AS/A-level Biology - Enzyme (1)

• All enzymes are globular proteins with a specific tertiary structure, which catalyse metabolic reactions in all living organisms.
• This means that they speed up chemical reactions, but are not used-up as part of the reaction.
• Enzymes are relatively large molecules, consisting of hundreds of amino acids which are responsible for maintaining the specific tertiary structure of the enzyme.
• Each enzyme has a specific active site shape, maintained by the specific overall tertiary structure.
• Therefore the tertiary structure must not be changed.

(1) Extracellular enzyme action

• It occurs outside the cell, which produces the protein.
• For example, some enzymes in digestive systems are extracellular as they are released from the cells that make them, onto food within the digestive system spaces.

(2) Intracellular enzyme action

• It occurs inside the cell, which produces the enzyme.
• For example, some enzymes in digestive systems are found in the cytoplasm of cells or attached to cell membranes and the reaction takes place inside the cell.

• The shape of the enzyme's active site is complementary and specific to the shape of the substrate molecule - the substrate is usually smaller.
• The substrate collides with the active site so the substrate 'key' fits into the active site 'lock' like a lock and key.
• The substrate is then held in one place so the reaction can go ahead, forming an enzyme-substrate complex.
• The products are formed and it becomes the enzyme-product complex.
• The products formed are a different shape from the substrate so they no longer fit into the active site so they move away.

• The substrate molecule collides with a complementary and specific enzyme active site.
• The enzyme changes shape slightly to make the active site fit more closely around the substrate.
• The substrate is held due to oppositely charged groups on the substrate and the active site are found near each other.
• The enzyme-substrate complex is formed.
• The change in enzyme shape places a strain on the substrate molecule so that it destabilises it and the reaction occurs more easily.
• The products are made and it becomes the enzyme-product complex.
• The products formed are a different shape from the substrate so they no longer fit into the active site so they move away.

• The activation energy is the minimum level of energy required to enable a reaction to take place.
• Enzymes work by lowering the activation energy of reactions.
• This means reactions can proceed quickly at temperatures much lower than boiling point as less energy is required for the reaction.

When it comes to AS/A-level Biology, what are the effects of pH, temperature, enzyme concentration and substrate concentration on enzyme activity?

(1) pH

• At different pH, the hydrogen ions interfere with the hydrogen bonds and ionic bonds holding the tertiary structure in place.
• Therefore the shape of active site of an enzyme can change and so the substrate will no longer fit.
• All enzymes have their own optimum pH.
• This is the pH at which the rate of reaction is highest.
• At optimum pH, the concentration of hydrogen ions in solution gives the tertiary structure of the enzyme the best overall shape, so that the shape of active site is complementary to the substrate.

(2) Temperature

• As the temperature increases, the kinetic energy increases so the substrate and enzyme molecules move around more quickly.
• This makes the substrate collide with the enzyme's active site much more often, forming more enzyme-substrate complexes.
• Applying heat makes the molecules vibrate.
• These vibrations put a strain on the weaker bonds (hydrogen and ionic bonds) and they break.
• The tertiary structure and the active site of the enzyme will change shape, as the bonds that are responsible for holding the tertiary structure in place breaks.
• Therefore the substrate can no longer fit into the active site so the enzyme becomes denatured.

(3) Enzyme concentration

• As the concentration of enzyme increases, more active sites become available.
• Therefore it is easier for substrate molecules to collide with the active site so more enzyme-substrate complexes form, forming more products, increasing the rate of reaction.
• If the concentration of enzyme increases further, the rate of reaction will reach its maximum value and stays constant.
• This is where all substrate molecules are occupying enzyme active sites, so any further increase in enzyme concentration will have no effect on the rate of reaction - the concentration of substrate becomes the limiting factor.

(4) Substrate concentration

• As the concentration of substrate increases, collisions between the enzyme's active site and the substrate molecules occurs more often.
• More enzyme-substrate complexes form, so more product is formed - the rate of reaction increases.
• If the concentration of substrate increase further, the rate of reaction will reach its maximum value and stays constant.
• At this point all the active sites are occupied and so any further increase in substrate concentration will have no effect on the rate of reaction - the concentration of enzyme becomes the limiting factor.

pH:

• Use buffer solutions that maintain pH at a set level by keeping the H+ concentration in solution constant. - Reaction rate depends on the pH as if it is not at optimum pH, the tertiary structure can change and change the shape of the active site of the enzyme.

Temperature:

• Carry out enzyme-controlled reactions in a water bath with a thermostat or use a polystyrene sleeve for insulation to keep the temperature constant.
  - Fluctuations in temperature will affect the enzyme-controlled reaction so readings taken will not reflect the action of the independent variable being tested.

Enzyme concentration:

• Use accurately measured volumes of enzyme in solution.
  - Reaction rate depends on the concentration of enzyme molecules present.
  - Using accurately measured volumes of enzyme solution gives a constant concentration of enzyme molecules.
• If using an enzyme in a living tissue, use constant measurements of the mass of tissue.
  - In living tissue, you must assume that all pieces of tissue contain the same number of enzyme molecules.
• If using whole pieces of tissue, use constant measurements of the surface area.
  - The number of enzyme molecules in contact with substrate molecules will affect the reaction rate.
  - If the surface areas of the pieces of tissues are different, the number of enzyme molecules exposed directly to substrate will be different.

Substrate concentration:

• Use accurately measured volumes or mass of substrate.
  - Reaction rate depends on the concentration of substrate molecules present.
  - Using accurately measured volumes or masses of enzyme solution gives a constant concentration of substrate molecules.

In part 2, you will know more about the effects of competitive and non-competitive inhibitors on the rate of enzyme-controlled reactions, with reference to both reversible and non-reversible inhibitors.

References:

1. https://www.scientificamerican.com/article/exploring-enzymes/

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Drafted by Bonnie (Biology)