Enzymes

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  • Created by: Elliew176
  • Created on: 12-05-17 14:39

The role of enzymes in reactions

·       Living organisms need to be built and maintained. This involves the synthesis of large polymer-based components e.g cellulose forms the walls of plants and long protein molecules form the filaments of muscles in animals. The cell components are synthesised and assembled into cells which then forms organs and tissues which form the whole organism. The chemical reactions required for grown are anabolic (building up) reactions. They are all catalysed by enzymes.

·       Cells use the energy in food molecules to power chemical reactions. However, the energy cannot be used directly. It must be transferred first to an energy-carrying intermediary, such as ATP. When chemical bonds in ATP are broken, energy is released. This energy provides the energy needed for chemical reactions.  Energy is released from large organic molecules like glucose consisting of many catabolic (breaking down) reactions. These reactions are also catalysed by enzymes.

·       These large organic molecules come from digestion of food made up of even larger molecules like starch. Digestion is catalysed by a range of enzymes.

Metabolism is the sum of all of the different reactions and reaction pathways happening in a cell or an organism and can only happen as a result of the control and order imposed by enzymes

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The role of enzymes in catalysing intracellular an

Intracellular enzymes:

  • Enzymes that act within cells are called intracellular enzymes.
  • Hydrogen peroxide is a toxic product of many metabolic pathways. The enzyme catalase ensures hydrogen peroxide is broken down to oxygen and water quickly to prevent accumulation. It is found in both animal and plant tissues.

 Extracellular enzymes are enzymes that are secreted by a cell and functions outside of that cell. 

Digestion of starch – the digestion of starch begins in the mouth and again in the small intestine. It is broken down in two steps, each using a different enzyme. Different enzymes are needed because each enzyme only catalyzes one specific reaction.  Starch polymers are broken down into maltose which is a disaccharide. The enzyme involved in this stage is called amylase which is produced by the salivary glands and the pancreas. It is released in saliva in the mouth and in pancreatic juice into the small intestine. Maltose is then broken down into glucose which is a monosaccharide. The enzyme involved in this stage is called maltase. Maltase is present in the small intestine.

 Digestion of proteins – trypsin is a protease, a type of enzyme that catalyzes the digestion of proteins into smaller peptides which can then be broken down further into amino acids by other proteases.  Trypsin is produced in the pancreas and released with the pancreatic juice into the small intestine where it acts on proteins. 

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Mechanism of enzyme action

Mechanism of enzyme action

·         Molecules in a reaction move and collide randomly.

·          For a reaction to happen, molecules need to collide in the right orientation. When temperatures and pressures are applied the speed of the molecules increase so the number of successful collisions and the overall rate of reaction.

·         Energy needs to be supplied for most reactions to start. This is called the activation energy. Enzymes help the molecules collide successfully and therefore reduce the activation energy

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Lock and key hypothesis

 Lock and key hypothesis

·         An area within the tertiary structure (enzymes are mainly globular proteins - protein molecules where the tertiary structure has given the molecule a generally rounded, ball shape) of the enzyme has a shape that is complementary to the shape of a specific substrate molecule. This is called the active site.

·         Only a specific substrate will fit the active site of an enzyme. When the substrate is bound to the active site and enzyme-substrate complex is formed. The substrate then reacts and the product is formed in an enzyme-product complex.

·         The products are then released leaving the enzyme unchanged and able to take part in subsequent reactions. The substrate is held so that the right atom groups are close enough to react. The R-group within the active site of the enzyme will also interact with the substrate forming temporary bonds. These put strain on the bonds within the substrate which also helps the reaction along. 

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Induced fit hypothesis

Induced fit hypothesis

·         Research shows that the active site of the enzyme changes shape slightly as the substrate enters. This is called the induced fit hypothesis and is a modified version of the lock and key hypothesis.

·          The initial interaction between the enzyme and substrate is relatively weak but these weak interactions rapidly induce changes in the enzymes tertiary structure that strengthen binding, putting strain on the substrate molecule.

·         This can weaken a particular bond or bonds in the substrate which lowers the activation energy for the reaction.  

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Factors affecting enzyme activity - temperature

Temperature: the rate of reaction increases when the temperature is increases. More het means more kinetic energy so molecules move more quickly. This makes the enzymes more likely to collide with the substrate molecules. The energy also increases which means collisions are more likely which results in a reaction. If the temperature gets too high, the reaction stops.

The rise in temperature makes the enzyme molecules vibrate more. If the temperature goes above a certain level, the vibration breaks some of the bonds that hold the enzyme in shape and this damages the tertiary structure of the enzyme. The active site changes shape and the enzyme and substrate no longer fit together. At this point, the enzyme is denatured so it no longer functions as a catalyst. This is because the active site changes shape and is no longer complementary to the substrate.

The optimum temperature is the temperature at which the enzyme has the highest rate of activity. For human bodies, this is usually around 40 degrees Celsius. Once enzymes have denatured above the optimum temperature, the decrease in rate of reaction is rapid.

The temperature coefficient, Q10, of a reaction is a measure of how much the rate of reaction increases with 10 degrees Celsius rise in temperature. Usually the rate of reaction doubles with a 10 degree Celsius temperature increase.

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Factors affecting enzyme activity - PH

PH: all enzymes have an optimum PH value. Most human enzyme work best at a pH of 7. Above and below the optimum pH, H+ and OH- ions found in acids and alkalis can mess up the ionic bonds and hydrogen bonds that hold the enzymes tertiary structure in place. This makes the active site change shape so the enzyme is denatured.

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Factors affecting enzyme activity - enzyme and sub

Enzyme concentration: the more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and forms an enzyme-substrate complex. So increasing the enzyme concentration increases the rate of reaction.

Substrate concentration: the higher the substrate concentration, the faster the reaction. This is because more substrate molecules means a collision between substrate and enzyme is more likely and more active sites will be used. This is only true up to a saturation point though. After that, there are so many substrate molecules that all the active sites are full and adding more substrate molecules doesn’t make a difference to the rate of reaction.  Substrate concentration decreases with time during a reaction so if no other variables are changed, the rate of reaction will decrease over time. This makes the initial rate of reaction the highest rate of reaction.

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Enzyme inhibitors - competitive inhibition

1) Competitive inhibitor molecules have a similar shape to that of the substrate molecules.

2) They compete with substrate molecules to bind to the active site but no reaction takes place.

3) Instead they block the active site so no substrate molecules can fit in it.

4) How much the enzyme is inhibited depends on the relative concentrations of the inhibitor and the substrate.

5) If there is a high concentration of the inhibitor, it will take up nearly all the active sites and hardly any of the substrate will get to the enzyme. If there is a higher concentration of substrate then the substrates chances of getting to an active site before the inhibitor increase. So increasing the concentration of substrate will increase the rate of reaction.

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Enzyme inhibitors - non-competitive inhibition

   1)      Non-competitive inhibitor molecules bind to the enzyme away from its active site. The site they bind to is the allosteric site.

   2)      This causes the active site to change shape as it alters the tertiary structure of the enzyme so the substrate molecules can no longer bind to it because the active site no longer has a complementary shape to the substrate.

  3)      They don’t compete with the substrate molecules to bind to the active site because they are a different shape.

  4)      Increasing the concentration of substrate won’t make any difference to the reaction rate – enzyme activity will be inhibited.

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