Difference Between Apoenzyme and Coenzyme

The main difference between apoenzyme and coenzyme is that apoenzyme is the protein component of an enzyme, whereas coenzyme is the organic compound many enzymes require for catalytic activity. 

Apoenzyme and coenzyme are essential in the catalytic activity in biological systems. However, apoenzyme is catalytically inactive. In comparison, holoenzyme, which includes apoenzyme and cofactor, is catalytically active.     

Key Areas Covered

1. What is Apoenzyme
– Definition, Facts, Features
2. What is Coenzyme
– Definition, Facts, Features
3. Similarities Between Apoenzyme and Coenzyme
– Outline of Common Features
4. Difference Between Apoenzyme and Coenzyme
– Comparison of Key Differences
5. FAQ – Apoenzyme and Coenzyme
– Answers to frequently asked questions

Key Terms  

Apoenzyme, Coenzyme

What is Apoenzyme

Apoenzyme is the inactive form of the enzyme that activates upon cofactor binding. Typically, apoenzymes are present in the conjugate (complex) enzymes. Simple enzymes are another type of enzyme that only comprises the protein component. An apoenzyme itself is unable to catalyze a reaction without a cofactor. Only the combination of apoenzyme and cofactor produces the catalytically active holoenzyme.

Figure 1: Apoenzyme

Furthermore, holoenzyme refers to the apoenzyme and the complete and catalytically active cofactor. A cofactor can be either a metal ion or a small organic molecule. Most metal ions are tightly bound to the enzyme through a covalent or non-covalent bond. They are known as prosthetic groups. Small organic molecules are known as coenzymes. Coenzymes can be either tightly or loosely bound to the enzyme. Prosthetic groups can be Co, Cu, Mn, Mg, Fe, Ni, etc. ions. Coenzymes can be NAD, NADP, FAD, biotin, folic acids, etc.

What is Coenzyme

A coenzyme is a small organic molecule that acts as a cofactor. It binds to the enzymes, assisting the function of the enzyme. While facilitating enzymatic action, coenzymes serve as intermediate carriers of electrons, specific atoms, or functional groups between biochemical reactions. Some functional groups transferred by coenzymes are hydride ions, hydrogen atoms, methyl groups, and acyl groups. Once bound to a chemical group, the structure of the coenzyme is changed. Therefore, coenzymes can be considered as the second substrate of the enzyme. Hence, they are called co-substrates. Coenzymes need to regenerate to participate in the reaction again and again.

Figure 2: Coenzyme  

Moreover, some coenzymes include NAD (nicotine adenine dinucleotide), NADP (nicotine adenine dinucleotide phosphate), FAD (flavin adenine dinucleotide) (Vit.B2), CoA (coenzyme A), CoQ (coenzyme Q), thiamine (vitamin B1), pyridoxine (vitamin B6), biotin, folic acid, etc.  

Similarities Between Apoenzyme and Coenzyme

• Apoenzyme and coenzyme are two components of an enzyme.
• They occur in conjugated enzymes.
• The apoenzyme, along with the coenzyme, is called the holoenzyme.

Difference Between Apoenzyme and Coenzyme

Definition

Apoenzyme refers to an enzymatically inactive protein part of an enzyme, which requires a cofactor for its activity. In contrast, coenzyme refers to an organic molecule that binds to the active sites of specific enzymes to assist in the catalysis of a reaction.

Type

Apoenzyme is a protein component of the enzyme, while coenzyme is an organic compound.   

Function

Apoenzymes are responsible for the specificity of the enzyme towards the substrate, while coenzymes assist enzymes in turning substrates into products. 

FAQ: Apoenzyme and Coenzyme

What is the difference between enzyme and coenzyme? 

An enzyme is a protein that increases the rate of a reaction without altering the reaction process. In comparison, a coenzyme is an organic, non-protein molecule required for the enzyme’s catalytic activity.  

What is the difference between apoenzyme and Proenzyme? 

Zymogen or Proenzyme is the inactive form of the enzyme. It becomes the apoenzyme after the removal of extra amino acids. 

What is the difference between coenzyme and cofactor? 

A coenzyme is an organic or metalloorganic compound that serves as the non-protein component of the enzyme. In comparison, cofactors are non-protein compounds that activate enzymes. 

Conclusion

In brief, apoenzyme and coenzyme are two components of an active enzyme. Apoenzyme is the inactive protein component of the enzyme. It is responsible for keeping the specificity of the enzyme towards the substrate. In comparison, a coenzyme is the organic component of an enzyme that activates the enzyme. Therefore, the type and function are the main differences between apoenzyme and coenzyme.  

References:

1. Apoenzyme – definition and examples – biology online dictionary. Biology Articles, Tutorials & Dictionary Online. (2023, May 29). 
2. Coenzyme | definition, function & examples – video & lesson … – study.com. (n.d.-a). 

Image Courtesy:

1. “Image from page 42 of “Cytology” (1961)” By Internet Archive Book Images – Own Work (Public Domain) via Flickr
2. “OSC Microbio 08 01 ApoHolo” By CNX OpenStax – Own Work (CC BY 4.0) via Commons Wikimedia