Edexcel IGCSE Biology · Spec 2.10-2.13
Enzymes
How enzymes work using the lock-and-key model, enzyme specificity, and the effect of temperature and pH on activity.
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Enzymes
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Explained
Enzymes, and the two things that stop them working
An enzyme is a protein that acts as a biological catalyst. It speeds up a reaction in a living organism and is not used up, so one enzyme molecule works over and over again.
Being a protein is the reason for everything else on this page. A protein is a folded chain, and the shape it folds into is what does the work. Anything that spoils the shape spoils the enzyme.
The lock and key model
Part of the folded enzyme forms a pocket called the active site. Only one kind of molecule, the substrate, fits into it, in the way that only one key fits a lock.
The substrate binds to the active site, forming an enzyme substrate complex. The reaction takes place, the products leave, and the active site is free to take another substrate molecule.
This is why enzymes are specific. Amylase breaks down starch and does nothing to protein, because protein does not fit its active site. A question asking why an enzyme is specific wants the active site and the complementary shape, not simply the word specific repeated back.
What an enzyme actually does is lower the activation energy, the amount of energy the reacting molecules need before they can react. It does not add energy and it does not force anything to happen; it makes the same reaction possible at the temperature the body happens to be.
Temperature
As temperature rises, the enzyme and substrate molecules gain kinetic energy and move faster, so they collide more often and the rate increases. That continues up to the optimum, which is around 37 degrees Celsius for most human enzymes.
Above the optimum the rate falls sharply. The heat breaks the bonds holding the folded shape, the active site changes shape, and the substrate no longer fits. The enzyme is denatured.
Two things about denaturing are worth guarding. It is permanent, so cooling the enzyme back down does not restore it. And the enzyme is not killed, because it was never alive. It is a molecule, and the right word is denatured.
pH
The same curve shape appears, for the same reason. Each enzyme has an optimum pH, and moving away from it in either direction slows the reaction, because the bonds holding the folded shape are disrupted and the active site changes.
Most human enzymes work best at about pH 7. The important exception is pepsin, the protease in the stomach, which has an optimum of about pH 2 and needs the hydrochloric acid there to work at all. Enzymes in the small intestine work at about pH 8, which is why bile is alkaline and neutralises the acid arriving from the stomach.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks candidates to describe results showing how pH affects an enzyme, for three marks, and lays out five points: that the optimum pH is 7; that the time taken increases away from the optimum, or decreases as pH rises to 7 and then increases; that the enzymes denature; that the shape of the enzyme changes; and that the substrate therefore does not fit the active site, or that enzyme substrate complexes will not form.
Its accept list is generous about wording and precise about meaning. Around 7, between 5 and 9, and is neutral are all allowed for the optimum. Active site shape changes is allowed. So is enzyme and substrate are no longer complementary, and so is substrate and active site do not bind.
What every one of those accepted phrases has in common is that it mentions shape or fit. The mark is not for the word denature on its own; it is for saying what denaturing does. Denatured is one marking point, and the shape change and the failure to fit are two more.
The same paper asks how a method could be improved and credits the idea that judging the end point by eye is subjective, not standardised, or a matter of individual opinion, then credits using a colour chart, a standard colour or a colorimeter to fix it. It also credits testing smaller pH intervals, because the optimum may lie between two values that were tested.
That last point is a useful habit. If your results show pH 7 is fastest and you only tested 5, 7 and 9, you have not shown that 7 is the optimum. You have shown it is the best of the three you tried.
Reading the graphs
The temperature graph and the pH graph both rise to a peak and then fall, which makes them easy to confuse. The difference is in the shape and in the explanation of the rise.
On the temperature graph the rise is gradual, caused by increasing kinetic energy and more frequent collisions, and the fall after the optimum is steep. On the pH graph the curve is usually symmetrical, and the rise is not caused by kinetic energy at all. It is the active site becoming closer to its correct shape as the pH approaches the optimum.
Explaining the pH rise in terms of particles moving faster is a common error and earns nothing, because pH does not change how fast molecules move.
Spec 2.10-2.13
What you need to know
- Explain how enzymes work as biological catalysts
- Describe the effect of temperature on enzymes
- Describe the effect of pH on enzymes
Active recall
Quick check
Answer each question before opening the answer.
What is an enzyme and how does the 'lock and key' model work?
A protein that acts as a biological catalyst. The substrate fits the specific active site (like a key in a lock), forming an enzyme-substrate complex.
What happens to an enzyme at high temperature?
It denatures — the active site changes shape so the substrate no longer fits and the reaction stops.
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