amino acids as the monomers of a polypeptide chain and the resultant hierarchical levels of structure that give rise to a functional protein

What this dot point is asking

VCAA wants the monomer (amino acid), the bond that joins them (peptide bond), and the four hierarchical levels of protein structure, with a clear link between sequence and function.

The answer

A protein is a polymer of amino acids that folds into a specific three-dimensional shape and performs a specific role. There are roughly 20 standard amino acids that make up proteins in living organisms.

Amino acids: the monomer

Every amino acid has the same core:

• A central alpha carbon.
• An amino group (-NH2).
• A carboxyl group (-COOH).
• A hydrogen atom.
• A variable R group (side chain) that gives each amino acid its chemical character (polar, non-polar, acidic, basic).

Amino acids join by a condensation reaction that forms a peptide bond between the carboxyl group of one and the amino group of the next, releasing water. A chain of amino acids is a polypeptide.

The four levels of protein structure

Primary structure. The linear sequence of amino acids in the polypeptide, read from the N-terminus to the C-terminus. It is determined directly by the order of codons in mRNA. The primary sequence dictates all higher levels of folding.

Secondary structure. Local repeating folds stabilised by hydrogen bonds between backbone N-H and C=O groups (not between R groups). The two main motifs are:

• Alpha helix. A right-handed spiral with hydrogen bonds every four residues.
• Beta pleated sheet. Parallel or antiparallel strands held together side by side by hydrogen bonds.

Regions without regular structure are called random coils or loops.

Tertiary structure. The full three-dimensional fold of a single polypeptide, stabilised by interactions between R groups:

• Hydrogen bonds between polar R groups.
• Ionic bonds (salt bridges) between oppositely charged R groups.
• Hydrophobic interactions clustering non-polar R groups in the interior.
• Disulfide bridges (covalent) between two cysteine residues.

Tertiary structure produces the functional shape of single-chain proteins such as myoglobin and most enzymes.

Quaternary structure. Two or more polypeptide chains (subunits) assembled into a functional complex, held together by the same kinds of R-group interactions as tertiary structure. Examples include haemoglobin (four subunits: two alpha, two beta, each with a haem group) and insulin (two chains held by disulfide bridges).

Why structure determines function

The folded shape creates specific binding sites (active sites in enzymes, antigen-binding sites in antibodies, receptor pockets, ion channels). If the fold is disrupted, the binding site changes shape and the protein loses function. This is why denaturation (heat, extreme pH, heavy metals) inactivates proteins: weak bonds break, the protein unfolds, and function is lost. Primary structure (covalent peptide bonds) is usually retained during denaturation.

Examples in context

Example 1. Insulin structure at Walter and Eliza Hall Institute. WEHI's diabetes research group studies insulin, a 51-amino-acid hormone with classic four-level structure. Primary: the A and B chains have specific amino acid sequences. Secondary: each chain contains alpha-helices stabilised by hydrogen bonds. Tertiary: the two chains fold into compact 3D shapes held by three disulfide bridges between cysteine residues. Quaternary: in storage, six insulin monomers and two zinc ions form a hexamer in pancreatic beta-cell granules. When insulin is secreted into the bloodstream, the hexamer dissociates into monomers that bind insulin receptors on liver and muscle. Modern recombinant insulin analogues (lispro, glargine) modify the primary sequence to change tertiary structure and absorption rate.

Example 2. Spider silk in CSIRO bio-materials research. CSIRO's bio-materials team studies golden orb-weaver spider silk (Nephila edulis) to develop ultra-strong fibres for medical sutures and aerospace. Silk proteins have a repetitive primary sequence dominated by glycine and alanine. Secondary structure is mostly beta-sheets that align parallel to the fibre axis under tension. Tertiary structure includes amorphous flexible regions that absorb shock. Quaternary structure forms when multiple silk monomers aggregate in the spinning duct under pH and ion changes. The strength-to-weight ratio is five times that of steel. CSIRO has expressed spider silk genes in goats and bacteria, illustrating how primary sequence dictates all higher levels of structure.

Try this

Q1. Name the four levels of protein structure and identify the bond responsible for each. [4 marks]

• Cue. Primary (peptide bonds), secondary (hydrogen bonds), tertiary (hydrogen, ionic, disulfide, hydrophobic interactions), quaternary (same as tertiary, between subunits).

Q2. Insulin contains two polypeptide chains held together by disulfide bridges. (a) Identify the level of structure represented by the two-chain arrangement. (b) State which amino acid contributes to disulfide bonds. [2 marks]

• Cue. (a) Quaternary structure. (b) Cysteine (via its SH group).

Q3. Refer to a protein denatured by boiling. (a) State which levels of structure are disrupted. (b) Explain why the primary structure usually survives. (c) Predict whether the protein will regain function after cooling. [2+2+2 marks]

• Cue. (a) Secondary, tertiary, quaternary structures (non-covalent bonds break). (b) Peptide bonds are covalent and not disrupted by heat under normal conditions. (c) Small simple proteins may refold; large complex ones usually cannot, and function is lost.