CDS Biology · Biochemistry
Proteins, Enzymes and Lactic Acid
What a protein is made of, why heat changes it for good, what isoenzymes are, and why a sprinter's muscles fill with lactic acid.
Why this matters
Five CDS questions from 2017 (II) to 2025 (I). Two are the same sprinter's cramp, asked in 2018 (I) and 2023 (I). The others ask why egg white turns solid on heating, what isoenzymes are, and which protein gives silk its shine.
Concept 1 of 2: Proteins, denaturation and isoenzymes
A protein is a long chain of amino acids folded into an exact shape, and its shape is what lets it do its job. Heat or acid can break the weak bonds that hold the fold. The chain then unfolds and tangles with its neighbours, and it cannot fold back. That is why a boiled egg never turns raw again. Enzymes are proteins too, and a cell can make several slightly different versions of one enzyme that all do the same job.
Definition
Proteins:
- Built from amino acids joined by peptide bonds. Every protein contains nitrogen.
- Denaturation: heat, strong acid or alcohol unfolds the protein so it loses its shape and its function. Boiling egg white (albumin) makes it solid and white; lemon juice curdles milk protein.
- Enzymes are protein catalysts: they speed up a reaction and are left unchanged at the end.
- Isoenzymes (isozymes) are different forms of an enzyme that catalyse the same reaction but differ in their amino acid sequence. Lactate dehydrogenase is the usual example.
- A coenzyme is something else: a small non-protein helper (often made from a vitamin) that an enzyme needs to work.
| Protein or term | What it is or does |
|---|---|
| Albumin of egg white | Turns solid and white on heating: thermal denaturationQ CDS 2024 (I): heating changes the egg white because its proteins are denatured, not its fats or sugars. |
| Collagen | Fibrous protein of skin, tendon, bone and cartilage |
| Keratin | Protein of hair, nails and the outer skin |
| Haemoglobin | Carries oxygen in red blood cells |
| Isoenzymes | Forms of one enzyme: same reaction, different amino acids |
| Coenzyme | Non-protein helper an enzyme needs |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (I) 2025 — General Knowledge · Q78Moderate
Example 1 · Biochemistry · Proteins, Enzymes and Lactic Acid
The enzymes which catalyze the same reaction but have different amino acid compositions are commonly referred as :
Isoenzymes, not isomers or isotopes
Isoenzymes are forms of one enzyme with different amino acid sequences. Isomers are compounds with the same formula, isotopes are atoms of one element with different masses, and coenzymes are helpers. Only the first is about enzymes doing the same job.
Cooked egg white is a protein change
Egg white is mostly water and protein. Heat denatures the protein; there is almost no fat or sugar in the white to change.
Concept 2 of 2: Respiration without oxygen: lactic acid and alcohol
Cells get energy by breaking down glucose. With enough oxygen they break it all the way to carbon dioxide and water and get a lot of energy. When oxygen runs short, as in the leg muscles of a sprinter, they stop half way and the leftover becomes lactic acid. Lactic acid building up in the muscle is what causes the cramp. Yeast also works without oxygen, but it makes alcohol and carbon dioxide instead.
Definition
Breaking down glucose:
- Glycolysis, in the cytoplasm, splits glucose into pyruvate. This step needs no oxygen.
- With oxygen (aerobic): pyruvate goes into the mitochondria and is broken down to carbon dioxide and water, releasing much more energy.
- Without oxygen in muscle: pyruvate becomes lactic acid. Its build-up causes cramp and pain after hard exercise.
- Without oxygen in yeast (fermentation): pyruvate becomes ethanol and carbon dioxide. This is how bread rises and how alcohol is made.
| Where glucose is broken down | Oxygen | End products |
|---|---|---|
| Mitochondria of most cells | Enough | Carbon dioxide, water and much energy |
| Muscle of a sprinter | Short supply | Lactic acid and a little energyQ Asked twice, CDS 2018 (I) and 2023 (I): the cramp is from lactic acid, never pyruvic acid or ethanol. |
| Yeast | None | Ethanol, carbon dioxide and a little energy |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q1Easy
Example 2 · Biochemistry · Proteins, Enzymes and Lactic Acid
A sprinter feels cramps and pain in the thigh muscles after a run. This is due to accumulation of
Lactic acid, not pyruvic acid
Pyruvic acid is the step before: in a muscle short of oxygen it is turned into lactic acid, and it is lactic acid that piles up and causes the cramp.
Muscles do not make alcohol
Ethanol comes from fermentation in yeast. Human muscle working without oxygen makes lactic acid only.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Proteins, denaturation and isoenzymes6 rows
| Protein or term | What it is or does |
|---|---|
| Albumin of egg white | Turns solid and white on heating: thermal denaturationQ CDS 2024 (I): heating changes the egg white because its proteins are denatured, not its fats or sugars. |
| Collagen | Fibrous protein of skin, tendon, bone and cartilage |
| Keratin | Protein of hair, nails and the outer skin |
| Haemoglobin | Carries oxygen in red blood cells |
| Isoenzymes | Forms of one enzyme: same reaction, different amino acids |
| Coenzyme | Non-protein helper an enzyme needs |
Respiration without oxygen: lactic acid and alcohol3 rows
| Where glucose is broken down | Oxygen | End products |
|---|---|---|
| Mitochondria of most cells | Enough | Carbon dioxide, water and much energy |
| Muscle of a sprinter | Short supply | Lactic acid and a little energyQ Asked twice, CDS 2018 (I) and 2023 (I): the cramp is from lactic acid, never pyruvic acid or ethanol. |
| Yeast | None | Ethanol, carbon dioxide and a little energy |
Watch out for (4)
- Isoenzymes, not isomers or isotopes→ Proteins, denaturation and isoenzymes
- Cooked egg white is a protein change→ Proteins, denaturation and isoenzymes
- Lactic acid, not pyruvic acid→ Respiration without oxygen: lactic acid and alcohol
- Muscles do not make alcohol→ Respiration without oxygen: lactic acid and alcohol
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