5.2 Enzymes
  • An enzyme is an organic catalyst that is mostly made up of proteins and is produced by living cell organisms.
  • Substances needed for an enzyme reaction are called substrates.
  • Substrates will bind with enzymes at a specific site (active site) and form an enzyme-substrate complex.
Enzyme nomenclature:
  • Name of enzyme is derived by adding '-ase' to the name of the substrate it catalyses.
  • Example: lactase, protease, and amylase.
  • A few enzymes that do not follow this naming system.
  • Example: trypsin, pepsin and renin
General characteristics of enzymes:
  • Enzymes are needed in small amount.
  • Enzymes are not breaking down at the end of the reaction.
  • Action of enzymes are specific due to presence of active sites.
  • Most of chemical reactions catalyzed by enzymes are reversible.
  • Enzymes are sensitive to temperature and pH.
  • Some enzymes require cofactors in their activities.
  • Enzymes activities can be slowed or stopped by inhibitors such as lead.
Intracellular and extracellular enzymes:
Intracellular enzymes Extracellular enzymes
  • Enzymes are synthesized in a cell for their own use.
  • Example: The hexokinase enzyme is used in the glycolysis process during cellular respiration.
  • Enzymes that are secreted outside the cell.
  • Example: The trypsin enzyme is produced by the pancreatic cells and secreted into the duodenum to break down polypeptides.

Extracellular action:

  • In the nucleus, the information for the synthesis of enzymes is carried by the DNA in a form of codes.
  • mRNA is formed to translate the codes into a sequence.
  • mRNA leaves the nucleus and binds with ribosome for the synthesis of protein to occur.
  • The synthesized protein is transported enters the lumen of the rough endoplasmic reticulum.
  • The protein is processed and packaged into a transport vesicle which buds off from the rough endoplasmic reticulum to transports the protein to the Golgi apparatus
  • In Golgi apparatus, the protein is modified to form an enzyme and is packaged in a secretory vesicle which transports the enzymes to the plasma membrane.
  • The secretory vesicle will fuse with the plasma membrane to release the enzymes out of the cell.
Mechanism of enzyme action:
  • The enzyme represented by a 'lock'
  • The substrate represented by a 'key'
  • Most reactions inside the cell require high activation energy.
  • Activation energy is the energy needed to break the bond in the substrate molecule before reaction can occur.
  • Enzyme function by lowering the activation energy.
Factors affecting the activity of enzymes:
Factor affecting the activity of enzymes
  • Temperature
  • Enzyme concentration (limiting factor: substrate concentration)
  • Substrate concentration (limiting factor: enzyme concentration)
  • pH
  • At low temperature, the rate of enzymatic reaction is low.
  • The rate of enzyme reaction increases as the temperature increases.
  • This is because of the activation energy of the substrate molecules increases.
  • Therefore, more collision between the enzymes substrate molecule increases the formation of an enzyme-substrate complex.
  • The reaction is the maximum at the optimum temperature.
  • After the optimum temperature, the rate of reaction decreases because the enzyme is denatured in which the bonds that form the structure of the enzyme are changed.
  • This causes the active site to lose its shape.
  • Therefore, the enzyme-substrate complex can no longer be formed.
  • Optimum pH is the pH at which the rate of reaction is at the maximum.
  • Small changes in the pH value of a medium will cause the enzyme to be denatured.
  • The shape of the active site will change.
  • Therefore, the enzyme-substrate complex cannot be formed again.
  • Different enzymes have different optimum pH:
    • The optimum pH of pepsin is pH 2
    • The optimum pH of amylase is pH 7
    • The optimum pH of trypsin is ph 8.5
Substrate concentration:
  • The higher the concentration of substrate, the higher the rate of reaction as more substrate molecules bind to the active site of the enzymes to form the enzyme-substrate complex.
  • The rate of reaction becomes low when it reaches the maximum point because all of the active sites have been filled up.
  • At this point, the enzyme concentration is the limiting factor.
Enzyme concentration:
  • The higher the concentration of enzymes, the higher the rate of reaction as more active sites for substrate molecules to bind to and form the enzyme-substrate complex.
  • The rate of reaction becomes low when it reaches the maximum point because all substrate molecules have bound to the active sites.
  • At this point, the substrate concentration is the limiting factor.