Proteins are large biological molecules built from smaller units called amino acids. They make up enzymes, muscles, antibodies, transport channels, and many structural materials in living cells. Their function depends not only on which amino acids are present, but also on how the chain folds into a precise three-dimensional shape.
Understanding proteins connects chemistry to biology, medicine, nutrition, and biotechnology.
Each amino acid has the same basic backbone, with an amino group, a carboxyl group, a hydrogen atom, and a variable R group attached to a central carbon. Amino acids join by dehydration synthesis, forming peptide bonds and releasing water. The resulting polypeptide folds through interactions such as hydrogen bonding, ionic attractions, disulfide bridges, and hydrophobic effects.
Protein structure is described at four levels: primary, secondary, tertiary, and quaternary.
Key Facts
- General amino acid structure: H2N-CH(R)-COOH, where R is the side chain.
- Peptide bond formation: amino acid + amino acid -> dipeptide + H2O.
- A protein with n amino acids contains n - 1 peptide bonds in a single unbranched chain.
- Primary structure is the amino acid sequence, written from the N-terminus to the C-terminus.
- Secondary structure includes alpha helices and beta sheets stabilized mainly by backbone hydrogen bonds.
- Protein shape determines function, and denaturation changes shape without usually breaking peptide bonds.
Vocabulary
- Amino acid
- An amino acid is a molecule with an amino group, a carboxyl group, and a variable side chain that can link to form proteins.
- Peptide bond
- A peptide bond is a covalent bond between the carboxyl carbon of one amino acid and the amino nitrogen of another.
- Polypeptide
- A polypeptide is a chain of amino acids joined by peptide bonds.
- Denaturation
- Denaturation is the loss of a protein's normal folded shape due to changes such as heat, pH, or chemicals.
- Quaternary structure
- Quaternary structure is the arrangement of two or more polypeptide subunits in a functional protein.
Common Mistakes to Avoid
- Calling every amino acid side chain nonpolar is wrong because R groups can be nonpolar, polar, acidic, or basic, which strongly affects folding and function.
- Counting peptide bonds as equal to the number of amino acids is wrong for one linear chain because a chain with n amino acids has n - 1 peptide bonds.
- Thinking secondary structure is caused by R-group interactions is wrong because alpha helices and beta sheets are mainly stabilized by hydrogen bonds between backbone atoms.
- Assuming denaturation always breaks the protein into amino acids is wrong because denaturation usually disrupts folding interactions while leaving peptide bonds intact.
Practice Questions
- 1 A single polypeptide contains 78 amino acids. How many peptide bonds does it contain, and how many water molecules were released when it formed?
- 2 A peptide is made by joining 5 amino acids with average molar mass 110 g/mol each. If 4 water molecules are removed during bond formation, estimate the molar mass of the peptide using water = 18 g/mol.
- 3 A protein loses its enzyme activity after heating, but chemical tests show its peptide bonds are still present. Explain which level or levels of structure were most likely disrupted and why.