Enzymes

Enzymes

1. Biological catalysts

   • 'Biological' because they are made by living cells
   • 'Catalysts' because they speed up reactions without being changed/used up in the reaction
   • Types of Reactions
     • Anabolic
       • Building up molecules
         • E.g protein synthesis
     • Catabolic
       • Breaking down molecules
         • Digestion
   • Metabolic pathways
     • A sequence of enzyme controlled reactions in which the product of one reaction becomes the reactant in the next
   • They lower the amount of activation energy needed to make a reaction happen

2. Structure

   • Tertiary Structure proteins
     • Have:
       • Hydrogen bonds
       • Ionic Bonds
       • Disulphide Bridges
       • Hydrophobic interactions
     • If any of the bonds are broken the shape of the active site will change
     • 3d Shape
   • Globular (Metabolically active)
   • Soluble because hydrophilic R groups are on the outside of the structure
   • Sites of enzyme action
     • Extracellular
       • Enzymes are secreted from cell via exocytosis
         • E.g Amylase & Saprotrophic Fungi
           • Amylase made in salivary glands and released into mouth to act on food. Saprotrphic Fungi- secretes amylase,lipase and proteases on their food to be digested
     • Intracellular, in solution
       • E.g the enzymes that catalyse glucose breakdown in glycosis
     • Intracellular, Membrane bound
       • Attached to the membrane
         • E.G cristae in mitochondria

3. Active site models

   • Temporary bonding creates an enzyme-substrate complex which allows for the reaction
   • Lock & Key Model
     • Emil Fischer-1894
     • Each enzyme is substrate-specific
       • Enzymes can only catalyse 1 type of reaction
         • Needs to be a successful collision between the enzyme & substrate
   • Induced Fit model
     • Where the active site changes shape to accommodate the substrate
     • E.g Lysozymes

4. General Process

   • Enzyme & Substrate mixed together
     • Enzymes & Substrate in constant motion & collide
       • Substrate binds w/active site (in a successful collision)
         • Substrate is modified & then released
           • Active sites refill w/more substrate
             • The rate of reaction depends on No. of free enzymes
               • if all other conditions are optimal The enzyme concentration becomes the limiting factor in the above scenario
               • As reaction proceeds there is less substrate & more product
                 • The the substrate concentration will become the limiting factor
                 • Eventually, all substrate used up & graph line flattens out

5. Factors affecting enzyme action

   • Tempurature
     • Increasing temp will increase K.E of enzyme & substrate
       • Leading to more successful collisions
         • This can increase the rate of reaction
           • The optimum for most enzymes is roughly 37'c
             • Above this temp the increasing vibrations break the weak hydrogen bonds
               • Causing the enzymes to denature
             • At Low temps the enzyme is inactivated
   • pH
     • Range of optimum pH is small
       • Changes affect the ionic & hydrogen bonds
         • Small deviations cause minor changes which are reversible & only inactivate the enzyme
           • Large Deviations cause denaturation
   • Substrate Concentration
     • IF ENZYME CONCENTRATION IS CONSTANT
     • It is a key limiting factor
       • Higher concentrations means quicker rate of reaction until a point
         • As all the active sites are saturated adding more substrate will not increase the rate
   • Enzyme Concentration
     • As the enzyme concentration increases the rate of reaction increases

6. Enzyme Inhibition

   • Competitive
     • Have a molecular shape complementary to the active site
       • Temporarily taking the position of the substrate
         • The effect of the inhibitor can be overcome by increasing the substrate concentration
   • Non-competitive
     • Bind to the allosteric site of enzymes (i.e a site other than the active site)
       • Therefore they do not compete w/the substrate
         • They affect bonds and alter the overall shape of the enzyme
           • Meaning the substrate cannot bind with active site
             • As the inhibitor concentration increases, more enzymes denature
               • Leading to a slower rate & less product produced
                 • E.g Heavy metal ions (Pb lead, arsenic As)/Cyanide
                   • Some non-competitive inhibitors bind reversibly & irreversibly

7. Immobilised Enzymes

   • Are enzymes that are trapped on an inert matrix
     • E.g a gel capsule or in alginate beads
   • Example of reactions
     • Fermentation
     • Making lactose-free milk
     • In washing powders
       • Some people are allergic to biological washing powders. By encapsulating the enzymes. These are released only when being washed 
     • Making Baby food
       • Pectinases break down pectin in cell walls releasing the fruit juice etc
     • In Biosensers
   • Advantages
     • Products can be easily recovered without contaminating the enzyme
     • The immobilised enzymes can be reused (lowering costs)
     • They can tolerate a wider range of conditions enzymes are more stable when immobilised
       • E.g temp and pH
     • Enzymes can be removed & added which means that there is more control over the rate of reaction
     • Can be used in a continuous production systems
   • Disadvantages
     • Works more slowly than free enzymes
     • If encapsulated in beads- Substrate has to penetrate beads to contact enzyme: meaning sometimes only surface enzymes catalyse reactions

8. Biosensors

   • Devices which measure the concentration of a specific metabolite in a mixture
     • When the mixture passes over the enzyme, a reaction occurs
       • The energy released is proportional to the concentration of the substrate
         • This is converted into an electrical impulse
           • An accurate digital display is shown
             • Because enzymes are specific to a given substrate the results are highly accurate even at low concentrations
               • This can be used to diagnose & monitor those w/diabetes
   • Advantages
     • Can be quantitive
     • Can detect very small amounts of molecules
     • Detects only a specific product
     • Accurate

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