Polypeptides
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of proteins and monomers and polymers. You can test your knowledge on these below.
What type of reaction forms a dipeptide from two amino acids?
A condensation reaction.
What types of bonds stabilise the tertiary structure of a protein?
Hydrogen bonds, ionic bonds, and disulphide bonds.
What else is produced when a dipeptide forms from 2 amino acids?
Water
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains polypeptides or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Protein Composition and Formation
- Proteins are polymers made up of many amino acid monomers linked by peptide bonds.
- Amino acids contain a central carbon with: an amine group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a variable R group.
- Two amino acids form a dipeptide through a condensation reaction, releasing a molecule of water.
- The bond formed between the amino acids is called a peptide bond.
Levels of Protein Structure
Primary Structure:
- The specific sequence of amino acids in a polypeptide chain.
- Joined by peptide bonds formed in condensation reactions.
- Peptide bonds form between the carboxyl group of one amino acid and the amine group of the next
- Determines the protein’s ultimate shape and function as the primary structure will determine the position of the bonds that hold the tertiary structure together.
Secondary Structure:
- Folding or coiling of the polypeptide chain due to hydrogen bonding.
- Hydrogen bonds form between the C=O and N-H groups within the polypeptide backbone.
- Common structures include the alpha helix (α-helix) and beta pleated sheet (β-sheet).
Tertiary Structure:
- Further folding and coiling due to interactions between R groups, including:
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Hydrogen bonds
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Ionic bonds
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Disulphide bonds (strong, covalent bonds between cysteine residues) that typically do not break when a protein is denatured.
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The amino acid cysteine contains sulphur in its R group.
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- This structure determines the protein’s specific 3D shape and specific functional properties.
Quaternary Structure:
- Association of two or more polypeptide chains
- It may also include non-protein groups.
- Examples include haemoglobin, which consists of four polypeptide chains and the non-protein haem groups.
Importance of Protein Structure
- The structure of a protein is critical to its function as proteins rely on their specific 3D shape as determined by their primary structure.
- Denaturation occurs when increase in temperature or change in pH disrupt the bonds holding the protein’s tertiary structure together.
Key Biological Roles of Proteins:
- Enzymes (e.g., catalase)
- Structural components (e.g., collagen)
- Transport molecules (e.g., haemoglobin)
- Signalling molecules (e.g., insulin)
Key Terms
- Amino Acid: Building block of proteins containing an amino group, carboxyl group, hydrogen atom, and R group.
- Peptide Bond: Covalent bond between two amino acids formed via a condensation reaction.
- Dipeptide: Molecule formed when two amino acids join via a peptide bond.
- Polypeptide: A chain of amino acids linked by peptide bonds.
- Primary Structure: Sequence of amino acids in a polypeptide chain.
- Secondary Structure: Regular folding or coiling patterns due to hydrogen bonds (e.g., α-helix, β-sheet).
- Tertiary Structure: 3D folding due to interactions between R groups.
- Quaternary Structure: Complex structure formed by two or more polypeptides.
- Denaturation: Loss of a protein’s structure and function due to increase in temperature or a change in pH. The proteins tertiary structure changes as hydrogen and ionic bonds break
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Exam Tips
Always link protein structure to its function when answering exam questions. For instance, describe how the specific tertiary structure of an enzyme allows it to bind to a substrate due to its complementary active site.
Pay attention to command words like "describe" or "explain" and tailor your answer to address them directly.
No answer provided.
Describe the levels of protein structure, explaining how bonds contribute to the final 3D shape of a protein. (6 marks)
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The primary structure of a protein is the specific sequence of amino acids in a polypeptide chain
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held together by peptide bonds.
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The secondary structure forms due to hydrogen bonding between amino acids
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leading to α-helices or β-pleated sheets.
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The tertiary structure arises from further folding and coiling of the polypeptide, stabilised by hydrogen bonds, ionic bonds, and disulfide bonds.
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between R groups.
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The quaternary structure results from the association of multiple polypeptide chains.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!