Protein structure and function (SACE Stage 2 Biology)

What this dot point is asking

You need to describe the four levels of protein structure, explain how amino acid sequence determines the folded shape, and explain how shape determines function.

What proteins are made of

Proteins are polymers of amino acids. There are 20 standard amino acids; each has the same backbone but a different R-group (side chain) that gives it distinct chemical properties (some are polar, some non-polar, some charged). Amino acids join through peptide bonds in a condensation reaction, forming a polypeptide chain.

The four levels of protein structure

Primary structure

The primary structure is the specific sequence of amino acids in the polypeptide, determined directly by the gene. This sequence determines everything that follows.

Secondary structure

The secondary structure is local folding of the backbone, stabilised by hydrogen bonds, into regular patterns - the alpha helix and the beta-pleated sheet.

Tertiary structure

The tertiary structure is the overall 3D folding of the whole polypeptide. It is stabilised by interactions between R-groups, including hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges. This is the level that gives most single-chain proteins their functional shape.

Quaternary structure

The quaternary structure exists only in proteins made of more than one polypeptide subunit. It describes how those subunits fit together. Haemoglobin, with four polypeptide chains, is the classic example.

Shape determines function

A protein's biological function depends on its precise 3D shape. The shape creates specific binding sites - for example, the active site of an enzyme or the oxygen-binding sites of haemoglobin. If the shape is correct, the protein binds its target and works; if the shape is wrong, it cannot.

Proteins do an enormous range of jobs: enzymes (catalysts), structural proteins (collagen, keratin), transport proteins (haemoglobin, membrane channels), hormones (insulin), antibodies, and contractile proteins (actin and myosin).

The bonds that hold each level together

Examiners often ask which bonds maintain each level, so keep them straight:

• Primary structure is held by strong covalent peptide bonds between amino acids. These are not broken by heat or pH alone.
• Secondary structure (alpha helix, beta sheet) is held by hydrogen bonds between backbone atoms.
• Tertiary structure is held by interactions between R-groups: hydrogen bonds, ionic bonds, hydrophobic interactions and covalent disulfide bridges.
• Quaternary structure is held by the same R-group interactions, but between separate polypeptide subunits.

This hierarchy explains denaturation: heat and extreme pH break the weaker bonds (hydrogen, ionic) that hold the higher levels, unfolding the protein, while the peptide bonds of the primary structure survive.

Fibrous versus globular proteins

A useful classification: fibrous proteins (collagen, keratin) have long, repetitive, often insoluble structures suited to structural and support roles. Globular proteins (enzymes, haemoglobin, antibodies) fold into compact, roughly spherical, usually soluble shapes with precise binding sites suited to metabolic and transport roles. The contrast reinforces the central theme that shape is matched to function, and gives a ready-made example when a question asks you to relate structure to role.

Denaturation

The bonds holding tertiary structure together are relatively weak and depend on conditions. Denaturation is the loss of a protein's 3D shape (and therefore its function) caused by high temperature or extreme pH, which break the hydrogen and ionic bonds maintaining the fold. The primary structure (peptide bonds) usually stays intact, but the protein no longer works. Denaturation is often irreversible.

Linking to the rest of the topic

This dot point connects the DNA code to the working cell: the gene sets the amino acid sequence, the sequence sets the shape, and the shape sets the function. It directly underpins the study of enzymes, where active-site shape and denaturation are central.