11.13 - Naturally Occurring Polymers

Amino acids and the protein chain

A protein is a naturally occurring polymer, and its monomers are amino acids.

The structure of an amino acid

• An amino acid carries two different functional groups:
  1. An amino group, -NH₂, which is basic.
  2. A carboxyl group, -COOH, which is acidic.
• Between the two groups sits a central carbon atom, bonded to a hydrogen atom and a side chain (R group).
• Every amino acid follows this pattern, differing only in its side chain.
• The simplest amino acid is glycine, whose side chain is a single hydrogen atom (H₂NCH₂COOH).

How amino acids join into a chain

Amino acids join by condensation polymerisation:

1. The carboxyl group of one amino acid reacts with the amino group of the next.
2. The -OH of the carboxyl group and a hydrogen atom from the amino group are lost together as a molecule of water.
3. A covalent bond forms between the carbon atom of the carboxyl group and the nitrogen atom of the amino group.
4. Each end of the joined molecule still has a free functional group, allowing more amino acids to join on and extend the chain.

Amide links and the polypeptide chain

• The link formed between the carbon atom and the nitrogen atom is called an amide link (or peptide link).
• A polymer held together by amide links is a polyamide, making proteins natural polyamides.
• A chain of joined amino acids is called a polypeptide, and a protein is made of one or more polypeptide chains.
• Combining different amino acids in different orders gives proteins their specific structures and functions.