Protein Structure: Primary to Quaternary

0:26Primary structure

Level one is primary structure. It is simply the sequence of amino acids in a polypeptide chain. Think of beads on a string, where every colour is a different amino acid. The beads are joined by peptide bonds, which are strong covalent bonds.

And the order of the beads is set by the gene. Change the order, and you change everything above it.

0:49Secondary structure

Level two is secondary structure. Parts of the chain twist or fold into regular, repeating shapes. One is the alpha helix, a coil. The other is the beta pleated sheet, where the chain folds back and forth in zigzags. Both are held by hydrogen bonds between groups along the backbone of the chain.

Each hydrogen bond is weak, but there are lots of them, so the shape is stable. Notice this is still just local folding. The whole molecule has not found its final shape yet.

1:23Tertiary structure

Level three is tertiary structure. This is the whole polypeptide folded into its final three dimensional shape. Now the R groups, the side chains, take over. They attract and bond with each other, and pull the chain into shape. There are three bond types to know.

Hydrogen bonds. Weak, but numerous. Ionic bonds, between R groups with opposite charges. These are stronger than hydrogen bonds. And disulfide bridges, covalent bonds between sulfur atoms in the R groups of two cysteine amino acids that the folding brings close together. These are the strongest.

Together they hold the molecule in one precise shape. And the shape decides the job.

2:09Quaternary structure

Level four is quaternary structure. This only exists when a protein has more than one polypeptide chain. The quaternary structure is how those separate chains are arranged and held together. Haemoglobin is the classic example. Four polypeptide chains, each folded in its own tertiary shape, fitted together into one working protein.

A protein with just one chain has no quaternary structure. And no, quaternary does not mean a protein with more than one amino acid. Every protein has lots of those.

2:45Summary of the four levels

Quick summary of the bonds, because the examiner loves this. Primary is held by peptide bonds. Secondary is held by hydrogen bonds. Tertiary is held by hydrogen bonds, ionic bonds and disulfide bridges.

And quaternary is simply more than one chain, held by the same kinds of bonds between the chains.

3:06How one amino acid changes the shape

Now the part that wins marks. Why does one changed amino acid matter? A different amino acid has a different R group. So hydrogen bonds, ionic bonds or disulfide bridges form in different places, or not at all. The chain therefore folds differently, and the tertiary structure changes.

And a different shape means the protein can no longer carry out its function. Sequence decides bonds. Bonds decide shape. Shape decides function.

4:22Examiner trap and recap

The trap. Peptide bonds do not hold tertiary structure. They only hold the primary sequence. Tertiary is hydrogen bonds, ionic bonds and disulfide bridges. Recap. Sequence, then local folds, then final fold, then several chains. Next time, we link shape to job: why some proteins are globular, some are fibrous, and how the biuret test spots them.