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Difference Between Apoenzyme and Holoenzyme

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Last updated date: 27th Feb 2024
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Introduction to Apoenzyme and Holoenzyme Difference

In order to understand the difference between Apoenzyme and Holoenzyme, we must first be aware of enzymes, their functions, and cofactors. Let us first discuss these topics in detail.

What are Enzymes?

Proteins called enzymes serve as biological catalysts by speeding up chemical processes. The chemicals that interact with enzymes are known as substrates, and the enzyme converts the substrates into other molecules known as products. The bulk of metabolic processes in cells must be catalysed by enzymes in order to move at rates fast enough to sustain life. While some enzymes are made of RNA molecules, most enzymes are composed of proteins.


Why are Enzymes Needed?

RNA molecules perform the translation of DNA information into proteins. Each cell contains several enzymes that the body uses in various ways. Enzymes support the chemical reactions necessary to maintain the well-being of an organism. For instance, they provide an important function for the metabolism, which turns food and liquids into energy.


What are Cofactors?

To help with catalytic activity, many enzymes need an extra tiny molecule called a cofactor. A non-protein molecule known as a cofactor is used to carry out chemical processes that the body's normal 20 amino acids are unable to complete. Cofactors can be either tiny organic compounds like coenzymes or inorganic molecules like metals. 


Functions of Cofactors

Inorganic and organic molecules called cofactors serve as catalysts in enzyme processes, usually in the form of metal ions or coenzymes. The majority of coenzymes are vitamin derivatives that are organic, nonprotein, and soluble in water by phosphorylation. Coenzymes connect to apoenzyme protein molecules to create active holoenzymes.


What is Apoenzyme and Holoenzyme?

Apoenzyme: An enzyme known as an apoenzyme lacks the cofactor it needs to function. Apoenzymes are inactive enzymes that become active when they attach to an organic or inorganic cofactor. Since they are in charge of an enzyme's substrate selectivity, apoenzymes are crucial for enzymatic function. Apoenzymes are inactive enzymes until they attach to an organic or inorganic cofactor, which causes them to become active. The enzyme transferases, oxidoreductases, ligases, and isomerases are examples of apoenzymes and coenzymes-containing enzymes.


Holoenzymes: Holoenzyme is a fully developed, catalytically active enzyme. An apoenzyme with its cofactors combine into a holoenzyme. The holoenzyme comprises each of the components needed for an enzyme to operate, such as DNA polymerase III and RNA polymerase.


Apoenzyme(inactive) + Cofactor= Holoenzyme(active).


A conjugate enzyme is another name for a holoenzyme. The protein component of the enzyme, known as the apoenzyme, is inactive without cofactors.


Thus, the holoenzyme or conjugate enzyme is the name for the catalytically active apoenzyme-cofactor combination.


Some Functions of both Apoenzyme and Holoenzyme Include- 

  • A holoenzyme cannot operate correctly without apoenzymes. DNA polymerase, which consists of an apoenzyme and a cofactor, is one of the most significant holoenzymes. 

  • Deoxyribonucleotide polymerization is catalyzed by DNA polymerase to produce DNA. After reading the intact DNA strand, it creates new DNA strands, which aids in DNA replication. It creates a new strand that is similar to its partner strand and complementary to the template strand. Magnesium ions serve as DNA polymerase's substitute cofactor.

  • Another apoenzyme and cofactor-based holoenzyme is RNA polymerase. The process of transcription is carried out by RNA polymerase, which builds RNA chains from DNA templates.


Difference Between Apoenzyme and Holoenzyme 

S.No

Category

Apoenzyme

Holoenzyme

1

Definition

The catalytically inactive protein in an enzyme.

The compound of apoenzyme and cofactor which is catalytically active.

2

Activity

Initially inactive, it only becomes active after connecting to a cofactor.

Active and completely capable of facilitating a biological response.

3

Composition

Only the protein constituent is included in apoenzymes. 

It includes cofactors like metal ions or other chemical complexes called coenzymes together with proteins.

4

Cofactors

There are no cofactors present.

Cofactors like metal ions or coenzymes are present.

5

Examples

The holoenzyme's protein portion, such as carbonic anhydrase without the zinc ion(cofactor).

Carbonic anhydrase, DNA and RNA polymerases, etc.


Summary

There are two states of enzymes: apoenzyme and holoenzyme. The primary distinction between apoenzyme and holoenzyme is that holoenzyme is the catalytically active form of the enzyme, consisting of the apoenzyme and the cofactor, whereas apoenzyme is the catalytically inactive protein component of the enzyme. A metal ion or a tiny organic molecule can serve as a cofactor. The cofactor's primary job is to interact with the apoenzyme's structure in order to support the enzyme's activity.

FAQs on Difference Between Apoenzyme and Holoenzyme

1. What is the importance of Apoenzymes in the living body?

Apoenzymes participate in catalytic processes and go through modifications to enable the conversion of the substrate into a product. For instance, the apoenzymes of glutamate dehydrogenase, lactate dehydrogenase, malate dehydrogenase, and oxidoreductases are connected to NAD coenzymes. The coenzyme NAD has the ability to take up hydrogens released from various substrates. Therefore, the apoenzyme protein component is in charge of each holoenzyme's specificity as well as the ability to recognize its unique substrate.

2. What is the importance of Holoenzymes in the living body?

The holoenzyme is an enzyme ready to perform its catalytic role, which is to speed up certain chemical processes that are produced in various places. Depending on the precise activity of the holoenzyme, the functions may change. One of the most crucial is DNA polymerase, whose function is to make sure the DNA is duplicated accurately. A key holoenzyme in the defense against cellular toxicity is catalase. Its function is to break down the byproduct of cellular metabolism, hydrogen peroxide, into oxygen and water. A manganese ion and a HEMO prosthetic group, which is comparable to hemoglobin, are needed as cofactors to activate the catalase apoenzyme.

3. Explain apoenzyme and holoenzyme.

An enzyme complex that has been activated and combined is called a holoenzyme. Cofactor and apoenzyme must bind for a certain catalytic function. A holoenzyme is an apoenzyme in its active state. Co-factors can be organic or metal-organic molecules (coenzymes), inorganic ions, or organic ions. The two forms of enzyme interchange depending on the requirement of the body. If the substrate needs to be catalyzed, then the cofactor is supplied to the apoenzyme and the active holoenzyme can be generated.