Difference Between Metalloenzymes and Metal Activated Enzymes

The key difference between metalloenzymes and metal activated enzymes is that the metalloenzymes have a firmly bound metal ion as the cofactor whereas the metal ions in metal activated enzymes are not firmly bound.

The activity of some enzymes depends on metal ions because these metal ions act as cofactors. These enzymes are in two major categories as metalloenzymes and metal activated enzymes. Therefore, these enzymes are different from each other according to the presence or absence of tightly bound metal ions. Let us discuss more details on these enzymes.

1. Overview and Key Difference
2. What are Metalloenzymes
3. What are Metal Activated Enzymes
4. Side by Side Comparison – Metalloenzymes vs Metal Activated Enzymes in Tabular Form
5. Summary

Metalloenzymes are enzymes that contain a tightly bound metal ion. This metal ion forms coordinate covalent bonds with the amino acids of the enzyme or with a prosthetic group. Further, it acts as a coenzyme and imparts the activity of the enzyme. When considering the location of the metal ion in the enzyme, it usually occurs in a specific region on the enzyme surface. Therefore, the ion does not disturb the binding of the substrate with the active site. Sometimes, enzymes require more than one metal ion for its activity. In rare occasions, they require two different metal ions as well. The most common metals that involve in this are Fe, Zn, Cu, and Mn. The metalloenzymes containing metal centers other than iron (non-heme centers) are wide spread in nature.

Difference Between Metalloenzymes and Metal Activated Enzymes

Figure 01: Enzyme Action

Examples of metalloenzymes:

Amylase, thermolysin are bound with Ca2+ ions
Dioldehydrase, glycerol dehydratase are bound with Co2+
Cytochrome c oxidase, dopamine-b-hydroxylase contains Cu2+
Catalase, nitrogenase, peroxidase, succinate dehydrogenase contains Fe2+
Arginase, histidine-ammonia lyase, pyruvate carboxylase contains Mn2+

Metal activated enzymes are enzymes that have an increased activity due to the presence of metal ions. Most of the times, these metal ions are either monovalent or divalent. However, these ions are not tightly bound with the enzyme as in metalloenzymes. The metal can activate the substrate, thus engage directly with the activity of the enzyme. These enzymes require metal ions in excess. Ex: around 2-10 times higher than the concentration of the enzyme. It is because they cannot bond with the metal ion permanently. However, these enzymes lose their activity during its purification.

Examples of meta activated enzymes:

Pyruvate kinase requires K+
Phosphotransferases requires Mg2+ or Mn2+

The metalloenzymes are enzymes which contain a tightly bound metal ion. As a unique characteristic, they have a firmly bound metal ion as the cofactor. Moreover, these enzymes require either one or two metal ions bound on to a specific region of the surface of the enzyme for their activity. Metal activated enzymes are enzymes that have an increased activity due to the presence of metal ions that are not firmly bound. It is the key difference between metalloenzymes and metal activated enzymes. That is, unlike the metalloenzymes, the metal-activated enzymes do not have a firmly bound metal ion as the cofactor. In addition to that, these enzymes require a high concentration of metal ions around them.

Difference Between Metalloenzymes and Metal Activated Enzymes in Tabular Form

Summary – Metalloenzymes vs Metal Activated Enzymes

The enzyme, for which the activity depends on the presence of metal ions, is of two types; they are, metalloenzymes and metal activated enzymes. The difference between metalloenzymes and metal activated enzymes is that the metalloenzymes have a firmly bound metal ion as the cofactor whereas the metal ions in metal activated enzymes are not firmly bound.

Reference:

1. “Metalloprotein.” Wikipedia, Wikimedia Foundation, 23 July 2018. Available here
2. “Metalloenzyme.” Egyptian Journal of Medical Human Genetics, Elsevier. Available here

Image Courtesy:

1.’Enzyme action’By Muessig – Own work, (CC BY-SA 3.0) via Commons Wikimedia