One building plan for thousands of proteins

• All proteins are unbranched linear polymers built from the same 20 amino acids.
• Every amino acid has the same basic structure: a central (α) carbon bonded to

• a carboxyl group (–COOH),
• an amino group (–NH₂),
• a hydrogen atom, and
• a fourth group R that differs from one amino acid to another.

(The exception is glycine, whose R is simply a second hydrogen atom; the other 19 have a distinct R group.)

• With the help of suitable enzymes, amino acids are joined by dehydration (condensation) reactions into peptide bonds, forming a polypeptide.
• Proteins differ because of the number, kind and sequence of amino acids, the number of polypeptide chains in the protein, and the weak hydrogen bonds that fold the molecule into its special shape.
• Making a protein is a complicated process involving many kinds of molecules – nucleic acids, ribosomes, enzymes and amino acids.