1.3 Proteins

• The general structure of an amino acid consists of a -COOH carboxyl/ carboxylic acid group, a -R variable side group consisting of a carbon chain and may include other functional groups such as benzene ring or -OH (alcohol), and -NH 2 amine/ amino group.
• Polypeptides form through a condensation reaction that forms a peptide bond (-CONH-) and eliminates a molecule of water.
• There are four levels of protein structure: primary, secondary, tertiary, and quaternary.
• The primary structure of a protein is determined by the sequence, number, and type of amino acids in the polypeptide, which is determined by the sequence of codons on mRNA.
• The secondary structure of a protein is defined by hydrogen bonds formed between O 𝛿 - (slightly negative) attached to ‒ C=O & H 𝛿 + (slightly positive) attached to ‒NH.
• There are two types of secondary protein structure: α-helix and β-pleated sheet.
• The tertiary structure of a protein is a 3D structure formed by further folding of the polypeptide, which may include disulfide bridges, ionic bonds, and hydrogen bonds.
• Collagen has a stable alpha triple helix due to repeating sequence glycine-proline-other, forms fibres, and has high tensile strength due to H-bonds and staggered covalent bonds between fibres.
• Globular proteins are involved in metabolic processes such as enzymes and haemoglobin.
• Disulfide bridges in the tertiary structure of proteins are strong covalent S-S bonds between molecules of the amino acid cysteine.
• Haemoglobin binds to oxygen with variable affinity to transport it around the body in the bloodstream.
• Haemoglobin consists of 2 α-chains, 2 β-chains, and 4 prosthetic haem groups, is globular and water-soluble, and dissolves in plasma.
• Tertiary structure changes so it is easier for subsequent O2 molecules to bind (cooperative binding).
• Precise 3D structure of proteins is held together by the same types of bond as tertiary structure.
• Intermolecular force between H �� + of O-H or N-H & lone pair on O or N of an adjacent molecule is numerous and easily broken.
• Functional proteins may consist of more than one polypeptide.
• Fe2+ haem group forms coordinate bond with O2.
• Proteins may involve addition of prosthetic groups such as metal ions or phosphate groups.
• pH changes cause these bonds to break due to interaction with OH ions.
• Fibrous proteins can form long chains or fibres, with sequences of amino acids repeating, and are insoluble in water.
• Fibrous proteins are useful for structure and support, such as collagen.
• Collagen is a component of bones, cartilage, connective tissue, and tendons.
• Globular proteins are spherical and compact, with hydrophilic R groups facing outwards and hydrophobic R groups facing inwards, usually making them water-soluble.
• Ionic bonds in the tertiary structure of proteins are relatively strong bonds between charged R groups.