• A protein may consist of a single polypeptide or more than one polypeptide linked together
• Some proteins exist as a?single polypeptide chain?(of amino acids)
• Other proteins are made up of?two or more polypeptide chains?joined together
• Single polypeptide chain?proteins include?lysozyme, an enzyme present in mucus secretions and tears, that?kills bacteria?as part of our primary defences against pathogens
• Proteins with?two polypeptide chains?include
  • insulin, a hormone responsible for regulating blood glucose levels
  • integrins, a group of membrane proteins that span a?phospholipid bilayer?and act as a receptor
    • integrins' two polypeptide chains each have a?hydrophobic section?that sits in the membrane bilayer

     

  Proteins with three polypeptide chains?include?collagen, the main structural protein in skin, tendons, ligaments and the walls of blood vessels

• Proteins with?four polypeptide chains?include?haemoglobin, which binds oxygen in red blood cells and delivers it from the lungs to respiring tissues
• Each polypeptide chain in a multi-polypeptide protein is referred to as a?subunit?of the protein

The amino acid sequence determines the three-dimensional conformation of a protein

• Proteins perform their diverse roles because of their 3-D shape and structure
• This is known as the protein's?conformation
• The?precise sequence of amino acids?determines how the protein?folds?and aligns itself?as the individual amino acids are being added?at the ribosome
  • Amino acids are always added in the same sequence so a?protein can start to form its shape even?before it is fully formed
  • Bonds form?between parts of amino acids that can cause a?bridge?to form between one part of the chain and another
    • this creates?loops,?sheets,?helices?and?folds

    Many of the bonds that hold the protein's shape form between the various R groups?of individual amino acids

    • If an amino acid is not present in its usual place in the chain due to?mutation, this can drastically alter the protein's 3-D shape, and affect its function

     

  Haemoglobin is a globular?protein (forms a globe-shaped protein)

  • Some of haemoglobin's outer parts are?hydrophilic?to be in contact with water whilst its inner parts are made up of amino acids with?hydrophobic?R groups

  Collagen is a fibrous?protein (forms a rope-like protein for tensile strength)

  • It has a?repeating sequence of amino acids?to create a helical structure
  • The chain of amino acids?remains in an elongated conformation?to give fibrous strength

Globular and fibrous protein models illustrating the roughly spherical shape of globular proteins and the long, stranded shape of fibrous proteins

• Denaturation is the irreversible change of protein conformation caused by temperature and pH extremes
• The bonds that form?between different R groups?are?relatively weak?(compared to the peptide bonds that hold the amino acids in sequence)
• These bonds can be?broken easily,?which can cause the?conformation?of the protein to change
• The?altered protein shape?may affect its?function,?physical state?and general usefulness in its original role
• This is called?denaturation
• Heat?and?extremes of pH?are the most common causes of denaturation
  • Both cause?breaking of the weak bonds?between R groups

  A certain pH is considered as an optimum?for a particular protein, because at that pH, the protein's 3-D structure is not denatured

• Denaturation is almost always?irreversible
  • The protein?cannot be re-formed?in its original conformation by reversing the change in conditions
  • However,?small denaturations?and?renaturations are possible in certain proteins to respond to small fluctuations in pH eg. haemoglobin

The effect of heat and pH on the shape and function of a globular protein

Denaturation in action

• Denaturation can be seen most easily by looking at the?changes in an egg white?as the egg is fried or poached
• Egg white is mainly the protein?albumin
• The?hydrophobic amino acids?in albumin are at the centre of the molecule in its normal state, so albumin is soluble
• Heating causes the hydrophobic amino acids to appear?at the edges, where they cause the protein to become?insoluble
• A harder, solid layer forms, which is the?cooked white
• Similar events occur in the proteins of the?egg yolk?as it cooks
• Denaturation also occurs in the?stomach, where the low pH (pH2) causes?proteins in the diet to become denatured?on their way to being fully hydrolysed further down the digestive system
• The stomach enzyme?pepsin, a protein-digesting enzyme has an optimum pH of 2 for this reason
• Certain?extremophiles?have evolved to have proteins that are stable even at extreme pH or temperature
  • eg.?Thermus aquaticus, a?bacteria that lives in hot springs?at 80°C
  • This temperature would denature most other proteins

  Denaturation of enzymes?can be used as part of experiments to measure enzyme activity

  • For example,?an experiment to establish the optimum pH or temperature?of an enzyme eg. pepsin or lipase

  Many drugs are proteins that cannot be taken by mouth, because the protein will be?denatured by stomach acid

  • These drugs should be?delivered in another way?eg. by?direct injection?into the blood

  Exam Tip