2.3 Proteins

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe and draw the general structure of an amino acid, and explain how a peptide bond forms and breaks.
  2. Explain what is meant by primary, secondary, tertiary, and quaternary structure of proteins.
  3. Describe the types of interaction that hold protein molecules in shape: hydrophobic interactions, hydrogen bonding, ionic bonding, and covalent bonding (including disulfide bonds).
  4. State that globular proteins are generally soluble and have physiological roles, while fibrous proteins are generally insoluble and have structural roles.
  5. Describe the structure of haemoglobin as an example of a globular protein, including how its quaternary structure forms from two α-globin chains, two β-globin chains, and a haem group.
  6. Relate the structure of haemoglobin to its function, including the importance of iron in the haem group.
  7. Describe the structure of a collagen molecule and the arrangement of collagen molecules to form collagen fibres.
  8. Relate the structures of collagen molecules and collagen fibres to their function.

1. Amino Acids — The Building Blocks of Proteins

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What Are Amino Acids?

Proteins are polymers — large molecules made by joining many smaller units together. The small units that make up proteins are called amino acids. Amino acids are therefore the monomers (individual building blocks) of proteins.

There are 20 different amino acids commonly found in living organisms. All 20 share the same basic structure, but each one is slightly different because of a part called the R group (also called the variable side chain).

General Structure of an Amino Acid

Every amino acid has a central carbon atom bonded to four things:

  • An amine group (–NH₂) — this is the basic end of the molecule
  • A carboxyl group (–COOH) — this is the acidic end of the molecule
  • A hydrogen atom (–H)
  • An R group — this is the variable part; it is different in each of the 20 amino acids

The amine group, carboxyl group, and hydrogen are the same in every amino acid. Only the R group changes. This is what makes each amino acid unique and gives it its own chemical properties.

Drawing an amino acid:

        R
        |
H₂N — C — COOH
        |
        H

Key tip: In an exam, if you need to draw a general amino acid, make sure you include all four parts attached to the central carbon: the amine group (–NH₂), the carboxyl group (–COOH), a hydrogen (–H), and the R group.

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