1.4 Enzymes

• Explain how inhibitors can affect enzyme activity

  Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions. They are found

  naturally, but are also used artificially as drugs, pesticides and research tools. There are two

  kinds of inhibitors.

  
  

  1. A competitive inhibitor molecule has a similar structure to the substrate molecule, and so it can fit into the active site of the enzyme. It, therefore, competes with the substrate for the active site, so the reaction is slower. Increasing the concentration of substrate restores the reaction rate and the inhibition is usually temporary and reversible.

  
  

  2. A non-competitive inhibitor molecule is

  quite different in structure from the substrate and does not fit into the active site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the active site,

  so that it can no longer bind substrate

  molecules. Non-competitive inhibitors

  therefore simply reduce the amount of

  active enzyme.
• Explain how substrate concentration affects enzyme activity.

  As the substrate concentration increases, the

  rate increases because more substrate

  molecules can collide with active sites, so

  more enzyme-substrate complexes form. Maximum activity occurs when the enzyme is saturated i.e. when all enzymes are binding substrate.
• Explain how enzyme concentration affects enzyme activity.

  As the enzyme concentration increases the rate of the reaction also increases because there are more enzyme molecules (and so more active sites), available to catalyse the reaction therefore more

  enzyme-substrate complexes form.
• Explain how cofactors and coenzymes affect enzyme structure and function.

  Inorganic substances (zinc, iron) and vitamins (respectively) are sometimes needed for proper enzymatic activity. E.g. Iron must be present in the quaternary structure of haemoglobin in order for it to pick up oxygen.
• Explain how environmental conditions affect enzyme activity.

  1. High temperatures may denature enzymes and increase the enzyme's rate of reaction. The rate increases because the enzyme and substrate molecules both have more kinetic

  energy and so collide more often, and also because more molecules have sufficient energy to

  overcome the activation energy.

  Above the optimum temperature the rate decreases as more of the enzyme molecules denature.

  The thermal energy breaks the hydrogen bonds holding the secondary and tertiary structure

  of the enzyme together, so the enzyme loses its shape and becomes a random coil - and the

  substrate can no longer fit into the active site. This is irreversible. Remember that only the

  hydrogen bonds are broken at normal temperatures; to break the primary structure ( the peptide

  bonds) you need to boil in strong acid for several hours - or use a protease enzyme

  
  

  2. pH affects the rate of enzyme-substrate complex where extremely high or low pH values generally result in complete loss of activity for most enzymes. pH is also a factor in the stability of enzymes. As with activity, for each enzyme there is also a region of pH optimal stability. Enzymes have an optimum pH at which they work fastest. For most enzymes, this is about pH 7-8 (normal body pH), but a few enzymes can work at extreme pH,

  such as gastric protease (pepsin) in our stomach,

  which has an optimum of pH 1.
• What affects enzyme activity?

  environmental conditions, cofactors and coenzymes, enzyme inhibitors
• Explain how enzymes lower activation energy

  The larger the activation energy is, the slower the reaction will be. This is because only a few substrate molecules will have sufficient energy to overcome the activation energy barrier. Enzymes reduce the activation energy of a reaction so that the kinetic energy of most molecules exceeds the activation energy required and so they can react.
• Define activation energy

  the amount of energy required to get a chemical reaction started.
• Define apoenzyme and holoenzyme

  Apoenzymes are inactive enzymes without its non protein moiety while holoenzymes are active with its non protein component.
• Describe the induced-fit hypothesis.

  1. substrate entering active site enzyme but does not fit exactly

  2. active site change to more precise positions allowing stronger binding so that the substrate fits into it and an Enzyme-Substrate Complex can form

  3. catalyst reaction takes place and Enzyme-Product Complex produced

  4. products released, enzyme reverts back to original conformation allowing enzyme to undergo further chemical reaction
• The lock theory helps to explain what?

  Enzyme specificity (an enzyme would only

  work on one substrate) and the loss of activity when enzymes denature.
• Describe the lock and key theory.


  The active site has a rigid shape, only substrates with the matching shape can fit and the substrate is a key that fits the lock of the active site and an Enzyme-Substrate Complex will form.

  
  

  .
• What happens when a substrate binds to an enzyme?

  The enzyme and substrate form an ES complex (enzyme-substrate complex) and the enzyme will change shape, and eventually the enzyme will produce products from the substrate.
• Define substrate

  1. The reactant that binds to the enzyme in a biochemical reaction.

  
  

  2. In enzymatic reactions, the substance at the beginning of the process, on which an enzyme begins its action is called a substrate.
• Enzymes are not ______ or ______ during reaction but are _____.

  1. altered 2. consumed 3. reusable
• How do enzymes increase the rate of reaction?

  By lowering the activation energy.
• What is an active site?

  The area on an enzyme where a substrate or substrates attach to
• Define anabolism

  the synthesis of complex molecules in living organisms from simpler ones together with the storage of energy; constructive metabolism.
• Define metabolism

  The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways, which manage the material and energy resources of the organism.
• What are enzymes?

  globular proteins that catalyse metabolic reactions