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.

Worked example

Normal haemoglobin has glutamic acid (Glu, charged, polar) at position 6 of the beta chain. In sickle-cell anaemia, a single base mutation in the gene changes this codon so that valine (Val, hydrophobic, non-polar) is inserted instead.

• Primary structure. A single amino acid swap.
• Tertiary and quaternary structure. The new hydrophobic patch on the beta subunit binds another haemoglobin tetramer in low oxygen conditions, polymerising into long fibres.
• Function. Red blood cells distort into the sickle shape, block capillaries and lose flexibility.

One change in primary structure cascades into loss of function at the whole-organism level.

Common traps

Calling all R-group interactions "bonds at the secondary level." Secondary structure depends on backbone hydrogen bonds, not R group interactions. R group interactions stabilise tertiary and quaternary structure.

Forgetting disulfide bridges are covalent. Disulfide bridges are strong covalent bonds between cysteine residues. They are usually the last thing to break during denaturation.

Claiming every protein has quaternary structure. Quaternary structure exists only when two or more subunits associate. Myoglobin, for example, has only tertiary structure.

Mixing up primary structure direction. Primary sequence is always written from N-terminus (amino end) to C-terminus (carboxyl end), in the same order the ribosome adds amino acids.

In one sentence

A protein's function arises from a four-level hierarchy of structure (sequence, local folds, three-dimensional shape, multi-subunit assembly), all ultimately determined by the order of amino acids encoded in the gene.