What is the Difference Between Enzyme Activator and Enzyme Inhibitor

The main difference between enzyme activator and enzyme inhibitor is that enzyme activator is a molecule that binds to the enzyme, increasing its activity, whereas an enzyme inhibitor is a molecule that binds to the enzyme, decreasing its activity. Furthermore, proteins, peptides, lipids, small organic molecules, and ions can serve as enzyme activators while the two main types of enzyme inhibitors are reversible and irreversible inhibitors. 

Enzyme activators and enzyme inhibitors are two types of molecules that bind to the enzymes, allosterically regulating their activity. Enzymes are biological catalysts, which reduce the activation energy of biochemical reactions. 

Key Areas Covered 

1. What is an Enzyme Activator
     – Definition, Types, Role
2. What is an Enzyme Inhibitor
     – Definition, Types, Role
3. What are the Similarities Between Enzyme Activator and Enzyme Inhibitor
     – Outline of Common Features
4. What is the Difference Between Enzyme Activator and Enzyme Inhibitor
     – Comparison of Key Differences

Key Terms 

Activity Regulation, Enzyme Activator, Enzyme Inhibitor, Enzymes, Irreversible Inhibitors, Reversible Inhibitors

What is an Enzyme Activator 

An enzyme activator is a molecule that binds to an enzyme in order to increase the velocity of the reaction. Some enzyme activators include ions, small organic molecules, peptides, proteins, or lipids. Moreover, many enzymes contain specific sites for the binding of small inorganic ions, especially cations such as calcium ions. Here, the binding of these ions changes the conformation of the enzyme molecule, activating it. Therefore, these ions serve as cofactors. Significantly, some cations such as magnesium ions bind to the substrate as well. They decrease the negative charge of the substrate, facilitating the binding of the substrate to the enzyme.  

Figure 1: Allosteric Enzyme Regulation

Also, some of the heavy metal cations eliminate inhibitors. On the other hand, some of the chelating agents including EDTA and EGTA bind to the inhibitory cations, eliminating the inhibitory action. Furthermore, some enzyme activators like calmodulin, a calcium-binding protein, form a complex with the target enzyme, activating the enzyme. Fructose 2,6-bisphosphate is a small organic molecule which serves as an enzyme activator while increasing the rate of glycolysis. Also, hexokinase-1 and glucokinase are proteins which activate enzymes. 

What is an Enzyme Inhibitor 

An enzyme inhibitor is a molecule which binds to an enzyme, decreasing the activity of the enzyme. The two main types of enzyme inhibitors are reversible inhibitors and irreversible inhibitors. The main difference between reversible inhibitors and irreversible inhibitors is that reversible inhibitors bind to the enzyme by forming non-covalent interactions while irreversible inhibitors bind to the enzyme by forming covalent interactions.  

The four types of reversible inhibitors are as follows. 

1. Competitive inhibitors – inhibitors which compete with the substrate for the active site of the enzyme 
2. Uncompetitive inhibitors – inhibitors which bind to the enzyme-substrate complex 
3. Noncompetitive inhibitors – inhibitors which prevent the dissociation of the enzyme-substrate-inhibitor (ESI) complex 
4. Mixed inhibitors – inhibitors which bind to both enzyme and the enzyme-substrate complex 
   

   Figure 2: Reversible Inhibition

Furthermore, irreversible inhibitors contain reactive functional groups, which modify amino acid residues in the active site of the enzyme. N-ethylmaleimide is such an irreversible inhibitor that react with the -SH group of cysteine residues.  

Similarities Between Enzyme Activator and Enzyme Inhibitor 

• Enzyme activator and enzyme inhibitor are two types of molecules which bind to enzymes, regulating the enzyme activity. 
• Both help in the regulation of the rate of biochemical reactions based on the requirements of the body. 

Difference Between Enzyme Activator and Enzyme Inhibitor 

Definition 

Enzyme activator refers to a molecule that binds to an enzyme, increasing the activity while enzyme inhibitor refers to a molecule that binds to an enzyme, decreasing the activity. Thus, this is the main difference between enzyme activator and enzyme inhibitor. 

Types 

Another difference between enzyme activator and enzyme inhibitor is that enzyme activators can be either proteins, peptides, lipids, small organic molecules or ions while the two main types of enzyme inhibitors are reversible and irreversible inhibitors. 

Examples 

Some examples of enzyme activators are calcium and magnesium ions, calmodulin, EDTA, EGTA, fructose 2,6-bisphosphate, hexokinase-1, and, glucokinase while some examples of enzyme inhibitors are N-ethylmaleimide, DFMO, DFP, and most of the pharmaceutical drugs. Hence, this is another difference between enzyme activator and enzyme inhibitor. 

Conclusion 

Enzyme activators are the molecules which bind to the enzyme, increasing the activity of the enzyme. Inorganic ions such as calcium and magnesium are common types of enzyme inhibitors. On the other hand, enzyme inhibitors are the molecules which bind to the enzyme, decreasing the activity of the enzyme. The two main types of enzyme inhibitors are reversible and irreversible inhibitors. Therefore, the main difference between enzyme activator and enzyme inhibitor is the regulation of the enzyme activity.

References:

1. Lopina, Olga D. “Enzyme Inhibitors and Activators.” Intech Open, IntechOpen, 29 Mar. 2017, Available Here

Image Courtesy:

1. “Figure 06 05 05” By CNX OpenStax -(CC BY 4.0) via Commons Wikimedia  
2. “Types of inhibition en” By fullofstars – en:Image:Inhibition.png (PD) (Public Domain) via Commons Wikimedia