Quick questions on Protein structure: primary, secondary, tertiary and quaternary (VCE Biology Unit 3)

Every amino acid has the same core:

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.

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.

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.

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

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

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).

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

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

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