2.1.4 Enzymes

• Define the terms anabolic reactions, catabolic reactions, digestion and metabolism.

  • Anabolic reaction: reactions of metabolism that construct molecules from smaller units, requiring energy from the hydrolysis of ATP
  • Catabolic reaction: 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 simple, soluble and diffusible substances
  • 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 and if those chemical reactions cannot take place, the cells cannot function. If many cells are affected, then the organism will feel the effects.
• Define the terms intracellular and extracellular enzymes and Give one example for each.

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

  • catalase: hydrogen peroxide into oxygen and water
  • amylase: starch into sugars
  • trypsin: proteins into polypeptides, and then 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.


  components for reactions often come from larger molecules that cannot enter cells, but must be broken down, so that the monomers are used for metabolism.

  starch is broken down into individual monomers of glucose that can be used for respiration. proteins are broken down into individual monomers of amino acids, which can be used within cells to build the specific proteins needed.
• State the sequence of events in an enzyme-controlled reaction.


  substrate fits into the active site to form an enzyme-substrate complex. the active site changes shape to accommodate the substrate. the active site then changes shape to convert the substrates into products and it is now an enzyme-product complex. the products are no longer complementary to the active site and are released. the active site is now free for another substrate.
• Suggest how the R-groups of amino acids are involved in catalysing reactions.

  • active site may involve only a smaller number of amino acids, but R-groups are the key to the function
  • interactions between different R-groups cause the active site to change shape, which must be complementary to the substrate
  • also may be charged, which must be complementary
• State 5 factors that affect the rate of an enzyme-controlled reaction

  • temperature
  • pH
  • substrate concentration
  • enzyme concentration
  • inhibitors
• Explain why increasing temperature from below the optimum increases the rate of reaction.


  As temperature increases, enzymes and substrate molecules have more kinetic energy and move faster. This means that they form more enzyme-substrate complexes.
• Explain why a pH away from the optimum decreases rate of reaction.


  Enzymes' structure depends on interactions of R-groups. pH alters charges and therefore bonds, which changes the enzyme. This then results in the denaturation of the enzyme and changes shape of the active site.
• Explain how increasing substrate conc. affects the initial rate of enzyme-controlled reaction


  As substrate concentration increases, the rate of reaction also increases because there are free enzyme molecules to combine with the substrates. At the saturation point, the rate cannot increase any further due to enzymes working 'flat out'. There are no longer any free enzymes to combine with the extra substrate, so increasing the concentration will no longer have any effect.
• Explain how increasing the enzyme conc. affects the initial rate of an enzyme-controlled reaction.


  As the enzyme concentration increases, the rate increases, as there are free substrates to react with. The rate will continue to increase until there is no longer an excess of substrate. Any extra enzymes are left free as there is no substrate to bind with.
• 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 enzyme
  • coenzyme: if a cofactor is an organic molecules, it is known as a coenzyme.
  • cofactors/coenzymes may form part of an 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 bound to an enzyme
  • coenzymes are organic
  • prosthetic groups are permanently bound to the enzyme
• Describe 4 ways in which multi-step reaction pathways can be regulated by cells.

  • inhibitors: competitive and non-competitive
  • end product inhibition
  • inactive precursor enzymes
• Define the terms enzyme inhibitor, competitive inhibitor and non-competitive inhibitor

  • enzyme inhibitor: factor that prevents or reduces the rate of an enzyme catalysed reaction
  • competitive inhibitor: inhibitor that competes with the substrate to bind to the active site of an enzyme
  • non-competitive inhibitor: inhibitor that binds to an enzyme at an allosteric site
• Define the terms reversible inhibitor, irreversible inhibitor and allosteric site

  • reversible inhibitor: inhibitor that can temporarily bind to the enzyme, but also dissociate itself, so the enzyme returns to being functional
  • irreversible inhibitor: inhibitor that once bound to an enzyme cannot dissociate and permanently stops the enzyme functioning
  • allosteric site: place on an enzyme where the molecule that is not a substrate may bind, changing the shape of an enzyme and influencing its ability to be active
• State two examples of competitive inhibitors and Describe their action.

  • statins are competitive inhibitors of enzymes used to produce cholesterol and are prescribed to help lower cholesterol
  • aspirin inhibits the production of chemicals responsible for pain and fever
• State two examples of non-competitive inhibitors and Describe their action.

  • organophosphates are used as insecticides that inhibit enzymes used in nervous impulse transmission, which causes paralysis
  • proton pump inhibitors block the enzyme responsible for secreting hydrogen ions into the stomach. This prevents build up of acid, which could lead to ulcers
• Define the term end product inhibition and Describe its usefulness in controlling metabolic pathways.

  end product inhibition is where the product of a reaction inhibits the enzyme required for that same reaction. it's useful because if there is little product, there is little inhibition. therefore, if more is made and have more product, there is more inhibition and production slows down. negative feedback controls the level of product.
• Define the term inactive precursor enzyme and Explain why enzymes may be produced in this form.

  an inactive precursor enzyme is a protein precursor (an inactive protein or peptide) that can be turned into an active form and can do this by post-translational modification, such as breaking off a piece of molecule or adding on another molecule. some enzyme can cause damage to cells, which means they must only be activated under certain conditions.
• Describe 3 ways in which active precursor enzymes may be activated.

  • addition of cofactor
  • addition of another enzyme, e.g. protease removing part of the structure
  • change in conditions, e.g. pH
• Define the terms apoenzyme, holoenzyme and zymogens/proenzymes.

  • apoenzyme: inactive enzyme. Activation of an enzyme occurs upon the binding of an organic or inorganic cofactor
  • holoenzyme: an enzyme with its required cofactor
  • zymogen/proenzyme: inactive substance, which is converted into an enzyme when activated by another enzyme