Is Helicase An Enzyme? 9 Facts (Read This First!)

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The suffix -ase are added to the end of an enzyme depending upon the name of the substrate. Let us learn some facts about the enzymatic nature of helicase.

Helicase is an enzyme required for the metabolism of nucleic acid like replication, transcription, translation, repair and recombination. There are mainly two types of helicases, DNA and RNA helicase.

They are one among the group of hydrolysing enzymes. Since helicase enzyme breaks apart the DNA helix, it is named as helicase.

Let us see what kind of enzyme helicase is, where they are found, how they are produced and many other related questions in this article.

What are helicase enzymes?

All enzymes are proteins and all enzymes are divided into different classes according to their role. Let us get to know more about helicases enzyme.

Helicase are large group of enzymes belong to the class of Hydrolases which bring about the cleavage of substrate. They are basically motor proteins that move unidirectional during the process of replication. They help in separating double stranded DNA for the initiation of replication process.

Animal cell cytoplasm can be accessed by a class of molecular motors known as motor proteins or helicase enzymes. By hydrolyzing ATP, they transform chemical energy into mechanical work.

Where are helicase enzymes found?

All eukaryotes contain helicase enzyme for various metabolic reactions. Let us find out the exact location of helicase enzyme.

Helicase enzymes are found specifically in the nuclear region of a cell. In plants, helicases are found in chloroplast, nuclei and mitochondria. Inside the nucleus, helicases are found on the X chromosome.

Functions of helicase enzyme

Helicase is one of the important enzyme used in the metabolic pathway. Let us get to know more about the functions of it.

Some of the important functions of helicase enzymes are given below:

  • Using the power of ATP hydrolysis, which is characterized by the breaking of hydrogen bonds between annealed nucleotide bases, helicases are frequently utilized to separate the strands of a DNA double helix or a self-annealed RNA molecule.
  • They also help to facilitate homologous DNA recombination and eliminate proteins linked to nucleic acids.
  • Helicases make it easier for RNA to go through metabolic processes such translation, transcription, ribosome biogenesis, RNA splicing, transport, editing, and disintegration.
  • Each helicase moves gradually along the duplex’s single nucleic acid strand with a directionality that is unique to that enzyme.
  • Helicases may function considerably more quickly in vivo than in vitro, because auxiliary proteins help to destabilize the fork junction. 
  • All RNA-related processes, including transcription, splicing, and translation, involve RNA helicases, which are responsible for modifying the shape of RNA molecules.
  • All helicases have sequence motifs (sequence pattern of nucleotide and amino acids) inside of their core structure that are important in ATP binding, ATP hydrolysis, and translocation along the nucleic acid substrate.
  • RNA helicases play crucial role in recognizing virion RNAs.

Structure of helicases enzymes

Different types of helicase enzymes are present in eukaryotes depending upon their structure and function. Let us explore more about their structure.

Helicase have usually a ring-shaped hexameric structure. The primary structure of DNA helicase is made up of 3 polymers with fourteen long chains of amino acid residues. Six identical subunits are arranged hexagonally in the DNA helicase revealed from bacteriophage T7’s crystal structure.

Unexpectedly, the ring is not sixfold symmetric, but rather is compressed significantly. Except for the eukaryotic minichromosomal maintenance (MCM) helicase, all hexameric helicases have a distinctive ring-shaped structure and are homohexamers. The twelve known hexameric helicases mostly function in transcription, recombination, and DNA replication.

What kind of enzyme is helicase?

All enzymes are proteinaceous in nature. Let us explore what kind of enzyme helicase is.

Helicases enzymes are that enzymes which belongs to the class of hydrolases and the sub-class of translocases. The complex coiling of DNA molecule in chromosomes makes its unwinding a difficult task. Utilizing ATP hydrolysis as a source of energy, helicase enzymes aid in unravelling the double helix.

What is important about helicases enzymes?

Helicase enzyme plays a crucial role during DNA replication. Let us discover the importance of helicase enzymes.

Helicase enzyme is important because of its substrate specificity. Substrate specificity can be determined by the direction in which it moves on the molecule. Helicase usually move on nucleic acid molecule in a unidirectional way which can be either 5′-3′ or 3′-5′.

Helicases directionality is determined by the polarity of the strand to which it is attached. For instance, 5′3′ helicases attach very effectively to substrates with a 5′ overhang but very poorly to those with a 3′ ssDNA overhang. 

How are helicase enzymes produced and when?

Helicase enzymes are basically protein foldings. Let us explore how and when helicase enzymes are formed.

Helicases are formed from 464 amino acid residues comprising a long chain primary structure. During replication, the side chains of amino acids interact with each other and produces a tertiary structure of helicases which is functional.

DNA helicases unwind DNA at locations known as origin of replication or ori during DNA replication, where synthesis will begin. The replication fork, so named because the two strands of DNA seem forked as they are unzipped apart, is a structure that is created while DNA helicase proceeds to unwind the DNA.

What are the three models given for DNA unwinding by enzyme helicases?

Unidirectional translocation and base-pair separation mechanisms help in the catalytic unwinding of long segments of DNA. Let us look at the three models for DNA unwinding.

The three general models are proposed according to the interaction of helicase with DNA helix. These are :

  1. Wedge model
  2. Torsional model and
  3. Helix-destabilizing model.

Wedge Model

In wedge model, helicase binds to the unwinding region and tightly bound to the one strand of the DNA. The other strand does not specifically interacts with the helicase.

Torsional Model

In torsional model, the hexamer helicase engages with both the strands, one strand is tightly bound to the channel site while other strand also interacts with outer region of hexamer that generates torque and rotates both the strands.

Helix De-stabilizing model

In the helix-destabilizing model, the hexamer interacts with one of the separated strands in the central channel, and also interacts with the dsDNA on the outer parts of the hexamer. The duplex region is melted by the helicase, and it translocates unidirectionally along the DNA in the central channel.

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is helicase an enzyme
Three proposed models for unwinding DNA by helicase enzyme

Classification of  Helicases

Depending upon their function, helicases are classified into many types. Let us find out on what basis helicase enzymes are classified.

Helicase enzymes are distinguished on the basis of ring structure, type of helix and polarity.

Classification of helicase enzymes are detailed below:

Classified on the basis of formation of ring structure

The enzyme helicases are classified into six superfamilies (SF).  The helicases who do not form ring structure are classified into SF 1 and SF 2. Those, who do make ring structure are classified under SF3-SF6.

Classified on the basis of the type of helix

Helicases are also classified as α and β helicase depending upon the type of helix to be catalyzed by them. Helicases α generally interact with single stranded DNA while β helicase interacts with double stranded DNA.

Classified on the basis of polarity

Another classification of helicases is done on the basis of polarity as they work in unidirection along with nucleic acid. Translocation polarity are of two types : Type A works in 3’-5’ polarity while Type B works well in 5’-3’ polarity.

Conclusion

To wrap up this post, we conclude that helicase is an enzyme belongs to the class of hydrolases. Helicases enzyme helps in breaking the hydrogen binds of nucleic acids to move them apart for DNA replication. Apart from this function, helicases also take part in various metabolic pathways like DNA repair and recombination.

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