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Amino acids and peptide bonds — Chemistry, 14–17

See how amino acids join into chains that fold into proteins. Chemistry, 14–17 years.

Small units, long chains

An amino acid has an amino group and a carboxylic acid group, plus a side group that makes it different from the others. The carboxyl group of one can join the amino group of another, forming a peptide bond, −CO−NH−, and releasing water. Many linked amino acids make a polypeptide.

Why build peptide chains?

Cells need molecules that can make shapes, speed up reactions and carry messages. A single amino acid cannot do all these jobs, but a chain can fold into a precise three-dimensional form. Linking a limited set of amino acids in different orders creates a huge range of useful molecules.

Joining glycine and alanine

Glycine is C₂H₅NO₂ and alanine is C₃H₇NO₂. Remove OH from the carboxyl group of glycine and H from alanine’s amino group: these form H₂O. The remaining parts join as C₂H₅NO₂ + C₃H₇NO₂ − H₂O = C₅H₁₀N₂O₃, with a −CO−NH− peptide link.

A peptide bond is not ionic

It is easy to call the link between amino acids ionic because amino and acid groups can gain or lose protons. That is reasonable when thinking about amino acids in water, but a peptide bond is a covalent −CO−NH− link formed by sharing electrons. The chain can still contain charged side groups elsewhere.

Proteins in life and medicine

Your enzymes, muscles, antibodies and many hormones are proteins made from peptide chains. Digestion breaks these chains into amino acids, while cells join amino acids again when building new proteins. Medicines may copy, block or replace protein shapes, so the chemistry of the chain matters.

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