C.7.5

Explain competitive and non-competitive inhibition.
Inhibitors are substances that slow down the rate of reactions catalyzed by enzymes.

Competitive inhibition
Occurs through competitive inhibitors. A competitive inhibitor has a chemical and structural similarity to the substrate and **competes with** the **substrate for the position at the active site** of the enzyme (it resembles the substrate in shape but cannot react). The r**ate of the reaction slows down** because the active site is occupied by the competitive inhibitor, making the active site less accessible to the substrate.



The graph below show the effect of competitive inhibition on the rate of an enzyme catalyzed reaction. When a competitive inhibitor is added to an enzyme **Km will increase**. This is because more substrate is needed to reach Vmax because the reaction is slower due to the active sites being occupied by the inhibitor.

Non-competitive inhibition
Occurs through non-competitive inhibitors which **bind to the enzyme, but not on the active site** and therefore don't compete with the substrate. However, non-competitive inhibitor does cause the **enzyme's active site changes shape** and as a result, the substrate can no longer bind to it, **decreasing the rate of the reaction /enzyme activity** decreasing Vmax.



When a **non-competitive inhibitor** is added to an enzyme **Km will remain unchanged**. Because the non-competitive inhibitor does not bind to the enzyme on the active site it so does not effect the way the substrate interacts with it. Even though the shape of the active site has changed, and **Vmax has decreased**, the Km is still based on 1/2 of the new Vmax.

When the substrate concentration is increased, the effect of competitive inhibitors is lessened since there is increased competition for the active sites by the substrate.When the substrate concentration is increased, the effect of non-competitive inhibitors stays the same, since the enzyme's shape still remains changed and therefore the substrate cannot bind to it. This is shown in the diagram below: A helpful site: http://www.bio.mtu.edu/campbell/401lec13.pdf

With the __uninhibited reaction__ at low substrate concentrations as the substrate concentration increases the rate of the reaction increases in a linear manner because as there are more frequent collisions between the enzyme and the substrate. As the concentration of the substrate starts to increase further the rate of the reaction starts to decrease, because the frequency of collisions decreases as the substrate is getting used up. Eventually the rate of the reaction does not change with increasing substrate concentration because all of the active sites on the enzyme have been occupied.
 * Describing the shape of the curves:**

With the __competitive inhibitor__ that the slope of the line is flatter, showing that the rate of the reaction does not occur as rapidly which means that it will take longer to reach Vmax.

With the __non-competitive inhibitor__, the line is to the right of the original uninhibited reaction and becomes horizontal at a lower rate, because Vmax is achieved in less time.