Protein Structure: Effect of pH & Temperature (HL IB Biology)

• Proteins structure is sensitive to changes in the environment, particularly temperature and pH changes
• The precise structure of a protein is dependent on the ionic interactions, hydrogen bonds and other intermolecular forces between polypeptide chains being intact
• Denaturation may occur by temperature and pH extremes that interfere with these bonds
  • Denaturation is the irreversible change of protein conformation 
• 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 and denaturation
• The altered protein shape may affect its function, physical state and general usefulness in its original role
• A certain pH is considered as an optimum for a particular protein, because at that pH, the protein's 3D 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 e.g. haemoglobin

Denaturation of a protein diagram

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 e.g. 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 e.g. by direct injection into the blood