Coenzyme And Holoenzyme? 7 Facts (Read This First!)

In this article, we get know about 7 Important Facts regarding ‘Coenzyme And Holoenzyme?’, along with their origin, composition, characteristics, functions and examples.

Holoenzyme is a fully evolved, catalytically active enzyme. An apoenzyme with its cofactors make into a holoenzyme. The holoenzyme is made up of every component needed for an enzyme to function, such as RNA polymerase, DNA polymerase III.

Let us discuss some facts and try to understand “Coenzyme And Holoenzyme?”

  • What is known as holoenzyme?
  • Is coenzyme and holoenzyme same?
  • How coenzyme is related with holoenzyme?
  • Why coenzyme is related with holoenzyme?
  • Difference between holoenzyme and coenzyme
  • Composition of holoenzyme

Apoenzyme + Cofactor = Holoenzyme

A conjugate enzyme is another name for holoenzyme. Without cofactors, the apoenzyme, or protein constituent of the enzyme, is inactive.

The non-protein component of a holoenzyme, which is necessary for its function, is known as a cofactor. Cofactors can be organic compounds known as coenzymes or metal ions (Mg2+, Fe3+, Zn2+).

The majority of coenzymes are made from vitamins. In a biological reaction, they operate as a momentary mediator of groups and transport these groups.

Examples of this:

  • Coenzyme A (CoA) carries acyl groups and is generated from substances like pantothenic acid, etc.
  • Flavin adenine dinucleotide (FAD) transfers electrons and is generated from vitamin B2 (riboflavin).
  • Nicotinamide adenine dinucleotide (NAD) carries hydride ions and is derived from nicotinic acid (Niacin).

A cofactor that is very firmly, frequently covalently, linked to the apoenzyme is known as a prosthetic group. Heme prosthetic groups connected to cytochromes are an example of a prosthetic group. Enzymes having metallic cofactors include, for instance:

  • Catalase, peroxidase, and cytochrome oxidase : Fe2+ or Fe3+
  • Pyruvate kinase: K+
  • Carbonic anhydrase: Zn2+

Thus, the holoenzyme or conjugate enzyme is the name for the active catalytic apoenzyme-cofactor complex.


Coenzymes are tiny, non-protein, chemical or inorganic compounds that support an enzyme’s particular function. They serve as a temporary transporter of particular functional groups between enzymes. The apoenzyme is joined by coenzymes, which promote enzyme activity. Technically speaking, a cofactor is a sort of coenzyme.

As an example, consider riboflavin, thiamine, adenosine triphosphate (ATP), NADH, NADPH, and folic acid.

These tiny molecular weight compounds are remarkably heat stable and tightly linked to an enzyme. Most of them are derivatives of the vitamin B-complex.

CoenzymesDerived from Vitamin B-Complex
CoASHPantothenic acid
FAD, FMNRiboflavin
Pyridoxal phosphatePyridoxin
Thiamine phosphateThiamine
TetrahydrofolateFolic acid
Examples of Coenzymes

What is known as holoenzyme?

A complete and catalytically active version of an enzyme is called a holoenzyme. A holoenzyme is an apoenzyme plus its cofactor. The majority of cofactors are tightly rather than covalently bound. Covalent bonds can, however, be formed using organic prosthetic groups, such an iron ion or a vitamin. RNA polymerase and DNA polymerase, which both include several protein subunits, are examples of holoenzymes.


  • Simple enzymes and conjugated enzymes are the two categories of enzymes based on their chemical compositions.
  • Simple enzymes: just have amino acids in them.
  • Enzymes that are conjugated: simple enzyme + cofactor (required for biological activity).
  • Cofactor – (minus) conjugated enzyme = Apoenzyme, a protein subunit.
  • Holoenzyme: Simple enzyme plus cofactor.
  • Therefore, simple enzymes are also known as apoenzymes. When apoenzymes join with a non-protein group, also known as a cofactor, either covalently or non-convalently, a complete biologically activated enzyme known as a holoenzyme is created.

Is coenzyme and holoenzyme same?

Many enzymes may also have a non-protein cofactor that is necessary for their catalytic activity in addition to the protein component. Holo-enzymes are those enzymes that contain both protein and non-protein components. The terms apo-enzyme (or apo-protein) and cofactor, respectively, refer to the protein component and non-protein component of a holo-enzyme. Coenzymes are an organic, non-protein component of holo-enzymes.

How coenzyme is related with holoenzyme?

Proteins that catalyze (nearly invariably) biochemical reactions are known as enzymes. Therefore, they increase the rate of chemical reactions, usually by attaching to and bringing reactants close together or by taking part in the reaction process. As catalysts, they are regarded as being unmodified (or returned to their initial state) after the conclusion of the reaction and can therefore take part in numerous iterations of the reaction provided that there is an enough supply of reactants.

  • Some enzyme processes also contain other chemical, nonprotein components that can contribute to the reaction but cannot catalyze it on their own.
  • These are known as coenzymes and are frequently vitamins or vitamin derivatives if they are organic (contain carbon). Nucleotides or their derivatives, such as ATP, NAD, FAD, etc., make up another significant class of coenzyme.
  • They are known as cofactors if they are non-organic and typically consist of metal ions like copper, zinc, cobalt, etc.
  • A holoenzyme is an enzyme that contains both its coenzyme and cofactor.
  • Consequently, a coenzyme is a holoenzyme’s nonprotein component.

Why coenzyme is related with holoenzyme

An apoenzyme is a dormant enzyme; it only becomes active when it binds to an organic or inorganic cofactor. A holoenzyme consists of an apoenzyme and its cofactor. A holoenzyme is whole and functional in catalysis. The majority of cofactors are tightly rather than covalently bound.

Ex: DNA polymerase & RNA polymerase are two examples of holoenzymes that contain several protein components.

The whole complexes have every subunit required for activity.

  1. Deoxyribonucleotides are polymerized into a DNA strand by the holoenzyme DNA polymerase. DNA replication actively involves DNA polymerase. In order to create the new strand, it interprets the intact DNA strand as a template.
  2. The resulting polymerized DNA strand is similar to the template’s orginal partner strand and complementary to the leading strand. Magnesium ions are utilized by DNA polymerase for catalytic activity.
  3. Another holoenzyme that catalyzes the RNA synthesis is RNA polymerase. Transcription, the process of building RNA chains using DNA genes as templates, requires RNA polymerase. It polymerizes ribonucleotides at an RNA transcript’s 3′ end.

Key factors

A biological catalyst is an enzyme. They are essential in several biological processes and substrate-specific. With the exception of catalytic RNAs and ribozymes, most enzymes are proteins comprised of amino acids. Some enzymes also need a non-protein component to function. Enzymes are divided into two categories based on this:

  • Simple enzymes, such as trypsin, pepsin, etc., are only composed of proteins.
  • Conjugate enzymes, also known as holoenzymes, are made up of a protein and a non-protein component that are both necessary for the action. The inactive apoenzyme is the protein component of the holoenzyme.
  • The non-protein component, also known as a cofactor, is required for the enzymes to perform their catalytic function. For instance, organic molecules or coenzymes (NAD+, NADP+, FAD2+), metal ions (Mg2+, Fe3+, Zn2+), and prosthetic groups For instance, DNA polymerase, pyruvate kinase, ADH, and catalase.

So Holoenzyme is a compound of an active enzyme and a cofactor, or an apoenzyme bound to a cofactor.

Apoenzyme (Inactive) + Cofactor ⇌ Holoenzyme (Active)

Difference between holoenzyme and coenzyme

  • Coenzymes are necessary organic substances that associate with enzymes to aid in the catalysis of processes. Cellular processes depend on coenzymes.
  • Coenzymes support the actions of enzymes in turn. To assist enzymes in carrying out their tasks, they bind to them in a loose manner. Coenzymes are organic, non-protein molecules that support an enzyme’s catalysis, or the reaction.
  • Coenzymes perform its function by attaching to the side groups of enzymes that is involved in the reaction, or the active side. Covalent bonds are used to connect enzymes and coenzymes together because they are non-metal organic compounds.
  • The coenzymes only share electrons with enzymes, never losing or gaining electrons. When they establish this link, they merely facilitate the reaction’s occurrence by transferring and carrying electrons through the reaction.
  • Coenzymes do not form an essential component of the enzymatic process. The coenzyme returns to free circulation inside the cell until it has been needed again after the reaction because the covalent connections are broken..
  • Holoenzymes operate based on the composition and characteristics of the active holo state. Due to their structure and cofactors, holoenzymes often exhibit significant bioactivity. Similar to other enzymes, holoenzymes are known to catalyze chemical processes, but they do so with the extra speed and stability of having their implanted cofactor.

Composition of holoenzyme

A holoenzyme is made up of two parts: (1) the core enzyme as well as (2) the sigma factor.

The holoenzyme is the entire RNA polymerase from E. coli. The core enzyme and the sigma factor are the two parts that make up the holoenzyme. The holoenzyme can be represented by the symbol α2 β β’ σ. !

1. The Core Enzyme:

The core enzyme can create RNA using DNA as a template, but it cannot start transcription at the correct places.

Four polypeptides, of the following three categories, make up the core RNA polymerase:

(a) The α subunit:

It occurs in two main transcripts per core enzyme’s active site and is focused on adhering to the promoter DNA. Its particular method of binding DNA is unknown.

(b) The β subunit:

It functions as an enzyme with a single copy and is involved in interacting with incoming nucleotides to produce RNA. Instead of relying on the pairing behavior of bases, it appears that the correct nucleotides adjacent DNA bases can be identified based on their overall geometry in relation to one another.

(c) The β’ subunit:

To enable transcription to begin, it is engaged in interacting with the template DNA, or the single-stranded length of DNA created.

The functionality of the core enzyme as a whole has been influenced by each subunit, and at least these are some of the four following distinct functional sites should be present.

(i)  When DNA is being replicated one base at a time, a DNA unwinding site constantly splits the DNA daughter strand as the core enzyme moves.

(ii) The location that binds to the transcription-active “antisense strand,” or strand.

(iii) A second site binds to the DNA being transcribed. “sense strand,” which is the strand opposite the “antisense strand.”

(iv) A DNA rewinding region is responsible for wrapping the two dna strands into a common duplex.

2. The Sigma Factor:

The sigma factor contributes to the RNA polymerase’s stable attachment to the promoter DNA and perhaps to the start of transcription. It is not directly involved in transcription because it is expelled when the RNA chain approaches 8 or 9 bases. The core enzymes then continues transcription (i.e., RNA synthesis).

Eukaryotic RNA Polymerase:

All eukaryotes have the three distinct RNA polymerases RNA polymerase I, II, and III.

RNA polymerase I:

The nucleolus houses this enzyme, which is in charge of ribosomal RNA transcription.

RNA polymerase II:

The nucleoplasm, a region of the nucleus other than the nucleolus, is where this polymerase is found. This enzyme is a key player in the RNA polymerase process and is responsible for transcription of all mRNA-producing genes. The transcript in eukaryotes begins as mRNA precursors known as heterogeneous nuclear RNA (hn RNA), which are then converted into mRNA.

coenzyme and holoenzyme
RNA Pol II in action, displaying the POLR2A C-terminal CTD extension from Wikipedia

RNA polymerase III:

This enzyme transcribes the tRNA and 5S RNA genes and is found in the nucleoplasm.

RNA polymerase in the organelle:

In addition to the nuclear RNA polymerases I, II, and III, eukaryotic cells also have small quantities of RNA polymerase activity in intracellular organelles, including such mitochondria and plastids. Comparatively, few genes contained in these organelles must be transcribed by these polymerases. These enzymes resemble phage RNA polymerases, it seems.


In the above article, we studied about Coenzyme and Holoenzyme. The similarities and differences between them. Their origin, composition, functions and examples have been discussed in detail.


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