Difference Between Apoenzyme and Holoenzyme

Main Difference – Apoenzyme vs Holoenzyme

Enzymes are biological catalysts that catalyze biochemical reactions that take place inside the cell. An enzyme only becomes active when the cell needs to perform the biochemical reaction catalyzed by that particular enzyme. Apoenzyme and holoenzyme are two states of enzymes. The main difference between apoenzyme and holoenzyme is that apoenzyme is the catalytically-inactive, protein component of the enzyme whereas holoenzyme is the catalytically-active form of the enzyme, consisting of the apoenzyme and the cofactor. A cofactor can be either a metal ion or a small organic molecule. The main function of the cofactor is to bind with the structure of the apoenzyme to assist the function of the enzyme.

Key Areas Covered

1. What is Apoenzyme
     – Definition, Facts, Activity
2. What is Holoenzyme
     – Definition, Facts, Activity
3. What are the Similarities Between Apoenzyme and Holoenzyme
     – Outline of Common Features
4. What is the Difference Between Apoenzyme and Holoenzyme
     – Comparison of Key Differences

Key Terms: Apoenzyme, Coenzyme, Cofactor, Conjugated Enzyme, Holoenzyme, Prosthetic Group

What is an Apoenzyme

Apoenzyme is the inactive form of the enzyme that activates upon the binding of a cofactor. Typically, apoenzymes are present in the conjugate (complex) enzymes. Simple enzymes are another type of enzymes that are only composed of the protein component. The formation of the holoenzyme is shown in figure 1.

Figure 1: Formation of Holoenzyme

An apoenzyme itself is unable to catalyze a reaction without a cofactor. Only the combination of apoenzyme along with the cofactor produces the catalytically-active enzyme.

What is a Holoenzyme

Holoenzyme refers to the apoenzyme along with the cofactor that is complete and catalytically-active. A cofactor can be either a metal ion or a small organic molecule. Most metal ions are tightly-bound to the enzyme through covalent bond 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. Cofactors bound to the succinate dehydrogenase enzyme are shown in figure 2.

Figure 2: Cofactors of Succinate Dehydrogenase

Some examples of holoenzymes are DNA polymerase and RNA polymerase. These enzymes consist of multi-protein subunits. Thus, they are complete and complex. Only holoenzymes can catalyze a biochemical reaction.

Similarities Between Apoenzyme and Holoenzyme

• Apoenzyme and holoenzyme are two states of an enzyme.
• Both apoenzyme and holoenzyme are composed of a protein component.

Difference Between Apoenzyme and Holoenzyme

Definition

Apoenzyme: Holoenzyme refers to the apoenzyme along with the cofactor which is complete and catalytically-active.

Holoenzyme: Apoenzyme refers to the inactive form of the enzyme which activates upon the binding of a cofactor.

Catalytic Activity

Apoenzyme: Apoenzyme is the inactive form of the enzyme.

Holoenzyme: Holoenzyme is the catalytically-active form of the enzyme.

Content

Apoenzyme: Apoenzyme consists of the protein part of the enzyme.

Holoenzyme: Holoenzyme consists of the apoenzyme and one or several cofactors.

Complexity

Apoenzyme: Apoenzymes are incomplete enzymes that are less complex.

Holoenzyme: Holoenzymes are complete and complex enzymes.

Examples

Apoenzyme: The catalytic components of DNA polymerase enzyme are considered as apoenzymes.

Holoenzyme: The multi-subunit complex of DNA polymerase is considered as the holoenzyme.

Conclusion

Apoenzyme and holoenzyme are two states of enzymes that catalyze biochemical reactions inside the cell. Apoenzyme is the protein component of the enzyme, which is inactive. The apoenzyme becomes active with the binding of one or several cofactors. The active form of the apoenzyme is known as the holoenzyme. The main difference between apoenzyme and holoenzyme is the structure and the catalytic activity of each state of the enzyme.

Reference:

1. “Structural Biochemistry/Enzyme/Apoenzyme and Holoenzyme.” Wikibooks, Available here.

Image Courtesy:

1. “Enzymes” By Moniquepena – Own work (Public Domain) via Commons Wikimedia
2. “Succinate Dehydrogenase 1YQ3 Electron Carriers Labeled” By Richard Wheeler (Zephyris) – Based on PDB: 1YQ3. Labeled version of en:Image:Succinate Dehydrogenase Electron Carriers Unlabeled.png (CC BY-SA 3.0) via Commons Wikimedia