Enzymes

• Define the terms "enzyme", "substrate", and "product"

  Enzyme - Biological catalysts that interact with substrate molecules to facilitate chemical reactions. Usually globular proteins.Substrate - A substance used, or acted on, by another process or substance such as a reactant in an enzyme catalysed reaction.Product - The end result of the reaction; enzymes act on substrates and convert them into products.
• Explain why enzymes are necessary to life

  Enzymes control the processes and reactions of metabolism. Without them many reactions would take place too slowly to keep organisms alive.
• Define the terms "anabolic reactions", "catabolic reactions", "digestion" and "metabolism"

  Anabolic reactions - Reactions of metabolism that construct molecules from smaller units, requiring energy from the hydrolysis of ATPCatabolic reactions - Reactions of metabolism that break down molecules to form smaller units releasing energy.Digestion - The catabolic process in the digestive tract where ingested food is converted into simpler, soluble and diffusible substances that can be assimilated by the body.Metabolism - the chemical processes that occur within a living organism in order to maintain life
• Explain how enzymes can affect both the structure and function of cells and whole organisms


  Enzymes control the processes inside cells, if those chemical reactions cannot take place, the cell cannot function. If many cells are affected then the organism will feel the effects.
• Define the terms "intracellular enzyme" and "extracellular enzyme". Give one example of an intracellular enzyme and two examples of extracellular enzymes

  Intracellular enzyme - Enzymes that act inside the cell e.g. DNA polymeraseExtracellular enzyme - Enzymes that are secreted and act outside cells e.g. amylase, lipase (digestive enzymes)
• State the substrates and products for the enzymes catalase, amylase and trypsin

  Catalase - Hydrogen peroxide into oxygen and waterAmylase - Starch into sugarsTrypsin - Proteins into polypeptides (which are further hydrolysed into amino acids by other enzymes)
• Explain the role of extracellular enzymes in general. Summarise the digestion of starch and of proteins as examples of the role of extracellular enzymes

  Components for reactions often come from larger molecules that cannot enter cells but must be broken down so that the monomers can be used for metabolism. Starch is broken down into individual individual glucose monomers that can be used for respiration. Proteins are broken down into individual amino acids which can be used within cells to build the specific proteins needed.
• Define the terms "active site", "complementary shape", and "specific"

  Active site - Area of an enzyme with a shape complementary to a specific substrate allowing the enzyme to bind a substrate with specificityComplementary shape - The shape of the active site and the substrate match so that they fit togetherSpecific - Each enzyme has a single substrate that it works on and that will fit its active site (or group of substrates - group specificity)
• Explain why an enzyme only catalyses one type of reaction

  A substrate will only fit into an active site if it is complementary in both shape and charge and the reaction is only catalysed if the substrate binds with the active site. This means that each enzyme only catalyses one type of reaction.
• State the sequence of events in an enzyme-controlled reaction

  Substrate fits into active site to form an enzyme-substrate complex. The active site changes shape to accommodate the substrate and the complementary charges help the 'fit'.The active site changes shape to convert the substrate into products. This is now an enzyme-product complex.The products no longer complement the active site and are released. The active site is now free for another substrate.
• Describe the "lock and key" hypothesis of enzyme action

  The active site is the lock, the substrate the key. One key fits one lock. This assumes that both active site and substrate have a complementary fixed shape.
• Describe the "induced-fit" hypothesis of enzyme action

  This has replaced lock and key. It assumes the active site changes shape slightly to fit around the substrate. The analogy used is a hand and a glove. The glove (active site) moulds around the hand (substrate).
• Suggest how the R-groups of amino acids are involved in catalysing reactions

  The active site may involve only a small number of amino acids but the R groups are key to the function. The interactions between them cause the shape which must be complementary to the substrate. They also may be charged which must be complementary.
• Define the term "activation energy" and "rate of reaction"

  Activation energy - the energy required to initiate a reactionRate of reaction - how quickly or slowly the reaction takes place - measure of the change in concentration of the reactants or the change in concentration of the products per unit time
• Draw an energy-level graph to show how a reaction progresses with and without an enzyme present (the transition state model)

  
• State what the presence of an enzyme does to the activation energy for the reaction and explain why this increases the rate of reaction

  Enzymes lower the activation energy required for the reaction making it easier for it to take place and therefore happen faster.
• State 5 factors that affect the rate of an enzyme controlled reaction

  Temperature, pH, substrate concentration, enzyme concentration, inhibitors
• Draw a graph showing how temperature affects the initial rate of an enzyme-controlled reaction

  
• Explain why increasing the temperature from below the optimum up towards the optimum increases the rate of reaction

  As temperature rises, enzymes and substrate molecules have more kinetic energy and move faster, forming more enzyme-substrate complexes
• Define the term "temperature coefficient, Q10" and state its usual value for enzyme controlled reactions


  A measure of how much the rate of reaction increases with a 10°C temperature increase. Enzyme reactions normally 2 (rate doubles every 10°C increase)
• Explain why increasing the temperature up from the optimum decreases the rate of reaction abruptly

  Enzyme denatures which changes the shape of the active site so that the substrate cannot fit.
• Explain why Siamese cats are white with black tails, ears, paws and faces

  They have a mutation in the enzyme that catalyses melanin production that means it is denatured at normal body temperature. The extremities of the cat such as the tail however are cooler so melanin is produced and the fur is a darker colour.
• Draw a graph showing how pH affects the initial rate of an enzyme-controlled reaction

  
• Explain why a pH change away from the optimum decreases the rate of reaction

  Enzyme structure depends on interactions of R groups. pH alters charges and therefore bonds, changing the denaturing the enzyme and changing the shape of the active site.
• Draw a graph showing how substrate concentration affects the initial rate of an enzyme-controlled reaction

  
• Define the term "Vmax"

  Maximum initial velocity or rate of an enzyme-catalysed reaction
• Explain how increasing the substrate concentration affects the initial rate of an enzyme-controlled reaction

  As substrate concentration increases, the rate of reaction also increases as there are free enzyme molecules to combine with the extra substrate. At the saturation point rate cannot increase further due to the enzymes working 'flat out' - there are no longer any free enzymes to combine with extra substrate so increasing the concentration has no effect.
• Draw a graph showing how enzyme concentration affects the initial rate of an enzyme-controlled reaction

  
• Explain how increasing the enzyme concentration affects the initial rate of an enzyme-controlled reaction

  As enzyme concentration increases, rate increases as there is free substrate to react. Rate will continue to increase until there is no longer an excess of substrate and any extra enzymes are left free as there is no substrate to bind with.
• Describe and explain how to investigate any of the factors that affect the rate of enzyme-controlled reactions

  Using catalase, hydrogen peroxide and measuring the rate by measuring the volume of oxygen produced in a certain amount of time. Change the conditions by using buffer solutions or water baths etc.

  
• Define the term "cofactor" and "coenzyme" and give two possible roles of cofactors/coenzymes

  Cofactor - non-protein components necessary for the effective functioning of an enzymeCoenzyme - If the cofactor is an organic molecule it is known as a coenzymeThey may form part of the active site or transfer atoms or groups from one reaction to another in a multi-step pathway
• Describe the similarities and differences between cofactors, coenzymes and prosthetic groups

  Cofactors are inorganic and are not necessarily permanently bound to the enzymeCoenzymes are organicProsthetic groups are permanently bound to the enzyme
• Explain why the chloride ion necessary for the correct formation of the active site in amylase is called a cofactor not a coenzyme or prosthetic group


  It is not permanently bound to the enzyme. It is an inorganic ion.
• Explain why the zinc ion that forms an important part of the structure of carbonic anhydrase (an enzyme necessary of the metabolism of carbon dioxide) is called a prosthetic group not a cofactor or coenzyme.


  It is an inorganic ion permanently bound to the enzyme.
• Give two examples of coenzymes that are synthesised from vitamins in our diet

  NAD is synthesised from vitamin B3 and used in respiration to transfer hydrogen Coenzyme A is synthesised from vitamin B5 and used in the breakdown of carbohydrates and fatty acids in respiration.
• Describe 4 ways in which multi-step reaction pathways can be regulated by cells

  Inhibitors - competitive, non-competitiveEnd product inhibitionInactive precursor enzymes
• Define the terms "enzyme inhibitor", "competitive inhibitor", "non-competitive inhibitor", "reversible inhibitor", "irreversible inhibitor" and "allosteric site"


  Enzyme inhibitor - a factor that prevents or reduces the rate of an enzyme catalysed reactionCompetitive inhibitor - an inhibitor that competes with the substrate to bind to the active site of an enzymeNon-competitive inhibitor - an inhibitor that binds to an enzyme at an allosteric siteReversible inhibitor - An inhibitor that can bind temporarily to the enzyme but also dissociate itself so the enzyme returns to being functionalIrreversible inhibitor - An inhibitor that once bound to the enzyme cannot dissociate and permanently stops the enzyme functioningAllosteric site - The place on an enzyme where a molecule that is not a substrate may bind, thus changing the shape of the enzyme and influencing its ability to be active.
• Explain how a competitive inhibitor affects the rate of an enzyme-controlled reaction.

  They have a similar shape to the substrate so enter the active site forming an enzyme-inhibitor complex. No product is formed but while the inhibitor is in the active site, the substrate can't enter.
• State two examples of competitive inhibitors and describe their action

  Statins are competitive inhibitors of the enzymes used to produce cholesterol so are prescribed to help lower cholesterolAspirin inhibits the production of chemicals responsible for pain and fever
• Explain how a non-competitive inhibitor affects the rate of an enzyme-controlled reaction

  Do not bind to active site but to a different part of the enzyme, distorting the tertiary structure which changes the shape of the active site so that the substrate can't fit.