Enzymes are large globular proteins with specifity. They combine with an complementary substrate to form an enzyme-substrate complex (ESC).
They are biological catalysts - they lower the activation energy needed for a particular biological reaction to occur. The activation energy is the minimum amount of energy needed to start a reaction - this is usually provided by heating.
However, enzymes can lose their ability to function - this is called denaturing. This occurs when the hydrogen bonds are broken, and the tertiary structure changes. Therefore, it can no longer catalyse that specific biological reaction.
The active site
Substrates bind to the active site, this is where the reaction occurs. The active site is an indented area of around 6 to 10 amino acids which are found on the surface of the enzyme. The tertiary structure is responsible for the shape of the active site and it is held by:
- Hyden bonds
- Disulphide bonds
- Ionic bonds
- Van der Waals forces
The allosteric site
This area is different the active site. It is an area on the enzyme which allows molecules to either activate or inhibit the enzyme. The reaction does not occur here.
Lock and Key Theory
The lock and key theory explains how enzymes are large globular proteins with specificity.
This theory was produced by Fischer and states that the enzyme's active site and the substrate are complementary. Therefore, no change occurs to form the enzyme-substrate complex.
Induced fit model
The induced fit model was produced by Koshland, it is similar to the lock and key theory but it has some key differences.
It states that the presence of the substrate molecule in the active site induces a slight change in the shape of the active site. This causes it to mold around the substrate.
The enzyme does not undergo any permanent chemical changes during the reaction. Therefore, once the product detatches, it can catalyse another reaction with the same type of substrate molecule.
Factors affecting enzyme action
Temperature
Increasing the temperature means that the particles have more kinetic energy - this increases the rate of reaction.
However, there are limitations as at the maximum temperature, the hydrogen bonds break. The rate of reaction decreases as the shape of the active site has changed. Therefore, the enzyme cannot catalyse the specific biological reaction - this is because they have denatured.
pH
All enzymes have an optimum pH they can work at. This is because the ionic and hydrogen bonds are affected by the presence and absence of H+ or OH-
If the pH is not within the optimum range, the enzymes can denature.
Substrate concentration
Increasing the concentration of the substrate increases the rate of reaction until the enzymes become saturated - this means that they are working at full capacity. Then, the rate of reaction plateaus unless more enzymes are added to the substrate. However, increasing the substrate concentration does not affect the rate of reaction when the enzymes are saturated. This is becuase the point of saturation has been reached.
Enzyme concentration
All the time the substrate is in excess, increasing the concentration of the enzyme will increase the rate of reaction.