evidence for the particular enzyme. Body temperature is about 37 °C, but this is not a universal optimum for every enzyme.
Below the optimum, collisions become more frequent
- As temperature rises below the optimum 最适, enzyme and substrate molecules move faster. More frequent successful collisions produce more enzyme–substrate complexes per unit time.
- The optimum is the temperature giving the highest measured rate under the stated conditions. Above it, increasing loss of active-site shape can outweigh the benefit of faster motion.
Read the optimum from the peak for the enzyme shown, rather than assuming it is always 37 °C.
Temperature and pH
rate peaks at the optimum
There's an optimum temperature; too hot and the enzyme is denatured and stops working.
The temperature at which an enzyme works fastest is its:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The optimum is where the rate is highest.
An enzyme is tested at 20, 30, 40 and 50 °C. Its measured rates are 2, 5, 8 and 1 arbitrary units respectively. Which tested temperature gives the highest rate, in °C?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
The largest measured rate is 8 at 40 °C. This is the best tested temperature; the exact optimum could lie between tested temperatures.
Why does warming an enzyme reaction below its optimum usually increase rate?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Faster molecular movement increases successful collisions while the active site remains suitable.
pH affects the active site too
- Each enzyme has an optimum pH. Moving away from it can change the active site and reduce activity; sufficiently extreme pH can cause denaturation.
- Different enzymes have different optima. An acidic optimum does not mean every enzyme works fastest in acid. Compare rates using the same enzyme, substrate amounts and temperature.
The graph gives this enzyme's response. A different enzyme can have its peak elsewhere.
Moving the pH far from an enzyme's optimum will:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The wrong pH changes the active site shape.
Enzyme A is fastest at pH 2; enzyme B is fastest at pH 8. Which conclusion follows?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Use the peak for the particular enzyme; there is no universal optimum pH.
Distinguish cold from denaturing 变性
- An enzyme that is denatured 变性 has an altered active-site shape, so its substrate no longer fits effectively. In these school experiments, simply cooling it does not restore its activity.
- Ordinary cooling reduces molecular movement and successful collisions without denaturing the enzyme. Warming a cold sample back to suitable conditions can restore a faster rate.
Being above the optimum does not mean every enzyme molecule is instantly destroyed. The effect depends on the enzyme, the temperature and the exposure time. Use measured evidence; do not infer permanent damage just from a slow rate.
If an enzyme gets too hot, it:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Excessive heating can change the active-site shape, so the substrate no longer fits effectively.
A denatured enzyme can recover its shape once it cools down.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Denaturing is permanent; cooling does not restore the active site.
Test whether the effect is reversible
- Split the same enzyme solution into equal samples. Keep one cool, heat another strongly for a fixed time, and retain an untreated control. Then return all to the same test temperature.
- Add equal fresh substrate amounts at the same pH and compare initial rates. The common test conditions help separate a lasting change from the immediate effect of temperature.
These are illustrative outcomes. The comparison needs an untreated control, fresh substrate and the same final conditions.
After all three samples return to 30 °C, the cooled sample and untreated control react at similar rates, but the strongly heated sample has almost no activity. This supports reversible slowing in the cold and lasting denaturation after heating. The conclusion comes from the recovery test, not from a temperature alone.
Why should heated and cooled enzyme samples be tested at the same final temperature, with equal fresh substrate and the same pH?
Common temperature and pH remove immediate differences in reaction conditions; fresh equal substrate prevents depletion from explaining a low rate. Comparing with an untreated control then helps identify a lasting effect of earlier treatment.
Explain the recovery-test results
- A cooled sample recovers its activity at the common test temperature. A strongly heated sample does not, even with fresh substrate.
- Explain each result using molecular movement or active-site shape. Identify why equal pH, substrate amounts and test temperature make the comparison useful.
Cooled, strongly heated and untreated samples of the same enzyme are returned to 30 °C and given equal fresh substrate at the same pH. The cooled sample and control react similarly; the heated sample has almost no activity. Explain the evidence.
Ordinary cooling slowed molecular movement and successful collisions reversibly, so the cooled sample recovers. Persistent loss of activity in the heated sample supports denaturation: its active-site shape changed so the substrate no longer fits effectively. Equal test temperature, pH and fresh substrate make the rate comparison useful.
Link the rate to its cause
- Below the optimum, warming increases successful collisions; excessive heating can denature the enzyme.
- Optimum temperature and pH belong to the particular enzyme and experimental conditions.
- Ordinary cooling slows activity reversibly. Failure to recover under suitable conditions supports lasting denaturation.
An unfamiliar enzyme reacts slowly at 60 °C. What further evidence best supports lasting denaturation?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A recovery test with a control supports lasting loss of activity. A temperature alone does not establish the response of every enzyme.