Efficient pyrophosphorolysis by a hepatitis B virus polymerase may be a primer-unblocking mechanism

Sinisa Urban, Severin Urban, Karl P. Fischer, D. Lorne Tyrrell

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


Effective antiviral agents are thought to inhibit hepatitis B virus (HBV) DNA synthesis iirreversibly by chain termination because reverse transcriptases (RT) lack an exonucleolytic activity that can remove incorporated nucleotides. However, since the parameters governing this inhibition are poorly defined, fully delineating the catalytic mechanism of the HBV-RT promises to facilitate the development of antiviral drugs for treating chronic HBV infection. To this end, pyrophosphorolysis and pyrophosphate exchange, two nonhydrolytic RT activities that result in the removal of newly incorporated nucleotides, were characterized by using endogenous avian HBV replication complexes assembled in vivo. Although these activities are presumed to be physiologically irrelevant for every polymerase examined, the efficiency with which they are catalyzed by the avian HBV-RT strongly suggests that it is the first known polymerase to catalyze these reactions under replicative conditions. The ability to remove newly incorporated nucleotides during replication has important biological and clinical implications: these activities may serve a primer-unblocking function in vivo. Analysis of pyrophosphorolysis on chain-terminated DNA revealed that the potent anti-HBV drug β-L-(-)-2′,3′-dideoxy-3′thiacytidine (3TC) was difficult to remove by pyrophosphorolysis, in contrast to ineffective chain terminators such as ddC. This disparity may account for the strong antiviral efficacy of 3TC versus that of ddC. The HBV-RT pyrophosphorolytic activity may therefore be a novel determinant of antiviral drug efficacy, and could serve as a target for future antiviral drug therapy. The strong inhibitory effect of cytoplasmic pyrophosphate concentrations on viral DNA synthesis may also partly account for the apparent slow rate of HBV genome replication.

Original languageEnglish (US)
Pages (from-to)4984-4989
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number9
StatePublished - Apr 24 2001
Externally publishedYes


  • Antiviral inhibition
  • Core particles
  • Lamivudine/3TC
  • Proofreading

ASJC Scopus subject areas

  • General


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