Amino Acids and Proteins

Amino acids are the building blocks of proteins — one of the most important classes of biological molecule. At A-Level, you need to understand the structure of amino acids, their acid–base behaviour, how they link together to form proteins, and how proteins can be broken down and identified.

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Structure of Amino Acids

An amino acid contains two functional groups attached to the same carbon atom (the α-carbon):

• An amino group (–NH₂) — a basic group
• A carboxyl group (–COOH) — an acidic group

The general structure is:

H₂N–CH(R)–COOH

where R is the side chain (also called the R group). The identity of R is what distinguishes one amino acid from another. There are 20 naturally occurring amino acids used by living organisms to build proteins, each with a different R group.

The simplest amino acid is glycine, where R = H. Others include alanine (R = CH₃), serine (R = CH₂OH), and cysteine (R = CH₂SH).

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Zwitterions and the Isoelectric Point

Because amino acids contain both an acidic group (–COOH) and a basic group (–NH₂), they undergo an internal acid–base reaction in which the –COOH donates a proton to the –NH₂:

H₂N–CH(R)–COOH → ⁺H₃N–CH(R)–COO⁻