Proteins:
- Structure of an amino acid:
- The amino acid monomers join together in a condensation reaction to form peptide bonds. The polymer formed is called a polypeptide.
- How do they form?
- Primary structure: the sequence of amino acids in the polypeptide chain. Amino acids are connected by peptide bonds. Most proteins do not function in their primary form.
- Secondary structure: the shape the molecule folds because of hydrogen bonding between the C=O of one amino acid and the N-H of the amine group of another – an helix or a pleated sheet.
- Tertiary structure: the final 3D shape of the molecule, held together by ionic bonds, interactions between hydrophilic R groups and strong disulphide bridges between R groups containing sulphur.
- Quaternary structure: if the protein contains more than one polypeptide chain.
- Fibrous proteins remain as long chains, often with several polypeptides cross-linked for extra strength. They are insoluble and are important structural molecules e.g. keratin, collagen.
- Globular proteins are folded into a compact spherical shape. They are soluble and are important metabolic moleculesg. enzymes, antibodies, and some hormones.
- The specific sequence of specific amino acids determines the shape of the protein and, therefore, its function.
- Test for proteins:
- Biuret solution turns blue to purple/lilac in the presence of protein.
