1213 Week 19 T

1213 Week 19 T

1. Enzyme Catalysed Reactions

   • E + S <-> ES <-> E + P
   • the relationship between time and product formed is exponential
   • as time goes on the rate of reaction slows
     • concentration of the substrate decreases over time so there are less collisions
     • velocity of the back reaction increases
     • product may inhibit the enzyme
     • enzyme may be labile
   • concentration against initial reaction rate is hyperbolic
   • Vmax = maximum reaction rate
   • Km = concentration of substrate where half of the enzyme active sites are occupied (half Vmax) (conc units)
   • Michaelis-Menten equation is velocity=Vmax[S] / ([S] + Km)
   • if the substrate dissociates quicker than the rate of product formation Km becomes the substrate dissociation constant
     • if this occurs then the smaller Km the higher the affinity of the substrate to the enzyme
   • it is difficult to measure Vmax because there is poor solubility of substrate at high concentrations
   • the Lineweaver-Burk plot removes the issue of insolubility
     • on the plot
       • y axis = 1/v
       • x axis = 1/[S]
       • the slope = Km/Vmax
       • the intercept = 1/Vmax

2. Allosteric Enzymes

   • allosteric - the binding site for the effector molecule is not the active site of the substrate
   • co-operative binding = occurs if the number of binding sites of a molecule that are occupied by a specific type of ligand is a non-linear function of this ligand's concentration
   • cooperative binding produces a sigmoidal graph of initital reaction rate against substrate concentration
   • mechanisms that control enzyme activity
     • feedback inhibition by a product
     • gene expression - amount of enzyme altered
     • phosphorylation/dephosphorylation
     • proteolysis - irreversible activation and altering life span through changes in rate of enzyme degradation

3. Inhibition of Enzyme Activity

   • irreversible inhibitors
     • form a covalent or they bind too tightly so dissociation is extremely slow
     • enzyme activity can only be restored by synthesis of new enzymes
   • reversible inhibitors
     • competitive
       • have a similar shape too the substrate
       • bind to the active site
       • can be overcome by increasing the substrate concentration
       • alter the slope of the Lineweaver-Burk plot but not the y axis intercept
       • Vmax remains the same
       • Km changes
     • non-competitive
       • bind to a site different to the active site of the substrate
       • can not be overcome by increasing substrate concentration
       • decrease the efficiency of the enzyme rather than stopping the reaction
       • Vmax changes
       • Km doesn't change
       • alter the slope of the Lineweaver-Burk plot but not the x axis intercept
   • organophosphates
     • target enzyme - acetylcholinesterase
     • uses - chemical warfare, insecticides
     • physiological effects
       • nerve agents
       • produce a cholinergic toxidrome as a result of excessive activation of the PSNS
       • DUMBBELS
         • D - diarrhoea
         • U - urination
         • M - miosis
         • B - bronchorrhoea
         • B - bronchospasm
         • E - emesis
         • L - lacrimation
         • S - salivation

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