By Wu Zhang, Year 12
Enzymes are vital biological catalysts that speed up chemical reactions, lowering activation energies and catalysing reactions without being consumed in the process. Their function depends on the complex 3-dimensional folding of polypeptide chains, enabling the active site to have its specific chemical properties, complementary to the substrate’s shape and properties. However, the chemical properties of these active sites may be altered in the presence of inhibitors, causing decreased effectiveness or permanent dysfunction. These inhibitors can cause hazards, especially in biological systems, but they’re also important because they help cells regulate reactions efficiently, and they are also used in medicine, agriculture, and biochemical research. The two main types are competitive inhibitors and non-competitive inhibitors. Although both reduce enzyme activity, they do so in different ways.
A competitive inhibitor reduces enzyme activity by competing with the substrate for the enzyme’s active site, where substrates normally bind due to their complementary properties and shape. Competitive inhibitors often have a similar shape to the substrate, so they can fit into the active site. However, they are not converted into products. Instead, they block the substrate from binding, reducing the rate of reaction, although it can be overcome by increasing substrate concentrations to outcompete the inhibitor.
For example, methanol is a simple organic molecule that can be toxic, since the enzyme alcohol dehydrogenase converts it into harmful products. However, ethanol can act as a competitive inhibitor because it also binds to alcohol dehydrogenase due to their similar structures and properties (+1 CH2). If ethanol binds to the active site first, methanol cannot bind and is not converted into toxic products as quickly. This gives the body more time to remove methanol safely. Therefore, ethanol slows the harmful reaction by competing with methanol for the same enzyme, which is especially useful in medical scenarios. If more ethanol molecules are present, they are more likely to outcompete methanol for the active site, meaning that the enzyme can still eventually reach its maximum rate if enough substrate is added. In enzyme kinetics, competitive inhibition increases the apparent Km (Michaelis value, indicating the substrate concentration at which the reaction velocity is half its maximum), because more substrate is needed to reach half of the maximum rate, but it does not change the eventual Vmax (maximum rate at which enzymes can catalyse a reaction with all its active sites fully saturated).
In contrast, a non-competitive inhibitor does not bind to the active site. Instead, it binds to another part of the enzyme called an allosteric site. This changes the enzyme’s shape, including the shape of the active site, since any changes to the overall structure can cause an imbalance in the active site region. As a result, the substrate may still bind, but the enzyme is less able to catalyse the reaction.
A common example is inhibition by heavy metal ions, such as mercury ions, lead ions, or copper ions. These ions can bind to certain groups in enzymes, especially sulfhydryl groups found in the amino acid cysteine. This disrupts the enzyme’s three-dimensional structure and reduces its activity. Unlike competitive inhibition, adding more substrate usually does not solve the problem because the active site itself has been altered.
The key difference between the two types of inhibition is therefore the location of binding. Competitive inhibitors bind to the active site, directly preventing substrate binding. Non-competitive inhibitors bind to an allosteric site, indirectly reducing enzyme activity by changing the enzyme’s shape. Understanding these inhibitors helps explain how enzymes are regulated and how chemical substances can control biological reactions.
Works Cited
“Alcohol Dehydrogenase – an Overview | ScienceDirect Topics.” Www.sciencedirect.com, www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/alcohol-dehydrogenase.
