Enzymes

• ·       nzymes are biological catalysts – They speed up reactions that would otherwise take much longer at lower temperatures and pressure.

  ·       They are globular proteins (spherical proteins that are usually water soluble).

  ·       Anabolic reactions - building up.

  ·       Catabolic reactions - breaking down.

  ·       Metabolism- All the reactions occurring in a cell or an organism .
• ·       Activation energy - the energy needed to cause a reaction to occur.

  ·       Activation energy can be so high it prevents a reaction from occurring.

  ·       Enzymes lower the activation energy
• Lock and Key Theory

  ·       The tertiary structure of an enzyme has a site that the substrate fits into - the active site.

  ·       Only a specific substrate fits into the active site, like a lock and key.

   

  ·       The substrate is held in the active site by a range of bonds (hydrogen bonds and ionic bonds). This forms the enzyme substrate complex.

  ·       The substrate is held in a way so that that the right atom groups are close enough to react with one another.
• Lock and Key 2:

     As well as this the R – groups within the active site interact with the substrate forming temporary bonds. This put strain on the bonds in the substrate, thus lowering the activation energy.

  ·       The opposite occurs when an enzyme is used to build up materials.
• Induced fit hypothesis:

  ·       This is a modified version of the lock and key hypothesis.

  ·       Recent evidence shows that the enzyme’s active site changes slightly as the  substrate enters.

  ·       The initial interaction between the enzyme and the substrate is relatively weak.

  ·       These weak interactions cause changes in the enzymes tertiary structure.

  ·       As the enzyme changes shape the bonds between the enzyme and substrate strengthen.

  ·       This puts a strain on the substrate and weakens the bonds within it, causing a lowering of the activation energy.
• Intracellular anabolic reactions

  ·       This can make polymers from monomers eg the production of glycogen by glucose -1-phosphate uridylyltransferase.

  ·       The enzyme holds the substrates together reducing any repulsion between them, this then allows them to bond.
• Intracellular catabolic reactions

  ·       Hydrogen peroxide (H2O2)   is a toxic by product of cellular reactions.

  ·       Capitalise breaks down hydrogen peroxide into oxygen O2 and water H2O.
• Extracellular enzymes

  ·       Amylase breaks down starch into maltose and Maltase breaks down maltose into glucose.

  ·       Trypsin catalyses the hydrolysis of peptide bonds, breaking down large polypeptides into smaller ones.
• Temperature

  ·       As the temperature rises so does the kinetic energy

  ·       There are more collisions between the enzyme and substrate meaning more enzyme substrate complexes are formed

  ·       A faster reaction.
• Temperature coefficient Q 10 of a reaction

  ·       It is a measure of how much the rate of reaction increases with a 10 oC rise in temperature.

  ·       In enzyme controlled reactions this usually causes the reaction rate to double.
• Denaturing enzymes

  ·       Very high temperatures caused the weak bond in the enzyme (protein) (hydrogen and ionic bonds) to break.

  ·       Causing the active site to lose its shape and the substrate to no longer fit
• Optimum temperature

  ·       The temperature at which the enzyme has the highest rate of activity.

  ·       For most enzymes in the human body this is around 40 oC.

  ·       Thermophilic bacteria (found in hot springs) Have an optimum temperature of 70 oC.

  ·       Psychrophilic Organisms eg bacteria found in Arctic regions have an optimum temperature of 5 oC.
• ·       When enzymes denature, there only needs to be a slight change in the active site for the enzyme not to work.

  ·       This happens to all of the enzymes at around the same temperature so the rate of reaction drops rapidly.
• pH

  ·       Enzymes are affected by changes in pH.

  ·       Hydrogen bonds and ionic bonds form between the R groups in amino acids holding the protein in its specific 3D shape.

  ·       Changes in pH outside the optimum pH cause the active site to change shape. If the change in pH is only small and returns to the optimum pH. The enzyme will return to its original shape – renaturation.
• Denaturing enzymes

  ·       Large changes in pH irreversibly change the active site.

  ·       Hydrogen ions interact with polar and charged R groups.

  ·       Changing of the concentration of hydrogen ions changes the degree of this interaction, and how the R groups interact with each other.

  ·       As the pH changes the R groups are less able to interact with one another and the bonds break.

  ·       As Hydrogen ions are charged, they also disrupt the hydrogen bonds in the enzyme.
• ·       The most substrate available the higher the collision rate, therefore the faster the reaction.

  ·       The more enzymes available, the more active sites that are not occupied, therefore the more enzyme substrate complexes that can be formed.
• ·       V max - the maximum rate of an enzyme controlled reaction.

  ·       The V max can be increased by adding more enzymes or increasing the temperature.  Both of these would increase the rate of reaction but would be limited by the substrate concentration
• Control of metabolic activity in cells

  ·       If all the reactions were happening in a cell at the same time chaos would occur.

  ·       Reaction pathways are controlled by different enzymes, therefore if you control the activity of the enzymes you can regulate product formation.

  ·       Enzymes can be activated by cofactor or coenzymes or in activated by inhibitors.
• Inhibitors

  ·       There are two types of inhibitors – competitive and non competitive.

  ·       Inhibitors can be reversible or irreversible.

   

  Reversible inhibitors

  ·       These tend to bind to enzymes with weak bonds such as hydrogen bonds that can be broken easily.

  Irreversible inhibitors

  ·       Attach to the enzyme with strong covalent bonds that are difficult to break without damaging the enzyme.
• Competitive Inhibitor:

  ·       The inhibitor has a similar shape to the substrate.

  ·       It fits into the active site and blocks the substrate.

  ·       As the inhibitor and the substrate are competing for the active site the inhibitor will slow down the rate of reaction.
• Competive inhibitor 2:

  ·       The degree of inhibition is determined by the relative concentration of the substrate and the inhibitor.· A competitive inhibitor reduces the rate of the reaction but it does not change the V max.

  · If the substrate concentration is increased then as the active site becomes available more substrate molecules than inhibitor molecules are in the vicinity ‘waiting’ to attach to the active site.
• Non-competitive inhibitor:

  • the inhibitor binds to the enzyme at a location other than the active site - the allosteric site
  • the binding of the inhibitor causes the tertiary structure of the enzyme to change
  • the active site is no longer complementary to the substrate
• End – Product Inhibition

  ·       These help to control metabolic pathways.

  ·       The product of the reaction inhibits the enzyme that produces it.

  ·       It is a form of negative feedback.

  ·       This means that excess products are not made and resources are not wasted.
• Cofactors, Coenzymes and Prosthetic groups

  ·       Non protein ‘helper’.

  ·       They may form part of the active site of the enzyme or they may transfer atoms or groups from one reaction to another.

  o   Coenzymes – organic e.g. vitamins

  o   Cofactor – inorganic  e.g. zinc, iron, copper

  o   Prosthetic group – Bind tightly and permanently to the active site
• Cofactor

  ·       Amylase contains a chloride ion that is necessary in creating the correct shape of the active site so it can break down the starch.

  Coenzymes

  ·       Vitamin B3  is used to synthesise NAD ( a coenzyme used to transfer Hydrogen atoms between molecules involved in respiration.
• Prosthetic group

  ·       Similar to cofactors and coenzymes as they are needed for certain enzymes to work. However, they are bound tightly to the enzyme.

  ·       Can be organic or inorganic

  ·       E.g. Zinc ions sit in the active site of the carbonic anhydrase allowing it to convert carbon dioxide into carbonic acid, protons and bicarbonate ions.
• Inactive precursor enzymes

  ·       Enzymes secreted from cells in their inactive form.

  ·       This prevents damage to the cell.

  ·       The precursor enzymes then have their tertiary structure changed allowing the active site to accept substrates.

  ·       The change can be due to the addition of a cofactor, a change in the pH, a change in the temperature or by the action of an enzyme.
• Activation by another enzyme

  ·       Protease cleaves bonds in the enzyme causing it to be activated.

  Activation of enzymes by a change in pH

  ·       Pepsinogen is converted to pepsin by the low pH in the stomach.

  ·       The Hydrogen ions cause bonds to break in the enzyme changing it from it’s inactive form of pepsinogen to its active form of pepsin.