What you need to know
- 2.6(i–ii)amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. structure and formation of peptide bonds in polypeptides.
- 2.6(iii–iv)Levels of protein structure, stabilising bonds, haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂. and collagenA fibrous protein of three polypeptide chains wound into a triple helix. It gives strength to tendons, skin and artery walls..
The textbook teaches proteins in section 1A, but the relevant specification statements are in Topic 2. Structures of individual named amino acids are not required.
By the end, you should be able to…
- Describe the general structure of an amino acid and formation of a peptide bondThe bond formed by condensation between the carboxyl group of one amino acid and the amine group of the next..
- Distinguish primary, secondary, tertiary and quaternary structureThe arrangement of two or more polypeptide chains in one protein, such as the four chains of haemoglobin..
- Explain how haemoglobin and collagen structures support their functions.
Build your understanding
Amino acids and peptide bonds
The general amino acid structure has a central carbon attached to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable R groupThe variable side chain of an amino acid. There are 20 different R groups in proteins.. The R group distinguishes different amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group.. This conventional general formula is sufficient here; you do not need to memorise the structures of individual amino acids.
Amino acids and peptide bonds (continued)
A condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. reaction between the carboxyl group of one amino acid and the amino group of another forms a peptide bondThe bond formed by condensation between the carboxyl group of one amino acid and the amine group of the next. and releases water. Many amino acids linked in this way form a polypeptideA chain of many amino acids joined by peptide bonds.. A protein contains one or more polypeptide chains folded into a functional structure.
Sequence determines folding
primary structureThe sequence of amino acids in a polypeptide chain. is the sequence of amino acids in a polypeptide. It determines which R groups occur and where they lie, influencing the interactions possible during folding.
secondary structureThe coiling (α-helix) or folding (β-pleated sheet) of a polypeptide, held by hydrogen bonds. includes α-helices and β-pleated sheets, stabilised by hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. between groups in the polypeptide backbone. tertiary structureThe overall three-dimensional folding of a polypeptide, held by hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions. is the overall three-dimensional folding of one polypeptide. Hydrogen bonds, ionic interactions and covalent disulfide bridges, together with interactions involving hydrophobic R groups, help stabilise it.
quaternary structureThe arrangement of two or more polypeptide chains in one protein, such as the four chains of haemoglobin. describes the association of two or more polypeptide subunits. Not all proteins have it. The final arrangement of groups gives a protein its properties, so changing a sequence can change folding and function. Not every sequence change has the same effect.
Haemoglobin: a globular transport protein
Adult haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂. has four polypeptide subunits, two α chains and two β chains, each associated with a haem group containing an iron ion. Each haem group can bind one oxygen molecule reversibly, so one haemoglobin molecule can carry up to four oxygen molecules.
Its compact globular structure and outward-facing hydrophilic groups suit an aqueous environment within red blood cells. Reversible binding allows oxygen to be loaded and released. Link the haem groups to oxygen binding and the four subunits to quaternary structure.
Collagen: a fibrous structural protein
A collagenA fibrous protein of three polypeptide chains wound into a triple helix. It gives strength to tendons, skin and artery walls. molecule contains three polypeptide chains wound into a triple helix. Glycine occurs at every third position; its small R group allows the chains to fit closely together. Hydrogen bonds help stabilise the association of the chains. Many collagen molecules are arranged into fibrils and fibres, with covalent cross-links between molecules adding strength.
The long, strong, insoluble fibres resist stretching and provide tensile strengthThe pulling force a material can withstand before it breaks, usually given per unit cross-sectional area. in tissues such as tendonTough, inelastic connective tissue that attaches a muscle to a bone.. Collagen therefore illustrates a different structure–function relationship from a compact, soluble globular proteinA compact, roughly spherical, soluble protein, such as an enzyme or haemoglobin.. Its triple helix should not be confused with a single α-helixA spiral secondary structure of a polypeptide, held by hydrogen bonds between nearby amino acids..
NOT: four levels do not mean four chains. Collagen’s triple helix is not one α-helix.
- Haemoglobin: two α and two β chains, each with a haem group
- Each haem binds one O₂, so one molecule binds up to four
- Collagen: three chains, glycine every third residue
- Ordinary denaturation need not break peptide bonds
Two traps this diagram blocks
- Do not say every protein has four polypeptide chains because there are four levels of structure.
- Do not say heating always breaks the primary sequence. Denaturation changes the shape; peptide bonds can remain.
From the diagram alone
Question
Explain how a change in primary structure could stop haemoglobin binding oxygen. [3]
Show a model answer
A different amino-acid sequence can change an R group and the interactions that fold the polypeptide. The shape of a subunit or the position of a haem group may change. Oxygen then may not bind reversibly at that haem.
| Level | What it describes |
|---|---|
| Primary | Amino acid sequence |
| Secondary | Local backbone folding: α-helix or β-pleated sheet |
| Tertiary | Overall three-dimensional shape of one polypeptide |
| Quaternary | Association of multiple polypeptide subunits |
Make your reasoning visible
Track the level of structure
Peptide bonds link amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. in the primary sequence. hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. stabilise secondary structureThe coiling (α-helix) or folding (β-pleated sheet) of a polypeptide, held by hydrogen bonds.. Do not claim that ordinary denaturation necessarily breaks peptide bonds or changes the primary sequence.
Match the feature to the job
For haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂., connect haem groups and reversible binding to oxygen transport. For collagenA fibrous protein of three polypeptide chains wound into a triple helix. It gives strength to tendons, skin and artery walls., connect triple helices, fibrils and cross-links to tensile strengthThe pulling force a material can withstand before it breaks, usually given per unit cross-sectional area..
Avoid this misconception
The claim: All proteins have four polypeptideA chain of many amino acids joined by peptide bonds. chains because there are four levels of structure.
The correction: The levels describe organisation. Only proteins with multiple polypeptide subunits have quaternary structureThe arrangement of two or more polypeptide chains in one protein, such as the four chains of haemoglobin., and their number of subunits varies.
Build up the levels of protein structure
Each level of protein structure is built on the one before. Order them from single amino acids to a protein made of several chains.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the levels in order, from amino acids to a working protein such as haemoglobin.
0 of 5 in the right place
Not every protein has a quaternary structure. Hydrophobic interactions also help hold the tertiary structure.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain how a change in primary structure can alter protein function.
Show suggested answer
A changed amino acid sequence may alter the position or identity of R groups. This may change interactions during folding, changing the three-dimensional shape and properties needed for the protein’s function.
Practice question
Retrieval and explanation · suggested answer
Compare one structural adaptation of haemoglobin with one of collagen, linking each to its function.
Show suggested answer
Haemoglobin has iron-containing haem groups that bind oxygen reversibly, allowing transport and release. Collagen forms long triple-helical molecules assembled into cross-linked fibrils, giving tensile strength to resist stretching.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two amino acid substitutions occur in different copies of a protein. One replaces an R group with a chemically similar group; the other introduces a charged group into a hydrophobic region. Must they have equal effects?
Show a hint
Consider both the chemical change and its position.
Explore the reasoning
No. A chemically similar substitution may preserve important interactions. A charged group in a hydrophobic region may disrupt folding more strongly. The effect depends on the location and role of the residue, so experimental evidence is needed to determine the actual effect.
Recall this lesson
Lesson recall questions
Question 1 of 5: List four types of bond or interaction that can hold a protein's three-dimensional shape.
Amino acids to protein structure
Practice question
Retrieval and explanation · suggested answer
List four types of bond or interaction that can hold a protein's three-dimensional shape.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A small sequence change, a large consequence
1A · ProteinsSickle cell disease illustrates how a change in haemoglobin can affect red blood cells. It connects molecular structure with cell behaviour and shows why researchers study the consequences…Specification 2.6(iii) · 2.6(iv)Read & use the scienceClose article
Why this matters
Sickle cell disease illustrates how a change in haemoglobin can affect red blood cells. It connects molecular structure with cell behaviour and shows why researchers study the consequences of changes in protein sequences. This example connects protein chemistry with a question researchers can investigate: how can one changed residue influence interactions between protein molecules? The surrounding amino-acid sequence, folding and oxygen conditions matter, so the change should not be described as every protein simply unfolding. At this stage, use it to connect primary structure with properties and function. Inheritance patterns and the clinical management of sickle cell disease are further topics, rather than extra facts required for the chemistry lesson.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.6(iii) understand the significance of a protein’s primary structure in determining its secondary structure, three-dimensional structure and properties (globular and fibrous proteins and the types of bonds involved in its three-dimensional structure)
- 2.6(iv) know the molecular structure of a globular protein and a fibrous protein and understand how their structures relate to their functions (including haemoglobin and collagen)
Use the science
Why would seeing a changed amino acid sequence alone be insufficient to predict the full effect on an organism?
Explore a possible response
The effect depends on the residue’s role, folding, interactions with other molecules and the cellular context. Measurements of protein behaviour and cells are needed to connect a sequence change to an organism-level effect.
Explore the source
- YourGenome: sickle cell anaemiaYourGenome
Can AI predict a protein’s shape?
1A · ProteinsThe 2024 Chemistry Nobel recognised protein structure prediction and computational protein design. These advances turn the connection between an amino-acid sequence and a three-dimensional molecule into a research…Specification 2.6(iii) · 2.6(iv)Read & use the scienceClose article
Why this matters
The 2024 Chemistry Nobel recognised protein structure prediction and computational protein design. These advances turn the connection between an amino-acid sequence and a three-dimensional molecule into a research tool. A predicted structure can suggest where another molecule might bind, or help researchers decide which experiment to do next. It is still a prediction: proteins move, interact and work in particular chemical environments. A convincing picture alone cannot demonstrate that a protein catalyses a reaction, survives a temperature change or behaves normally inside a cell. Structural prediction helps ask sharper biological questions; experimental measurements establish what the molecule actually does.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.6(iii) understand the significance of a protein’s primary structure in determining its secondary structure, three-dimensional structure and properties (globular and fibrous proteins and the types of bonds involved in its three-dimensional structure)
- 2.6(iv) know the molecular structure of a globular protein and a fibrous protein and understand how their structures relate to their functions (including haemoglobin and collagen)
Use the science
Two variants are predicted to have similar overall shapes. Why might their biological activities still differ, and what experiment would you propose?
Explore a possible response
A small change at a binding site, charge or flexibility can alter interactions without changing the overall fold. Compare a relevant measured activity under controlled conditions.
Explore the source
- The Nobel Prize in Chemistry 2024: Press releaseRoyal Swedish Academy of Sciences / Nobel Prize