IB Biology HLProteins & Their StructurePaper 1 & 2~11 min read
Protein Structure: pH & Temperature
A protein only works while it is holding the right shape, and that shape is held by bonds so weak that a hot pan or a splash of acid can break them. Once they go, the protein is usually finished for good.
📚 What you need to know
The precise shape of a protein depends on ionic interactions, hydrogen bonds and other forces between and within polypeptide chains staying intact.
These bonds form between R groups and are weak compared with the peptide bonds holding the chain together.
Extremes of temperature or pH interfere with those bonds and cause denaturation.
Denaturation is a change of conformation — the protein loses its shape, so it loses its function.
Denaturation is almost always irreversible: the protein cannot be re-formed by putting the conditions back.
Each protein has an optimum pH at which its 3D structure is not denatured, e.g. pepsin at pH 2.
Small denaturations and renaturations do happen in some proteins, such as haemoglobin responding to small pH changes.
Extremophiles have proteins that stay stable in conditions that would denature most others, e.g. Thermus aquaticus at 80°C.
What is actually holding the shape
Think of a protein as a long chain that has been folded up and then pinned in place. There are two very different kinds of bond involved, and telling them apart is the whole topic.
Bond
Where it is
How strong
Peptide bonds
Along the backbone, joining amino acid to amino acid
Strong covalent bonds — heat and pH do not break them
Hydrogen bonds
Between R groups, and along the backbone in helices and sheets
Weak, easily broken
Ionic bonds
Between positively and negatively charged R groups
Weak, and depend on the charges staying as they are
Hydrophobic interactions
Between non-polar R groups tucked away from water
Weak, disrupted by movement of the chain
The peptide bonds hold the amino acids in sequence. The weak bonds hold that sequence in a folded 3D shape. Denaturation only attacks the second group.
This is the single most common confusion in the whole topic. Picture the peptide bonds keeping the amino acids in a straight line, and the weak bonds folding that line into a working shape. Denaturation unfolds the shape. It does not cut the chain.
Denaturation
Definition
Denaturation is a change in the conformation (3D shape) of a protein, caused by extremes of temperature or pH breaking the weak bonds between R groups.
Same chain, same amino acids, same order. The only thing that has changed is the folding — and that is enough to stop it working.
How heat does it
Heat gives the molecule more kinetic energy, so the chain vibrates more.
The vibration breaks the weak hydrogen and ionic bonds between R groups.
The chain unfolds, so the shape changes — and in an enzyme that means the active site no longer fits its substrate.
How pH does it
Acidic and basic R groups carry charges. A change in pH adds or removes hydrogen ions, which changes those charges.
Once the charges change, the ionic bonds between them are lost and hydrogen bonding is disrupted.
The protein folds differently, so again the shape and the function are lost.
Optimum pH
Each protein has a pH at which its 3D structure is not denatured and it works best. That is its optimum pH. Move away in either direction and activity falls off, because more and more of the weak bonds are disturbed.
Pepsin is built for the stomach, trypsin for the small intestine. Neither would survive long in the other’s home.
Denaturation you can see
Egg white
The clearest everyday example is frying or poaching an egg. Egg white is mostly a protein called albumin.
In its normal state the hydrophobic amino acids sit at the centre of each molecule, away from water, so albumin is soluble and the white is clear and runny.
Heating unfolds the molecules, so those hydrophobic amino acids end up on the outside.
Now they avoid the water and stick to each other instead, so the protein becomes insoluble and a harder, solid, white layer forms.
The same thing happens to the proteins in the yolk as it cooks.
No amount of cooling will turn it back — that is what irreversible means.
Your own stomach
The stomach is about pH 2, which denatures the protein in your food before it is fully hydrolysed further down the gut.
That is useful: an unfolded protein is far easier for enzymes to get at.
The stomach’s own enzyme, pepsin, has an optimum pH of 2 for exactly this reason. It is built to work in acid that would wreck most other proteins.
Extremophiles
Some organisms have evolved proteins that stay stable at extreme pH or temperature.
Thermus aquaticus is a bacterium that lives in hot springs at 80°C, a temperature that would denature most other proteins.
Why this matters for medicine. Many drugs are proteins, and a protein swallowed as a tablet would be denatured by stomach acid before it ever reached the blood. That is why insulin is injected rather than taken by mouth.
Denaturation is also a handy lab tool. If you want to find the optimum pH or temperature of an enzyme like pepsin or lipase, you deliberately push it past its limits and measure where the activity falls away. The falling half of the curve is denaturation being measured.
Worked examples
WE 1
Explaining denaturation by heat
Explain why an enzyme stops working when it is heated well above its optimum temperature. (4 marks)
Point 1: energy
Heating gives the molecule more kinetic energy, so the polypeptide chain vibrates more.
Point 2: bonds break
The vibration breaks the weak hydrogen and ionic bonds between R groups that hold the folded shape.
Point 3: shape changes
The chain unfolds, so the tertiary structure and the shape of the active site change — the enzyme is denatured.
Point 4: consequence
The substrate no longer fits the active site, so no enzyme-substrate complexes form and activity falls to zero.
More vibration → weak bonds break → active site changes shapesay which bonds break; “the bonds break” on its own rarely gets the mark
WE 2
Why an egg white turns solid
Explain why egg white changes from a clear liquid to a white solid when it is heated. (3 marks)
Point 1: the protein
Egg white is mainly the protein albumin, which is normally soluble because its hydrophobic amino acids are buried in the centre.
Point 2: heating
Heat breaks the weak bonds holding the fold, so the molecules unfold and the hydrophobic amino acids end up on the outside.
Point 3: the result
The unfolded molecules become insoluble and join to one another, forming the solid white layer.
Denaturation makes albumin insoluble, so it sets solidthe word “insoluble” is the key idea here — do not stop at “it denatures”
WE 3
Why insulin is injected
Insulin is a protein hormone. Suggest why it must be injected rather than swallowed as a tablet. (3 marks)
Point 1: the conditions
The stomach has a very low pH of about 2.
Point 2: the effect
That extreme pH changes the charges on R groups and breaks the ionic and hydrogen bonds holding the shape, so the insulin is denatured.
Point 3: the consequence
A denatured insulin molecule no longer fits its receptors, and it would also be hydrolysed to amino acids by protease enzymes, so it could not work. Injecting it puts it straight into the blood and avoids the stomach.
Stomach acid would denature it before it reached the bloodsaying “it gets digested” alone is a weak answer — name the denaturation and the low pH
💡 Exam tips
Define denaturation as a change in conformation, not as “the protein is destroyed” or “the bonds in the protein break”.
Always name the bonds that break: hydrogen and ionic bonds between R groups.
Always say the peptide bonds are not broken. That single sentence often separates a 3 from a 2.
Use irreversible, and mention that small renaturations are possible in some proteins such as haemoglobin.
Learn pepsin, optimum pH 2 and Thermus aquaticus, 80°C as ready-made examples.
For graph questions, describe the shape: activity rises to a peak at the optimum, then falls as denaturation begins.
⚠ Common mistakes
Saying the protein is “killed”. Proteins are molecules, not organisms. They are denatured.
Saying peptide bonds break. They do not — that would be hydrolysis, which is a different reaction.
Saying denaturation changes the primary structure. The sequence is unchanged; only the folding changes.
Claiming cooling reverses it. Denaturation is almost always irreversible.
Confusing the hydrogen bonds. In secondary structure they are along the backbone; the ones broken here also include those between R groups.
Assuming every enzyme likes pH 7. Pepsin’s optimum is pH 2, and it would be denatured at pH 7.
Up next: Amino Acid Diversity — we keep saying “it depends on the R groups”, so it is time to look at the R groups properly.
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