Holoenzyme and Enzyme? 11 Facts (Read This First)


In this article we are focusing on the topic “Holoenzyme and Enzyme? 11 Facts (Read This first), by giving the detailed facts and putting down the statements supporting the topic.

Enzymes exists in two forms inactive and active form. The inactive form of enzyme is known as zymogen and the active form is called holoenzyme. Holoenzyme consist of two components, namely apoenzyme and coenzyme. Apoenzyme regulates the specificity for substrates in combination with a coenzyme which is an enhancer of the catalytic activity of an enzyme.

Holoenzyme is known as a complete active biochemical form of an enzyme which is made up of two components, apoenzyme and a coenzyme which makes the enzyme catalytically active. They also determine the specificity of the enzyme with respect to different substrates in the reaction mixture by binding to the active site of the enzyme where the transformation of substrates to products takes place. 

Functions of holoenzymes

An enzyme in its active form is known as a holoenzyme which is catalytically active and participate in the reaction process where substrates are converted to the products. At the end of the reaction the constituents of the enzyme like, apoenzyme and coenzyme which are small protein molecules or functional groups are released from the active site of the enzyme into the blood stream to participate in another enzymatic reaction.

Enzyme needs cofactor for their complete activation and functions, without any cofactor an enzyme will be rendered inactive and is called as a zymogen. An example of zymogen can be trypsinogen which is an inactive form of trypsin. Cofactors include a non-protein molecules or metallic ion which improves the catalytic activity of an enzyme.

Structure of holoenzymes

A typical composition of holoenzyme includes, an enzyme plus a cofactor and an apoenzyme, which can be any non-protein molecules. There are variety of different holoenzymes each having their significant functions. The structural components of some holoenzymes are as follows:

  • DNA Polymerase III: It consists of there subunits, the alpha (α) subunit, the ε subunit and θ subunit.
  • RNA Polymerase II: It consists of multiple proteins that involves in DNA transcription. It consists of 12 subunits and its size is of 550 kilo Dalton.  It basically contains transcriptional factors, RNA polymerase II and regulatory proteins for gene expression which are together known as SRB proteins.
  • Protein Kinase A Holoenzyme: It exists in two form the inactive and active form. The inactive form of PKA consists of tetramer with two regulatory subunits and two catalytic subunits. It is a type of serine protein kinase.

Examples of holoenzymes

There are different types of holoenzymes known with multiple subunits and are found in both prokaryotes and eukaryotes. The examples of holoenzymes include;

DNA Polymerase III

DNA polymerase III is a holoenzyme which is primarily involved in the replication of prokaryotes. It works in combination with other DNA polymerases include, Pol I, Pol II, Pol IV and V.

holoenzyme and enzyme
Structure of DNA Polymerase Holoenzyme III Image from Wikipedia

RNA polymerase II

RNA Polymerase II is a holoenzyme which is found in eukaryotes which participate in the process of transcription, formation of an mRNA, snRNA’s and microRNA’s from DNA duplex. It is also known as DNA dependent RNA polymerase.

RNA Polymerase II Image from Wikipedia

Protein Kinase A Holoenzyme

PKA holoenzyme act as a receptor for cyclic adenosine monophosphate (cAMP) and is also a regulator for cardiac function and its morphology. It is activated by different types of ligands such as, hormones, neurotransmitters, and other signals.

What is the relation between holoenzyme and enzyme?

Holoenzyme is an enzyme bonded with cofactors and coenzymes that enhances its catalytic activity. Enzymes are inactive in their normal form; they are freely floating in the blood stream waiting to be activated by their specific binding partners. They are activated by the efficient binding of cofactors and coenzymes and then they are named as holoenzymes.  

Trypsinogen is an inactive form of the enzyme trypsin which is produced in the pancreas and is activated into the active form trypsin by the cleavage of amino-terminal activation peptide with the help of enterokinase in the small intestine.

Difference between Holoenzyme and Apoenzyme

HoloenzymeApoenzyme
Holoenzyme is a complex structure that forms by the combination of apoenzyme and its cofactors which are non-protein in nature.Apoenzyme is the inactive form of an enzyme which is converted to the activated form by the joining of certain cofactors.
It refers to the complete catalytically active form of the enzyme.It is known as an inactive enzyme.
It is composed of an enzyme and several cofactors.It is composed of only enzyme.  
Their structure is complex and active.Structure is less complex and inactive.
Examples include, DNA polymerase III and RNA polymerase II which are both multi-subunit structures.Examples include, trypsinogen, apo glucose oxidase, etc.
Difference between Holoenzyme and Enzymes

How is holoenzyme an enzyme?

Holoenzyme is a complete active form of enzyme which is involved in catalytically activating the chemical and biological reactions occurring inside the cells. Holoenzymes contain multiple subunits in different forms. Holoenzyme is formed of an inactive enzyme bonded with coenzymes.

Enzymes are made up of only proteins while the holoenzyme is a conjugated form of an inactive enzyme and a non-protein molecule called cofactor which is making the enzyme catalytically active.

Is holoenzyme an active enzyme?

Yes, holoenzyme is a complex of cofactors and coenzymes which together activates apoenzyme which is an inactive enzyme. The active form of apoenzyme is known as holoenzyme which performs in enhancing the speed of the biochemical reactions. Cofactor which is the binding partner of apoenzyme is a non-protein molecule or a metal ion.

The example of cofactor includes, ATP, minerals and vitamins. Coenzymes are organic molecules which include, nicotineamideadenine dinucleotide (NAD), nicotineamide adenine dinucleotide phosphate (NADP), Flavin adenine dinucleotide) FAD). These coenzymes are involved in electron transfer reaction.

Can all enzyme be the holoenzyme?

Yes, all the enzymes can be holoenzymes. Any enzyme cannot remain inactive for a long time in a living cell. Enzyme needs cofactors to be activated and without any cofactor they are termed as apoenzymes. Holoenzymes are the catalytically active enzymes performing in variety of biochemical reactions inside living organisms.

Where are holoenzymes found?

There are variety of holoenzymes found in the body which are made naturally by the conjugation of cofactors and coenzymes with the enzyme. One type of holoenzyme is CaMKII which are made up of 12 subunits is found at the synapses where the nerve cells join each other. PKA holoenzyme (Protein Kinase A) is found to be functionally activated in the heart where it regulates the cardiac functioning and morphology. DNA

How holoenzymes are produced and when?

Apoenzyme and its binding cofactors are involved in the production of holoenzymes. Holoenzymes are made up of polypeptides and inorganic or organic cofactor. Apoenzyme is an inactive enzyme which needs cofactors and coenzymes to turn to its active stage. After being activated by its binding partners an enzyme is now called a Holoenzyme which is catalytically active.

Holoenzymes are also called conjugated enzymes. Bacterial RNA polymerases are made up of certain proteins and multiple subunits. DNA polymerase III holoenzyme is found to be activated at the replication fork and is a part of replisome during prokaryotic DNA replication. RNA polymerase holoenzyme is found in eukaryotes and is activated during the process of formation of RNA known as translation process.

What is important about holoenzymes?

Holoenzyme is important because of its function as an active enzyme with a high catalytic activity. The main function of holoenzymes is the formation of products from the variety of substrates. Holoenzymes plays a significant role in various biological processes in both prokaryotes and eukaryotes.

DNA polymerase III and RNA polymerase II are a type of holoenzymes which are significant for the synthesis of mRNA, snRNA and microRNA in prokaryotes and eukaryotes.

Conclusion

To conclude the article, we can say that holoenzymes are the active form of enzymes and are significant in performing variety of biological reactions and are important for maintaining the normal functioning of the cells.

Sneha Sah

Hey! I am Sneha Sah, I have completed post graduation in Biotechnology. Science has always been fascinating to me and writing is my passion. As an academic writer my aim is to make Science easy and simple to learn and read. Reach me at- Snehashah14111996@gmail.com

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