Protein Structure and Function

An amino acid has a central carbon bonded to an amine group, a carboxyl group, a hydrogen atom and a variable R group. The R group distinguishes the twenty common amino acids. Condensation between the amine and carboxyl groups of neighbouring amino acids forms a peptide bond and releases water.

A dipeptide contains two amino-acid residues. Many residues form a polypeptide; one functional protein may contain one chain or several. The primary structure is the sequence, not the folded outline seen in a diagram. Different R-group interactions mean that changing even one residue can sometimes alter folding or binding, although not every substitution has a large effect.

Scroll the diagram to see the full view.

Secondary structure includes alpha helices and beta-pleated sheets stabilised by hydrogen bonds between parts of the polypeptide backbone. Tertiary structure is the overall three-dimensional folding of one polypeptide, maintained by interactions including hydrogen bonds, ionic interactions and disulfide bridges between suitable side chains.

Quaternary structure describes the association of multiple polypeptide subunits. Haemoglobin is an example of a multi-subunit protein; an enzyme with one polypeptide can have tertiary structure without quaternary structure. Fibrous proteins such as collagen and globular proteins have structures suited to different mechanical or metabolic roles. Name the feature and explain its contribution rather than writing “shape suits function”.

Heat or extreme pH can disrupt interactions maintaining protein conformation. This may change a binding site so a protein loses activity: denaturation. A typical denaturation explanation does not require peptide bonds to break or the primary sequence to change. Different proteins vary in stability, so avoid claiming every protein denatures at one temperature.

The biuret test detects peptide bonds: add the reagents providing alkaline copper(II) ions according to the laboratory method; a lilac or purple result supports protein/polypeptide being present. Free amino acids do not give the same positive response. Use controls and distinguish a positive chemical test from proof that a sample’s enzyme remains functional.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.