1.2 4

• What proteins enable


  • Form hair, skin and nails
  • Enzymes needed for metabolism and digestion
  • Hormones that control various body systems
  • Muscle fibres to contract
  • Antibodies that protect from disease
  • Blood clotting
  • Transport oxygen in the form of haemoglobin
• Proteins


  Macromolecules made up of many small monomer units called amino acids joined together by condensation reactions
• Amino acids that can combine to form proteins

  • About 20 different naturally occurring amino acids
• Amino acid structure


  Always an amino group (NH) and a carbonyl group (-COOH) attached to a carbon atom, with the R group varying between amino acids
• Forming proteins from amino acids

  1. Amino acids join together by a reaction between the amino group of one and the carboxyl group of another, forming a peptide bond and losing a water molecule

  2. More amino acids join to form polypeptide chains

  3. Polypeptide folds, coils or associates with other polypeptide chains to form a protein
• Peptide bond


  • Strong bond between amino acids
• Other bonds in proteins

  • Hydrogen bonds
  • Disulfide bonds
  • Ionic bonds
• Hydrogen bonds


  Form between the oxygen of the carboxyl group and hydrogen of the amino group, holding the protein structure together
• Disulfide bonds


  Form between two cysteine amino acids, much stronger than hydrogen bonds
• Ionic bonds


  Form between positive and negative amino acid side chains, holding the protein structure together
• Changes in temperature or pH

  Can break the bonds holding the 3D shape of proteins, causing denaturation
• Levels of protein structure

  • Primary - linear sequence of amino acids
  • Secondary - regular repeating structure held by hydrogen bonds
  • Tertiary - 3D folding of the secondary structure
  • Quaternary - arrangement of multiple polypeptide chains
• Fibrous proteins


  • Have little or no tertiary structure, are long parallel polypeptide chains, insoluble in water, very tough and suited for structural functions
• Globular proteins


  • Have complex tertiary and quaternary structures, soluble in water, involved in metabolic processes
• Primary structure


  The linear sequence of amino acids in a peptide
• Secondary structure


  The repeating pattern in the structure of the peptide chains, such as an α-helix or β-pleated sheets
• Tertiary structure


  The three-dimensional folding of the secondary structure
• Quaternary structure


  The three-dimensional arrangement of more than one tertiary polypeptide
• Fibrous proteins


  • Have little or no tertiary structure
  • Are long, parallel polypeptide chains with occasional cross-linkages that form into fibres
  • Are insoluble in water and very tough, which makes them suited to structural functions
• Fibrous proteins


  • Connective tissue in tendons
  • Matrix of bones
  • Structure of muscles
  • Silk of spiders' webs and silkworm cocoons
  • Keratin that makes up hair, nails, horns and feathers
• Collagen


  • A fibrous protein that gives strength to tendons, ligaments, bones and skin
  • The most common structural protein found in animals, up to 35% of the protein in the body
  • Has a tensile strength comparable to steel due to its unusual triple helix structure
• In the genetic disease osteogenesis imperfecta


  The collagen triple helix may not form properly, causing the bone to lack tensile strength and become brittle and easily broken
• Globular proteins


  • Have complex tertiary and sometimes quaternary structures
  • Fold into spherical (globular) shapes
  • Their large size affects their behaviour in water
• Haemoglobin


  • A very large globular protein made up of 574 amino acids arranged in polypeptide chains held together by disulfide bonds
  • It is a conjugated protein, with an iron-containing haem group that enables it to bind and release oxygen molecules
• Conjugated proteins


  Protein molecules joined with or conjugated to another molecule called a prosthetic group, which affects the performance and functions of the molecule
• Conjugated proteins


  • Haemoglobin with an iron-containing haem group
  • Chlorophyll with a magnesium-containing prosthetic group
• Glycoproteins


  Proteins with a carbohydrate prosthetic group, which helps them hold water and resist digestion by proteases
• Glycoproteins


  • Mucus
  • Synovial fluid in joints
• Lipoproteins


  Proteins conjugated with lipids, important in the transport of cholesterol in the blood
• Lipoproteins


  • Low-density lipoproteins (LDL)
  • High-density lipoproteins (HDL)
• Amino acids are joined together by peptide bonds to form dipeptides and polypeptides, but the 3D structures of proteins are the result of hydrogen bonds, disulfide bonds and other bonds between amino acids within the polypeptide chains