Holoenzymes are a complete and catalytically active conjugate enzymes while the apoenzymes are the inactive proteinaceous part of a holoenzyme. Let us discuss in details.
A holoenzyme is composed of two parts- apoenzymes and cofactors. Apoenzymes are the proteinaceous part of a holoenzyme. Cofactors are small, non-proteinaceous compounds which can be both organic or inorganic.
Cofactors are essential for making the apoenzyme catalytically active. Let us look into some more facts related to holoenzymes and apoenzymes.
Is holoenzyme same as apoenzyme?
Apoenzymes are a part of holoenzymes. Let us see how they are similar or dissimilar.
As a holoenzyme is a complete enzyme while an apoenzyme is a part in which it cannot take part in biological reactions alone, they cannot be said to be the same. They not only differ from each other structurally and functionally but composition wise as well.
Some examples of holoenzymes are DNA polymerase, RNA polymerase, reverse transcriptase, carbonic anhydrase, pyruvate kinase, pyruvate carboxylase and succinate dehydrogenase.
Now, that we know that apoenzymes are the proteinaceous part of a holoenzyme, let us discuss in details the two types of cofactors that are responsible for activating the apoenzymes- Prosthetic groups and Coenzymes.
Prosthetic groups
Prosthetic groups are non-proteinaceous compounds that bound to enzymes either covalently or non-covalently. Let us see how they support a biological reaction.
Prosthetic groups take part in only one reaction in either forward or backward direction at a time. Hence, it interchanges between its forms in a single catalytic cycle. Prosthetic groups may contain metallic ions. They are called metalloenzymes when attached to an apoenzyme.
Some examples of such metalloenzymes are zinc, magnesium and iron-sulfur clusters.
Coenzymes
Coenzymes are organic compounds that may or may not contain a metal. Let us get to know more about it.
Coenzyme exist in more than one states – oxidized and reduced states. They undergo different reactions to interchange into their many possible states. For example, they can undergo one reaction to convert from oxidized to reduced state and then a different reaction to converted back to oxidized state.
As, they keep getting recycled in a metabolic pathway by undergoing the antagonistic reactions, they are not required by the cell in huge quantities and are present in relatively low concentrations.
A lot of vitamins act as coenzymes for different enzymes participating in several metabolic pathways. Some examples of such cofactors are coenzyme A, vitamin B7 or biotin, retinal and vitamin C .
Why is holoenzyme an apoenzyme?
Apoenzymes can be referred to as holoenzymes in some case. Let us see what such special cases are.
An apoenzyme can be said to be the same as a holoenzyme in cases where, an enzyme complex is solely composed of proteins and is functionally active.
Some examples of such simple enzymes are lipases, amylases, trypsin, pepsin, helicase, ligases.
How is holoenzyme an apoenzyme?
Simple enzymes are apoenzymes that are capable of catalyzing biological reactions and thus can be referred to as holoenzymes. Let us know more about this.
Simple enzymes can be referred to as apoenzymes due to their structural composition and because they can catalyze a biological reaction. They can be referred to as a holoenzyme too.
Difference between holoenzyme and apoenzyme.
As has been mentioned already, holoenzymes and apoenzymes are both structurally and functionally different from each other. Let us discuss it in detail.
Some major differences between the holoenzymes and the apoenzymes are mentioned in the table given below.
| HOLOENZYME | APOENZYME | |
| Definition | Holoenzymes are complete enzymes that are catalytically active. | Apoenzymes are the inactive form of an enzyme. |
| Function | It catalyzes reaction in the metabolic processes. | It forms the major structural part of a holoenzyme. |
| Structural composition | It is composed of proteins, organic compounds and may or may not contain metals. | It is composed solely of proteins. |
| Components | An apoenzyme and a cofactor come together to form a holoenzyme. | It is not attached to any other component. |
| Function | It catalyzes metabolic reactions. | It is functionally inactive. |
| Examples | Polymerase complex, Carbonic anhydrase. | Catalytic part of the DNA polymerase. |
Why apoenzyme is important for holoenzyme?
Apoenzymes form the functional entity of a holoenzyme, although it requires a cofactor or a coenzyme to activate it. Let us see why apoenzyme is important for holoenzyme.
Apoenzyme is important for holoenzyme because these cofactors and coenzymes basically act like a switch that turn the function of an enzyme on and off by binding and separating from the apoenzymes. So, an apoenzyme is responsible for maintaining the specificity in the enzyme substrate reactions.
Conclusion
Apoenzymes and holoenzymes are structurally and functionally different from each other such that apoenzymes forms a part of the holoenzyme. To make a catalytically functional enzyme or holoenzymes, the apoenzymes need to conjugate with a cofactor that as non- proteinaceous in nature.
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Hi …I am Tulika Priyadarshini, I have completed my Master’s in Biotechnology. Writing gives me mental peace and satisfaction. Sharing the knowledge that I gain in the process is a cherry on the cake. My articles are related to Lifesciences, Biology and Biotechnology. Lets connect through LinkedIn-
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