Enzymes - Unit 2 OCR

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  • Enzymes
    • Action of Enzymes
      • Biological Catalysts
        • Speed up chemical reactions, catalyst is a substance that speeds up chemical reactions without being used up itself
        • Enzymes catalyse metabolic reactions in your body, action can be intracellular or extracellular
      • Structure
        • Globular proteins - active site with specific shape to determine enzymes tertiary structure, active site is part where substrate binds to
        • For enzyme to work substrate must fit in active site, if shape doesn't match active site reaction won't be catalysed meaning enzymes are very specific and work with very few substrates (usually one)
        • When substrate binds to active site an enzyme-substrate complex is formed
      • How They Speed Up Reactions
        • In chemical reaction certain amount of energy needs to be supplied before the reaction will start (activation energy) usually as heat
        • Enzymes reduce amount of activation energy needed often making reactions happen at lower temperatures than without enzyme
        • When substance binds to active site a enzyme-substrate complex is formed, lowering the activation energy
          • If two substrates need to join, attaching to enzyme holds them close reducing any repulsion so they bond easier
          • If enzyme catalysing a break down there is a strain on bonds in substrate to fit into active site meaning the substrate breaks easier
      • Models of Action
        • Lock and Key Model - substrate fits into enzyme in the same way a key fits in a lock, active site and substrate have complimentary shape
          • Doesn't give full story as although they must fit together initially, the complex changes shape slightly to complete the fit, locks substrate tighter
        • Induced Fit Model - substrate doesn't only have to be the right shape to fit it has to make the active site change shape in the right way
          • Prime example of how widely accepted theory can change when new evidence found
    • Factors Affecting Activity
      • Temperature - rate of enzyme controlled reaction increases as it increases so substrate molecules more likely to collide, energy of collision increases so more likely to end in reaction, rate increases until enzyme reaches optimum temperature where rate of enzyme controlled reaction is at its fastest
        • If it gets too high the reaction stops, rise makes enzymes vibrate more and if it goes above a certain level the vibration breaks the bonds holding enzyme in shape, the active site shape changes so substrate no longer fits causing the enzyme to denature : no longer function as a catalyst
      • pH - all have optimum pH value where rate of reaction is at its fastest, most human ones work best at pH 7 with exceptions, pepsin works best at pH 2 which is useful as found in stomach, above and below optimum the H+ and OH- ions in acid and alkalis can break ionic and hydrogen bonds that hold tertiary structure in place causing change in active site shape : denature
      • Enzyme Concentration- more enzymes there are in solution the more likely a substrate is to collide, increasing concentration increases rate of reaction, if amount of substrate is limited it reaches a point where there's more enzyme to deal with so adding more has no effect
      • Substrate Concentration- higher this is the faster the reaction, more substrate means collision is more likely so more active sites are used up, only true until saturation point where after there are so many substrate that enzymes have as much as they can cope with so adding more has no effect : enzyme concentration is limiting factor
      • Measuring the Rate
        • Measure how fast the product of reaction appears and use it to compare rate of reaction under different conditions
          • E.g. Catalase catalyses breakdown of hydrogen peroxide into water and oxygen,  collect oxygen produced and measure how much is given off in set period of time
        • Measure the disappearance of a substance over time and use it to compare rate of reaction in under different conditions
          • E.g. Amylase catalyses breakdown of starch to maltose, detect starch using potassium iodide and iodine, time how long it takes for sample of starch to disappear by adding iodine regularly, use times to compare between rates
    • Cofactors and Inhibitors
      • Cofactors and Coenzymes - some enzymes only work when another non-protein substance is bound to them : these non-proteins known as cofactors
        • Inorganic Cofactors - work by helping enzyme and substrate bind together, don't directly participate in reaction so aren't used up or changed : Manganese ions found in hydrolase
        • Organic Cofactors (Coenzymes) - participate in reaction and change as result, often act as carriers moving chemical groups between enzymes, continually recycles during process
      • Enzyme Inhibitors - prevents activity by binding to enzyme they inhibit, can be competitive or non-competitive
        • Competitive - similar shape to substrate, compete with it to bind to active site but don't react, block the active site so no substrate can fit, how much of active site inhibited depends of relative concentrations of inhibitor and substrate, high concentration of inhibitor makes it take up nearly all of active site and hardly any substrate reaches enzyme
        • Non-Competitive - bind to enzyme away from active site to change its shape so substrate cannot bind, don't compete with substrate for active site because they are different shape, increasing substrate concentration won't make a difference as activity will still be inhibited
        • Reversible and Non-Reversible - some can be reversed so not permanent, which one they are depends on bonds between enzyme and inhibitor : strong covalent bonds mean cannot be removed easily and are irreversible, weaker hydrogen or ionic bonds mean can be removed so reversible
      • Metabolic Poisons - interfere with metabolic reactions causing damage illness or death, often enzyme inhibitors
        • E.g. Cyanide is a non-competitive of cytochrome c oxidase (enzyme that catalyses respiration reaction), cells that can't respire die
          • E.g. Malonate is a competitive of succinate dehydrogenase
            • E.g. Arsenic is a non-competitive of pyruvate dehydrogenase
      • Drugs - some medicinal drugs are enzyme inhibitors
        • E.g. Some antiviral drugs  like reverse transcriptase inhibitors used to treat HIV work by inhibiting enzyme reverse transcriptase that catalyses replication of viral DNA, preventing the virus replicating
          • E.g. Some antibiotics like penicillin inhibits enzyme transpeptidase which catalyses formation of protein in bacterial cell wall, weakens the cell well preventing bacterium regulating its osmotic pressure so cell bursts and bacterium is killed


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