Why does competitive inhibition increase km

First one performs a set of V vs. Next, a second set of reactions is performed in the same manner as before, except that a fixed amount of the methotrexate inhibitor is added to each tube.

At low concentrations of substrate, the inhibitor competes for the enzyme effectively, but at high concentrations of substrate, the inhibitor will have a much reduced effect, since the substrate outcompetes it, due to its higher concentration remember that the inhibitor is at fixed concentration.

Graphically, the results of these experiments are shown above. Notice that at high substrate concentrations, the competitive inhibitor has essentially no effect, causing the Vmax for the enzyme to remain unchanged.

However, at lower substrate concentrations it does. It may not be obvious why we call the changed Km the apparent Km of the enzyme. It only appears to do so. This is because of the way that competitive inhibition works. The enzyme molecules that are not bound by methotrexate can, in fact, bind folate and are active. Methotrexate has no effect on them and their Km values are unchanged. Why then, does Km appear higher in the presence of a competitive inhibitor.

The reason is that the competitive inhibitor is reducing the amount of active enzyme at lower concentrations of substrate. Studies of competitive inhibition have provided helpful information about certain enzyme-substrate complexes and the interactions of specific groups at the active sites. As a result, pharmaceutical companies have synthesized drugs that competitively inhibit metabolic processes in bacteria and certain cancer cells.

Many drugs are competitive inhibitors of specific enzymes. A second type of inhibition employs inhibitors that do not resemble the substrate and bind not to the active site, but rather to a separate site on the enzyme rectangular site below. The effect of binding a non-competitive inhibitor is significantly different from binding a competitive inhibitor because there is no competition.

In the case of competitive inhibition, the effect of the inhibitor could be reduced and eventually overwhelmed with increasing amounts of substrate. This was because increasing substrate made increasing percentages of the enzyme active. With non-competitive inhibition, increasing the amount of substrate has no effect on the percentage of enzyme that is active. Indeed, in non-competitive inhibition, the percentage of enzyme inhibited remains the same through all ranges of [S].

This means, then, that non-competitive inhibition effectively reduces the amount of enzyme by the same fixed amount in a typical experiment at every substrate concentration used The effect of this inhibition is shown above. As you can see, Vmax is reduced in non-competitive inhibition compared to uninhibited reactions. This makes sense if we remember that Vmax is dependent on the amount of enzyme present. Reducing the amount of enzyme present reduces Vmax.

Additionally, KM for non-competitively inhibited reactions does not change from that of uninhibited reactions. This is because, as noted previously, one can only measure the KM of active enzymes and KM is a constant for a given enzyme. The Lineweaver-Burk double reciprocal plot for this set of data shows a series of parallel lines - both Km and Vmax are reduced:.

If the requirement is to increase the intracellular concentration of the substrate, then either a competitive or non-competitive inhibitor will serve, since both will inhibit the utilisation of substrate, so that it accumulates. However, if the requirement is to decrease the intracellular concentration of the product, then the inhibitor must be non-competitive.

As unused substrate accumulates, so it will compete with a competitive inhibitor, and the final result will be a more or less normal rate of formation of product, but with a larger pool of substrate. Increasing the concentration of substrate does not affect a non-competitive inhibitor. The inhibitor constant, Ki, is an indication of how potent an inhibitor is; it is the concentration required to produce half maximum inhibition. For a competitive inhibitor, the lines converge above the x axis, and the value of [I] where they intersect is -Ki.

For a non-competitive inhibitor, the lines converge on x axis, and the value of [I] where they intersect is -Ki. It is worth noting that in competitive inhibition, the percentage of inactive enzyme changes drastically over the range of [S] values used.

To start, at low [S] values, the greatest percentage of the enzyme is inhibited. At high [S], no significant percentage of enzyme is inhibited. This is not always the case, as we shall see in non-competitive inhibition. Kevin Ahern and Dr. Indira Rajagopal Oregon State University. Non-Competitive Inhibition Figure 4.