Question 22.4: How Enzymes Work The table below shows data for an enzyme-ca...
How Enzymes Work
The table below shows data for an enzyme-catalyzed reaction with and without an inhibitor:
| Substrate concentration (μmol/L) |
Activity (μmol product/min) w/o Inhibitor |
Activity (μmol product/min) + Inhibitor |
| 0.1 | 0.2 | 0.02 |
| 0.5 | 10 | 2.0 |
| 1.0 | 30 | 6.0 |
| 5.0 | 50 | 10 |
| 10 | 75 | 15 |
| 100 | 99 | 19.8 |
| 1000 | 100 | 20 |
What is the type of inhibitor that is indicated by these data?
Strategy
Remember that the key to distinguishing the type of inhibitor is to notice the effect on the maximum velocity. A competitive inhibitor, by its very name, means that the inhibition can be overcome with a large amount of competing substrate, but the same is not true for a noncompetitive inhibitor. We could graph out these data and see what the curve looks like, as we saw in Figure 22.9, but these data do not really require that. Note that the substrate is varied over a range of ten thousand–fold. When the substrate is 5.0 μmolar, the velocity is halfway to the maximum value seen with a concentration of 1000 μmolar. Also, there is very little difference between the velocity at 100 μmolar compared to 1000 μmolar. This means that 1000 μmolar is very close to saturating levels of substrate, so the maximum velocity of the reaction is about 100. With inhibitor, what we see is that the velocity is 5-fold lower with the highest substrate concentration used, and again the difference in velocity between 100 and 1000 μmolar substrate is not very large. Even without graphing it, we can see that this velocity with inhibitor is never going to reach the same level as without the inhibitor.
Since adding lots of substrate is unable to outcompete the inhibitor, this is a noncompetitive inhibitor
■ Quick Check 22.4
Which of the following is true regarding enzymes?
(a) They only speed up the forward reaction from substrates to products.
(b) They make a reaction thermodynamically favorable that would be unfavorable without them.
(c) They reduce the energy of the transition state.
(d) The reaction is catalyzed at the active site of the enzyme.
(e) The most common amino acids involved in catalysis are the nonpolar ones.
(f) Amino acids with charged sidechains are often found in the active site.
(g) Histidine is the most common amino acid found in the active site.