Factors affecting enzyme activity
- Substrate concentration
- Enzyme concentration
Velocity (V) – the number of substrate molecules converted to product per minute.
Initial Velocity (Vo) – rate of reaction measured as soon as the enzyme and substrate.
Max velocity is also known as Vmax
Km – reflects enzyme-substrate affinity. It is numerically equal to the substrate concentration at which the reaction velocity is equal to ½ Vmax.
- Small Km , high enzyme-substrate affinity due to a low [S] being needed
- High Km , low enzyme-substrate affinity due to a high [S] being needed
As substrate concentration[S] increases, so does the rate of enzyme-catalysed reactions until it reaches Vmax. This can be seen by the steady increase of the curve. The plateauing off of the curve at Vmax reflects saturation(X). This is due to all the active sites on the enzyme molecule being occupied with substrate. At this point the reaction cannot occur any faster and does not have any effect on the rate of the reaction, therefore the curve plateaus.
The substrate concentration, temperature, and pH are kept constant in order for the enzyme concentration to have full effect. As enzyme concentration increases, so does the rate of the enzyme-catalysed reactions. This is because more enzymes will be colliding with substrate molecules. However this would only be up to a certain concentration where the enzyme concentration would no longer be a limiting factor.
As temperature (heat energy) increases more moles of substrate will have enough activation energy, therefore the number of substrate moles that bind to the active site(s) to form product will increase substantially. This leads to a high rate of reaction as a result of high collision rate between substrate molecules and active sites due to a high kinetic energy. This is seen in the high increase of the curve. The curve then peaks towards optimal temperature(X) and then quickly decreases. At optimal temperature the bonds of the tertiary structured enzyme cannot take it anymore and become denatured. When denaturation occurs the hydrogen bonds, hydrophobic, electrostatic and van der waal interactions begin to break. This breaking is a co-operative process, therefore as one bond breaks, it makes it easier for the others to break. As a result of this the enzyme’s shape changes and can no longer bind to the substrate.
pH is the measure of hydrogen ions concentration in a solution. The higher the hydrogen ion concentration, the lower the pH, vice versa. Most enzymes function efficiently over a narrow pH range. A change in pH above or below this range reduces the rate of enzyme reaction considerably. Changes in pH lead to the breaking of the ionic bonds that hold the tertiary structure of the enzyme in place. The enzyme begins to lose its functional shape, particularly the shape of the active site, such that the substrate will no longer fit into it, the enzyme is said to be denatured. Also changes in pH affect the charges on the amino acids within the active site such that the enzyme will not be able to form an enzyme-substrate complex.