Amino acid structure

The big idea: Amino acids are the small subunits (monomers) that join together to build proteins.

Every amino acid is built around one central carbon atom. Bonded to that carbon are four things:

an amino group (—NH₂), a carboxyl group (—COOH), a single hydrogen (—H), and a variable R group (a side chain).

Three of these four are identical in every amino acid — only the R group changes.

Amino acid

The monomer (subunit) that proteins are built from. It has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable R group.

Central carbon

The single carbon atom at the centre of every amino acid; the four groups are all attached to it.

Amino group (—NH₂)

A nitrogen-containing group found in every amino acid. Its name gives the 'amino' in 'amino acid'.

Carboxyl group (—COOH)

An acidic group found in every amino acid. It gives the 'acid' in 'amino acid'.

R group (side chain)

The part of an amino acid that varies from one amino acid to the next. There are 20 different R groups, so there are 20 different amino acids.

Where the name comes from: The name amino acid is a clue to its two key groups:

amino = the amino group (—NH₂), and acid = the carboxyl group (—COOH).

Both of these groups appear in every amino acid.

If three of the four groups are the same in every amino acid, then the R group must be the part that makes them different.

There are 20 different amino acids in living things, and they differ only in their R group — everything else about the core structure is identical.

The variable R group: The R group is the side chain: a different chemical group sticking off the central carbon in each amino acid.

Because there are 20 different R groups, there are 20 different amino acids.

The R group can be small (just a hydrogen in glycine) or large; it can be charged, water-loving or water-hating. These differences in the R groups are what give finished proteins their wide range of properties.

Element composition — look for nitrogen: Amino acids (and the proteins built from them) always contain the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N).

The nitrogen is important: it comes from the amino group (—NH₂), and it is the element that carbohydrates and lipids do NOT have.

So if a molecule's elements include nitrogen, it could be a protein / amino acid — but never a carbohydrate or a fat.

A memory hook: R = 'Rest of it' = the part that's different. The amino group, carboxyl group and hydrogen are the fixed core; the R group is the changeable bit.

And: nitrogen = protein. Carbohydrates and lipids have C, H and O only — only proteins add N.

How this is tested: On Paper 1A a multiple-choice question often shows the structure of a named amino acid (such as leucine) and asks you to identify the part that is the variable R group — the side chain that makes one amino acid differ from another.

Another common Paper 1A format gives the element composition of two molecules and asks which one is the protein / amino acid — the answer is the one that contains nitrogen.

On Paper 2 you may be asked to draw or label the generalized structure of an amino acid, showing all four groups on the central carbon.

✓ What the diagram must show: All four groups must be on the one central carbon: amino group, carboxyl group, hydrogen and the R group.

Marking down usually comes from leaving out the R group or forgetting that the amino and carboxyl groups are both present.