Helicase

Lohman, T. M., Thorn, K., and Vale, R. D., 1998. Staying on track Common features of DNA helicases and microtnbnle motors.. Cell 93 9-12. 

Heat of combustion, 113 Heat of hydrogenation, 186 table of, 187 Heat of reaction, 154 Helicase, DNA replication and, 1106 Hell-Volhard-Zelinskii reaction, 849 amino acid synthesis and. 1025 mechanism of, 849 Heme, biosynthesis of, 966 structure of, 946 Hemiacetal, 717 Hemiketal, 717 Hemithioacetal, 1148 Henderson-Hasselbalch equation,... 

Dicer represents the key enzyme in the RNAi pathway. Dicer is also known as Helicase with RNAse motif, heRNA, Helicase-moi, K12H4.8-like, or KIAA0928. Dicer produces cleaves long double-stranded RNA into small pieces of about 21-23 nucleotides. These so-called siRNA duplexes produced by the action of Dicer contain 5 -phosphates and free 3 -hydroxylgroups... 

Tai CL, Chi WK, Chen DS, Hwang LH (1996) The helicase activity associated with hepatitis C virus nonstructural protein 3 (NS3). J Virol 70 8477-8484 Tong X, Chase R, Skelton A, Chen T, Wright-Minogue J, Malcolm BA (2006) Identification and analysis of fitness of resistance mutations against the HCV protease inhibitor SCH 503034. Antiviral Res 70 28-38... 

Fig. 3 A ribbon diagram of the HCV NS3/4A protease ICU1 (Yao et al, 1999). The serine protease domain is shown in cyan with the catalytic triad highlighted in yellow, and the helicase domain is...

The helicases are enzymes central to life itself. The nature of double-stranded DNA means that before a polymerase can begin to copy the appropriate region of the nucleic acid, the two strands have to be unwound the separation of the two strands is the function of the helicase (Fig. 2). An indication of the significance of this family of enzymes is seen in the so-called Werner syndrome, where the helicase function required in the suppression of inappropriate recombination events is defective and causes genomic instability and cancer (for a review see Cobb and Bjergbaek 2006). 

F%. 2 Herpesvirus DNA-rephcation fork showing the site of action of helicase/primase inhibitors... 

Since the pioneering work of Kleymann et al. (2002), Betz et al. (2002), Baumeister et al. (2007), and Crute et al. (2002), who showed that compounds identified as inhibitors of the helicase-primase enzyme complex could alleviate herpesvirus-induced disease in animal models, the attention of researchers developing antiviral compounds has been drawn more and more towards the virus-encoded helicases, particularly those of Herpes viruses and of RNA viruses such as Hepatitis C Virus (HCV) and SAKS coronavirus (SARS-CoV). Enzyme activity is usually assayed by measuring NTPase activity in the presence of an appropriate nucleic acid co-substrate although, more recently, novel fiuorimetric and luminescence principles have been applied to the measurement of strand unwinding and/or translocation of the protein along the nucleic acid (Frick 2003, 2006). 

Much of the literature pertaining to putative inhibitors of HCV helicase has recently been discussed in the excellent review published by Frick (2007). As he points out, one of the main problems with a helicase as target for antiviral drugs is the potential for general toxicity related to the highly conserved nature of the... 

One exciting approach is the development of short sequences of RNA that bind specifically to HCV helicase and/or the protease activity found in the same hepatitis C virus-encoded non-structural protein, NS3, and inhibit helicase at sub-micromolar concenttations (Umehara et al. 2005). These molecules could provide the basis for developing potent helicase inhibitors with improved pharmacotherapeutic properties. 

Helicase has also been a focal point for the development of antiviral chemotherapy of the coronavirus associated with severe acute respiratory syndrome (SARS) in humans. Although several experimental compounds with nucleic acid binding activity showing effective inhibition of SARS-CoV helicase were reported in 2005, there have been no reports of any further development since that time (Kesel 2005). It remains to be seen whether the S ARS-CoV compounds will be developed further, especially since no new infections have been observed in recent years. 

A recent review stated There are no HCV helicase inhibitors currently in development. Most experts believe that it will be difQcult, if not impossible, to develop helicase inhibitors (Hepatitis C Support project 2006). Whether or not this is a valid statement remains to be seen, but the potential success of compounds with a similar target in the herpesviruses suggests that the possibility of developing inhibitors of HCV helicase should not be dismissed quite so lightly. 

Beard PM, Duffy C, Baines JD (2004) Quantification of the DNA cleavage and packaging proteins UL15 and UL28 in A and B capsids of herpes simplex vims type 1. J Virol 78 1367-1374 Betz UA, Fischer R, Kleymann G, Hendrix M, Rubsamen-Waigmann H (2002) Potent in vivo antiviral activity of the herpes simplex vims primase-helicase inhibitor BAY 57-1293. Antimicrob Agents Chemother 46 1766-1772... 

Frick DN (2003) HeUcases as antiviral drug targets. Drug News Perspect 16 355-362 Frick DN (2006) Step-by-step progress toward understanding the hepatitis C virus RNA helicase. Hepatology 43 1392-1395... 

Frick DN (2007) The hepatitis C virus NS3 protein a model RNA helicase and potential drug target. Curr Issues Mol Biol 9 1-20... 

Maga G, Gemma S, Fattorusso C et al. (2005) Specific targeting of hepatitis virus C NS3 helicase. Biochemistry 44 9637-9644... 

Townsend L, Devivar R, Turk S, Nassiri M, Drach J (1995) Design, synthesis, and antiviral activity of certain 2,5,6-trihalo-l-(beta-d-ribofuranosyl)benzimidazoles. J Med Chem 38 4098 105 Turlure F, Devroe E, Silver PA, Engelman A (2004) Human cell proteins and human immunodeficiency virus DNA integration. Front Biosd 9 3187-3208 Umehara T, Fukuda K, Nishikawa F, Kohara M, Hasegawa T, NisUkawa S (2005) Rational design of dual-functional aptamers that inhibit the protease and helicase activities of HCV NS3. J Biochem 137 339-347... 

Examples (a) nucleosome K Huger, AW Mader, RK Richmond, DF Sargent, TJ Richmond. Nature 389 251-260, 1997 (b) DNA polymerases CA Brautigam, TA Steitz. Curr. Opin. Struct. Biol. 8 54-63, 1998 (c) single-stranded binding protein Y Shamoo, AM Friedman, MR Parsons, WH Konigsberg, TA Steitz. Nature 376 362-366, 1995 (d) restriction endonucleases RA Kovall, BW Matthews. Curr. Opin. Chem. Biol. 3 578-583, 1999 (e) DNA lig-ase S Shuman. Structure 4 653-656, 1996 (f) DNA helicases MC Hall, SW Matson. Mol. Microbiol. 34 867-877, 1999 (g) zinc-finger proteins Y Choo, JW Schwabe. Nat. Struct. Biol. 5 253-255, 1998. 

A replication fotk consists of four components that form in the following sequence (1) the DNA helicase unwinds a short segment of the patental duplex DNA ... 

Topoisomerases Relieve torsional strain that results from helicase-induced unwinding... 

Two proteins are initially involved in the nonho-mologous rejoining of a ds break. Ku, a heterodimer of 70 kDa and 86 kDa subunits, binds to free DNA ends and has latent ATP-dependent helicase activity. The DNA-bound Ku heterodimer recruits a unique protein kinase, DNA-dependent protein kinase (DNA-PK). DNA-PK has a binding site for DNA free ends and another for dsDNA just inside these ends. It therefore allows for the approximation of the two separated ends. The free end DNA-Ku-DNA-PK complex activates the kinase activity in the latter. DNA-PK reciprocally phos-phorylates Ku and the other DNA-PK molecule, on the opposing strand, in trans. DNA-PK then dissociates from the DNA and Ku, resulting in activation of the Ku helicase. This results in unwinding of the two ends. The unwound, approximated DNA forms base pairs the extra nucleotide tails are removed by an exonucle-... 

Primosome The mobile complex of helicase and primase that is involved in DNA replication. 

---