Enzymes

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Introduction to enzymes

Enzyme- Enzymes are globular proteins, with a specific tertiary structure, which catalyse metabolic reactions in living organisms.

Enzymes are also known as biological catalysts because they speed up chemical reactions.  They have a very specific 3D shape which is determined by their tertiary structure.  The active site is the most important part of an enzyme.  This is where the enzyme’s substrate binds.  One theory of enzyme action is called the lock and key theory because the enzyme’s active site and the substrate are complementary in shape and charge.

Intacelluar Enzymes- Enzymes that catalyse reactions inside of cells. e.g Catalase, ATPsynthase, ATPase, DNA helicase, DNA polymerase, RNA polymerase, Lysosome hydrolytic enzymes

Extracellular Enzymes- Enzymes that catalyse reactions outside of cells.e.g digestive enymes

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Lock and Key theory & Induced fit theory

Lock and Key-

  • substrate has a complementary shape and charge to active site
  • substrate binds to active site and forms an enzyme/substrate complex
  • substrate is converted into product
  • product is no longer complementary in shape and charge and so is released

Induced Fit-

  • substrate binds to active site
  • active site shape changes
  • to give a closer fit between active site and substrate
  • more bonds form between substrate and active site
  • forms enzyme/substrate complex
  • change in shape of active site destabilises / weakens bonds in substrate
  • activation energy reduced
  •  further shape change of active site / enzyme after products form
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Activation energy

Enzymes work by lowering the activation energy needed for the reactions they catalyse.

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Measuring rate of enzyme controlled reaction

Measure the rate at which the reactants are used or products are formed.  Gases are easiest to measure therefore the rate of this reaction can be measured by:

  1. The rate at which O2 is used up.

  2. The rate at which CO2 is formed.

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Effect of temp on enzymes controlled reaction

Description:As temperature increases, rate of reaction increases.

Explanation:Enzyme and substrate molecules gain more kinetic energy. Results in more successful collisions. More enzyme/substrate complexes formed per unit time. More product formed.

Description:As temperature increases beyond the optimum temperature, rate of reaction decreases.

Explanation:Enzyme molecules gain more kinetic energy.Tertiary structure bonds vibrate so much they break.The enzyme loses its specific 3D shape.The active site changes shape/denatures.The substrate can no longer fit as its no longer complementary to the active site.Results in fewer successful collisions.Fewer enzyme/substrate complexes formed per unit time.Less product formed.

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Effect of PH

Description:As pH decreases away from optimum and becomes more acidic, rate of reaction decreases.

Explanation:More H+ ions present disrupt bonds within enzyme active site. Active site and substrate no longer complementary and do not join. Fewer enzyme/substrate complexes formed per unit time and less product formed.

Description:As pH increases away from optimum and becomes more alkaline, rate of reaction decreases.

Explanation:More OH- ions present disrupt bonds within enzyme active site.Active site and substrate no longer complementary and do not join. Fewer enzyme/substrate complexes formed per unit time and less product formed.

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Effect of Enzyme Concentration

Section A Description:Increasing enzyme concentration increases rate.

Explanation:As enzyme concentration increases, more active sites become available.More enzyme/substrate complexes (and therefore product) can form, so rate of reaction increases.

Section B Description:Increasing enzyme concentration does not affect rate.  Rate stays constant.

Explanation:All substrate molecules are occupying enzyme active sites. At any one time all substrates are in an active site. The rate of reaction is the maximum possible for the fixed substrate concentration.

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Effect of Substrate Concentration

Section A Description:Increasing substrate concentration increases rate.

Explanation:As substrate concentration increases, more collisions occur between enzymes and substrate molecules.More enzyme/substrate complexes (and therefore product) can form, so rate of reaction increases.

Section B Description:Increasing substrate concentration does not affect rate.  Rate stays constant.

Explanation:All the enzyme molecules present are forming enzyme/substrate complexes as fast as possible.At any one time all active sites are occupied.The rate of reaction is the maximum possible for the fixed enzyme concentration.

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Competitive & Non- Competitive inhibitors

Inhibitor- Any substance or molecule that slows down the rate of an enzyme controlled reaction by affecting the enzyme molecule in some way.

Effect of competitve inhibitor on rate of reaction-

  • rate decreases
  • inhibitor and substrate have similar shape / structure
  • both will fit into the enzymes active site as both have complementary shape to active site
  • substrate molecule is blocked from entering active site
  • fewer enzyme/substrate complexes form

Effect of non- competitive inhibitor on rate of reaction

  • rate decreases
  • fits into allosteric site / a place other than active site
  • active site changes
  • substrate no longer fits / complementary to active site
  • enzyme/substrate complex does not form
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Drugs and Metabollic poisons

Penicillin- Inhibits a bacterial enzyme responsible for forming the cell’s peptidoglycan cell wall. Bacteria exposed to Penicillin have incomplete and weak cell walls; in solutions of high water potential the cells lyse and die.

Digitalis- A chemical originally extracted from the plant Foxglove, it is now used to treat irregular heart beats. Digitalis is a Non-Competitive inhibitor which binds to an enzyme involved in restricting cardiac muscle contraction.

Alpha- amanitin- A chemical found in the extremely toxic Death Cap Mushroom. It acts as a Non-Competitive Inhibitor for the enzymes DNA Polymerase and RNA Polymerase, involved in DNA replication and Transcription. In the presence of the inhibitor the cells are unable to make proteins  and therefore quickly die. 


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Cofactors

Cofactor- Any substance that must be present to ensure enzyme-controlled reactions take place at the appropriate rate.  Some cofactors are a part of the enzyme (prosthetic groups); others affect the enzyme on a temporary basis (coenzymes and inorganic ion cofactors).

Many enzymes consist of a protein and a non-protein (called a cofactor). These enzymes need a cofactor in order to function. Cofactors may be:

  • charged metal ions (activators), which temporarily bind to the active site of the enzyme,
  • organic molecules, usually vitamins or made from vitamins (coenzymes), which are not permanently bound to the enzyme molecule, but combine with the enzyme-substrate complex temporarily.
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Cofactors examples

Chloride ions (activator) -Amylase enzymes must contain a Cl - to adopt their functional shape, allowing the enzyme to hydrolyse Amylose to Maltose.

Calcium ions (activator)- Thrombin enzymes are responsible for blood clotting. The enzyme must contain a Ca2+ to change soluble fibrinogen proteins into insoluble fibrous fibrin proteins to form an effective clot.

Coenzyme A (coenzyme)- In respiration the substrate Pyruvate is broken down to Acetyl. Acetyl is very unstable and reactive, so Co-enzyme A helps remove it from the enzyme’s active site in a stable manner.

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