Substrate concentration
At a low substrate concentration there are many active sites that are not occupied, therefore this means that the reaction rate is low. When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases. Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed. At this moment enzymes are working continuously and if more substrate I added, the enzyme simply cannot work faster; substrate molecules are effectively ‘queuing up’ for an active site to become vacant. The enzyme is working at its maximum possible rate, known as V max. The graph below illustrates the effects of substrate concentration on a ‘reaction’.
Temperature activity
Enzymes work best at an optimum temperature. Below this, an increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase, so the rate will too. Above the optimum temperature, and the enzymes are denatured. Bonds holding the structure together will be broken and the active site loses its shape and will no longer work. Therefore at low temperatures, the reaction takes place only very slowly. This is because molecules are moving slowly. In order to have more impact in collisions with enzyme active site and the substrate, rising the temperature to a certain level is essential. Not only the amount of collisions increase, also they do so with more energy as this makes it easier for bonds to be broken so that reaction can occur. The optimum temperature for most human enzymes is around 40 C. A slight rise above this would begin to denature enzymes. This graph below is an example of a how temperature could affect the rate of a reaction.
pH activity
As with temperature, enzymes have an optimum pH. If the pH changes much from the optimum, the chemical nature of the amino acids can change. This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be denatured. PH itself is a measure of the concentration of hydrogen ions in a solution. The lower the pH, the higher the hydrogen ion concentration. Hydrogen ions can interact with the R groups of amino acids, affecting the way in which they bond with each other and therefore affect their 3D arrangement. The graph below illustrates the effect of pH on the rate of an enzyme-controlled reaction.
Enzyme inhibitors
Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable.
Reversible inhibitors:
Competitive reversible inhibitors: these molecules have a similar structure to the actual substrate and so will bind temporarily with the active site. The rate of reaction will be closer to the maximum when there is more ‘real’ substrate, (e.g. arabinose competes with glucose for the active sites on glucose oxidase enzyme).
Non-competitive reversible inhibitors: these molecules are not necessarily anything like the substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the enzyme though, so the reaction rate decreases.
Irreversible inhibitors:
These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction, for example, cyanide irreversibly inhibits the enzyme cytochrome oxidase found in the electron transport chain used in respiration. If this cannot be used, death will occur.
However metabolic reaction must be finely controlled and balanced, so no single enzyme can be allowed to ‘run wild’, constantly churning out more and more product. One way of ensuring that this cannot happen is to use end product of a chain reactions as an enzyme inhibitor. For example:-
As the enzyme converts substrate to product, it is slowed down because the end-product binds to another part of the enzyme and prevents more substrate binding. However, the end-product can lose its attachment to the enzyme and go on to be used elsewhere, allowing the enzyme reform into its active state. As product level falls, the enzyme is able to top them up again. This is end-product inhabitation and is an example of non-competitive reversible inhabitation.
