Helicase

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Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands (i.e. DNA, RNA, or RNA-DNA hybrid) using energy derived from nucleotide hydrolysis.

Contents 1 Function 2 Structural features 3 Superfamilies 4 Pop Culture 5 References 6 External links

[edit] Function

Many cellular processes (DNA replication, RNA transcription, DNA recombination, DNA repair, Ribosome biogenesis) involve the separation of nucleic acid strands. Helicases are often utilized to separate strands of a DNA double helix or a self-annealed RNA molecule using the energy from ATP or GTP hydrolysis. They move incrementally along one nucleic acid strand of the duplex with a directionality specific to each particular enzyme. There are many helicases (14 confirmed in E. coli, 24 in human cells) resulting from the great variety of processes in which strand separation must be catalyzed.^{[citation needed]}

Helicases adopt different structures and oligomerization states. Whereas DnaB-like helicases unwind DNA as donut shaped hexamers, other enzymes have been shown to be active as monomers or dimers. Their precise mechanisms of action remain unclear.

[edit] Structural features

The common function of helicases accounts for the fact that they display a certain degree of amino acid sequence homology; they all possess common sequence motifs located in the interior of their primary sequence. These are thought to be specifically involved in ATP binding, ATP hydrolysis and translocation on the nucleic acid substrate. The variable portion of the amino acid sequence is related to the specific features of each helicase. Based on the presence of the defined helicase motifs, it is possible to attribute a putative helicase activity to a given protein. However, the presence of these motifs does not necessarily imply that the protein indeed possesses helicase activity. Conserved motifs do, however, support an evolutionary homology among enzymes. Based on the presence and the form of the helicase motifs, helicases have been separated in 4 superfamilies and 2 smaller families. Some members of these families are indicated, with the organism from which they are extracted, and their function.

[edit] Superfamilies

• Superfamily I: UvrD (E. coli, DNA repair), Rep (E. coli, DNA replication), PcrA (Bacillus stearothermophilus, role not precisely known), Dda (bacteriophage T4, replication initiation).

• Superfamily II: RecQ (E. coli, DNA repair), eIF4A (Baker's Yeast, RNA translation), WRN (human, DNA repair), NS3^[1] (Hepatitis C virus, replication). TRCF (Mfd) (E.coli, transcription-repair coupling factor).

• Superfamily III: LTag (Simian Virus 40, replication), E1 (human papillomavirus, replication).

• DnaB-like family: DnaB (E. coli, replication), gp41 (bacteriophage T4, DNA replication),T7gp4 (bacteriophage T7, DNA replication).

• Rho-like family: Rho (E. coli, Transcription termination factor ).

[edit] Pop Culture

"If I were an enzyme, I would be DNA helicase so I could unzip your genes." is a pick up line usually suggested as a joke between members of the scientific community.^{[citation needed]} This same line is also the name of a popular Facebook group, with over 107,000 members as of March 14, 2007.

[edit] References

1. ^ Dumont S, Cheng W, Serebrov V, Beran RK, Tinoco Jr I, Pylr AM, Bustamante C, "RNA Translocation and Unwinding Mechanism of HCV NS3 Helicase and its Coordination by ATP", Nature. 2006 Jan 5; 439: 105-108.

• Bird L, Subramanya HS, Wigley DB, "Helicases: a unifying structural theme?", Current Opinion in Structural Biology. 1998 Feb; 8 (1): 14-18.
• Betterton MD, Julicher F, "Opening of nucleic-acid double strands by helicases: active versus passive opening.", Physical Review E. 2005 Jan; 71 (1): 011904.

[edit] External links

• MeSH DNA+Helicases
• MeSH RNA+Helicases