Amino acids, peptide bonds and polypeptides

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Amino acids, peptide bonds and polypeptidesSpec 2.6

In short

An amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. Amino acid monomers are joined by condensation reactions, which release water and form peptide bonds, to make polypeptides. A protein is one or more polypeptides folded into a specific shape.

Proteins are polymers made from amino acid monomers. All amino acids share the same basic structure around a central carbon atom: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group. About 20 different amino acids are used to make proteins, and they differ only in their R group. (You do not need to learn the structures of individual amino acids.)

The general structure of an amino acid with amino group, carboxyl group, hydrogen and variable R group, and two amino acids joining by a condensation reaction that releases water and forms a peptide bond. — A peptide bond forms in a **condensation** reaction between the carboxyl group of one amino acid and the amino group of the next.

Condensation and hydrolysis

Two amino acids join in a condensation reaction. The carboxyl group of one amino acid reacts with the amino group of the next. A molecule of water is released, and a covalent peptide bond (C–N) forms between them, to give a dipeptide. Repeating this builds a chain of many amino acids, called a polypeptide. The reverse reaction, hydrolysis, uses water to break peptide bonds.

amino acid + amino acid → dipeptide + water (condensation, forms a peptide bond)

A protein is made of one or more polypeptide chains, folded into a precise three-dimensional shape. The sequence of amino acids in each chain is determined by the sequence of bases in a gene.

Practical skill:

Recommended additional practical: use a semi-quantitative method to estimate protein concentration with biuret reagent and colour standards. Add sodium hydroxide, then copper(II) sulfate. A purple (lilac) colour shows peptide bonds; the more protein, the deeper the colour. Compare with standards of known concentration or use a colorimeter. Take care: sodium hydroxide is corrosive, so wear eye protection.

Common mistake:

The peptide bond is between the carbon of one amino acid's carboxyl group and the nitrogen of the next amino group. A condensation reaction releases water, and does not use it up.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between primary, secondary, tertiary and quaternary protein structure?

Primary structure is the sequence of amino acids joined by peptide bonds. Secondary is coiling into an alpha helix or folding into a beta pleated sheet, held by hydrogen bonds. Tertiary is the overall 3D folding, held by hydrogen, ionic and disulfide bonds. Quaternary is two or more polypeptides together.

How are the structures of haemoglobin and collagen related to their functions?

Haemoglobin is globular and soluble, with four polypeptides each holding an iron-containing haem group that binds oxygen. Collagen is fibrous and insoluble: three polypeptides form a triple helix with hydrogen bonds and cross-links, giving great tensile strength for support in tendons and skin.

Why do enzymes denature at high temperatures?

High temperatures give the enzyme molecule more kinetic energy, which breaks the hydrogen and ionic bonds holding the tertiary structure. The active site changes shape, so the substrate can no longer fit, and the enzyme-substrate complex cannot form. This denaturation is not reversible.

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