In addition to temperature and pH, other substances in the solution can also affect enzyme activity. Substances that decrease enzyme activity are called inhibitors, while substances that increase enzyme activity are called activators.
If an inhibitor forms a covalent bond with the enzyme that is difficult to break, the reaction is irreversible. For example, heavy metal ions and cyanide can change protein structure permanently. Irreversible inhibitors cannot be removed by physical means, but in some cases, enzyme activity can be restored by chemical methods. Organophosphate pesticides is poisonous, because it can bind covalently to acetylcholinesterase (AChE). Antidotes such as pralidoxime can displace them from the enzyme.
If the chemical bonds between the inhibitor and the enzyme are weak, such as hydrogen bonds or ionic bonds, the inhibition is temporary. They can easily regain activity by spontaneous bond breaking, dialysis, or ultrafiltration.
Competitive inhibitors are a common type of reversible inhibitor. Their structure is similar to that of the substrate, so they compete with the substrate for the active site of the enzyme. As the inhibitor continuously occupies the active site to form a reversible complex, the probability of substrate binding to the enzyme decreases, thereby reducing the rate of biochemical reaction. As long as there is much more substrate than the competitive inhibitor, the active site is less likely to be occupied by the inhibitor, and the substrate can be catalyzed by the enzyme as usual.
Non-competitive inhibitors are dissimilar to the substrate and do not occupy the active site. Instead, they bind to allosteric sites to changes enzyme shape slightly, making the active site unsuitable for substrate binding. As their concentration decreases, they will dissociate from enzyme to relieve the inhibition on their own. This reversible binding acts like a switch that controls the reaction rate. An important example is negative feedback in biochemical reactions. When there is an excess of products, enzyme activity is inhibited to regulate metabolic balance. The rate of glycolysis can be regulated by the concentration of ATP. When ATP levels increase, the structure of phosphofructokinase-1 (PFK-1) is altered to slow down glycolysis. When ATP decreases, inhibition is removed spontaneously to accelerate glycolysis.
Enzyme activators are molecules or ions that enhance the activity of enzymes. They can increase the rate of enzymatic reactions by facilitating the binding of substrates to the enzyme.
Metal ions: Certain metal ions can serve as enzyme activators. For instance, magnesium ions (Mg²⁺) are often required for the activity of many enzymes involved in ATP-dependent reactions. Zinc ions (Zn²⁺) are also known to activate certain enzymes such as carbonic anhydrase. Chloride ion (Cl⁻) is an activator of salivary amylase.
Proteolytic cleavage: In some cases, enzymes are synthesized as inactive precursor molecules called proenzymes. Activation of these enzymes occurs by proteolytic cleavage, that is some peptide bonds are cut to generate the active form of the enzyme. This mechanism is commonly observed in digestive enzymes like trypsin and pepsin.