Enzymes

• Enzymes are biological catalysts. They speed up chemical reactions without themselves being chemically changed at the end of the chemical reactions. Hence, they can be reused and are effective in small concentrations
• Catabolic reactions - consist of chemical reactions which break up complex molecules into simpler molecules. E.g. Hydrolysis

  Anabolic reactions - consist of chemical reactions which build up simpler molecules into complex molecules E.g. Condensation reaction
• Structure of enzymes

  • Most enzymes are proteins with unique 3-dimensional structure
  • Each enzyme has an active site, where substrate(s) bind(s)

  -Substrate is the molecule which an enzyme acts on

  -Each active site can only allow specific substrate(s) to fit in

  -The specificity of the enzyme is attributed to the complementary shape between the substrate and the active site
• Chemical reaction and activation energy

  
  

  Changing one molecule into another molecule requires deforming the reactant molecules before the reaction can proceed

  
  

  Activation energy - energy that must be supplied to the reactant molecules for them to react

  • This absorption of thermal energy increases the speed of the reactant molecules, so that they collide more frequently and forcefully in the correct orientation for chemical reaction to occur
  • Thermal agitation of the atoms within the molecules also makes the bond more likely to break
• • Heating speeds up a chemical reaction, but this is inappropriate for a biological system - high temperatures denature proteins and kill cells. Heating will also speed up all chemical reaction, not just the intended ones. Organisms therefore use enzymes as alternative
• Enzymatic reaction includes:

  • Effective collision between specific substrate(s) and enzyme at the correct orientation causes the substrate molecules to enter the active site of the enzyme
  • Substrate molecules bind to the active site of the enzyme and forms the enzyme-substrate complex
  • Formation of enzyme-substrate complex lowers the activation energy
  • Chemical reaction occurs and products formed
  • The enzyme-substrate complex dissociates to release the products and the chemically unchanged enzyme is ready for another cycle of chemical reaction
• Lock and key hypothesis

  • In lock and key hypothesis, substrate is the key while the enzyme is the lock
  • The substrate can fit exactly into the active site of the enzyme
  • The shape of the active site of the enzyme is complementary to the shape of the substrate
  • The substrate then binds to the active site of the enzyme, forming the enzyme-substrate complex
  • Once the products are formed, they no longer fit into the active site of the enzyme and are released into the surrounding medium. The enzyme is ready for another round of reaction
• Characteristics of enzymes

  • Enzymes speed up chemical reactions by lowering the activation energy
  • Enzymes remain chemically unchanged after the reactions. They can be used over and over again, hence, enzymes are required in small concentration
  • Enzymes are specific in action, due to its 3D shape (substrate specific)
• Induced fit model

  • The active site of enzymes is complementary in shape but not a perfect fit to the substrate it catalyses
  • However, when the substate bind to the active site of the enzyme, it induces a change in the shape of the active site, allowing the substrate to fit more tightly into the active site
  • Binding of the substrate to the active site of the enzymes occurs, enzyme-substrate complex is formed
  • Chemical reactions occur, products are formed
• Effects of varying temperature

  Enzymes have an optimum temperature - temperature at which the rate of enzyme activity is at its maximum

  • Rate of enzyme activity is low at low temp (near or below 0)
  • Enzymes are less active
  • Reversible condition - enzymes increases in activity when temp increases
• Effects of varying temperature

  • As temp increases, kinetic energy of substrate and enzyme molecule increases
  • Enzymes start to be more active
  • Substrate and enzyme molecules collide more often. This increases the number of effective collision and formation of enzyme-substrate complexes and thus rate of reaction increases
  • Reaction rate doubles for every 10°C rise in temp until optimum temp is reached
  • Rate of reaction is at its maximum at optimum temp
• Effects of varying temperature

  • As temp increases beyond optimum temp, rate of reaction starts to decrease
  • Enzyme is denatured. The enzymes loses its 3D shape and active site is unable to bind to the substrate. Once an enzyme is denatured, it is irreversible, and it cannot regain its function even when temp is lowered
  • As temp continues to increase, more enzyme molecules become denatured, which causes the rate of reaction to decrease further
• Effects of varying pH

  • Enzymes have an optimum pH - pH at which enzymes activity is at its maximum
  • Any pH that deviates from the optimum pH will cause the rate of reaction to decrease
  • At extreme pH, enzyme loses its 3D shape and active site is unable to bind to the substrate
  • Once an enzyme is denatured, it is irreversible, and it cannot regain its functions even when pH goes back to optimum
• Pepsin and trypsin are two types of protease

  Protease - any enzyme that catalyses the breakdown of proteins into polypeptides or polypeptides into amino acids

  Others include : peptidase, papain, elastase etc.
• Limiting factor - a factor that directly affects the rate of chemical reaction if its quantity is changed. The value of this factor has to be increased in order to increase the rate of the process
• Effects of varying enzyme concentration

  • Rate of reaction is dependent on the number of effective collision between enzyme and substrate molecules
  • At lower enzyme concentrations, adding more enzyme increases the rate of chemical reaction - enzyme concentration is limiting
  • When enzyme concentration increases, substrates more likely to bind to empty active site. More effective collision occurs, resulting in formation of more enzyme-substrate complexes
• Effects of varying enzyme concentration

  • At the plateau, rate of reaction becomes constant and enzyme concentration is no longer limiting factor - substrate concentration is limiting
  • There are not enough substrates to occupy all the active sites of the enzymes - substrate concentration is limiting
• Effects of varying substrate concentration

  • At low substrate concentrations, few substrate molecules present; hence there are many available enzyme active sites for effective collision to occur
  • Rate of reaction increases with an increase in substrate concentration until a point when further increase in substrate concentration will no longer increase the rate of reaction
  • Rate of reaction becomes constant and reaches a plateau
• Effects of varying substrate concentration

  • At higher substrate concentrations, increasing concentrations of substrate cannot increase rate of reaction - enzyme concentration is limiting factor
  • This is because all active sites of the enzyme molecules are saturated with substrate molecules, and the concentration of product formed per unit time remains the same