Hepatitis B virus (HBV) chronically infects >250 million people. It replicates by a unique protein‐primed reverse transcription mechanism, and the primary anti‐HBV drugs are nucleos(t)ide analogs targeting the viral polymerase (P). P has four domains compared to only two in most reverse transcriptases: the terminal protein (TP) that primes DNA synthesis, a spacer, the reverse transcriptase (RT), and the ribonuclease H (RNase H). Despite being a major drug target and catalyzing a reverse transcription pathway very different from the retroviruses, HBV P has resisted structural analysis for decades. Here, we exploited computational advances to model P. The TP wrapped around the RT domain rather than forming the anticipated globular domain, with the priming tyrosine poised over the RT active site. The orientation of the RT and RNase H domains resembled that of the retroviral enzymes despite the lack of sequences analogous to the retroviral linker region. The model was validated by mapping residues with known surface exposures, docking nucleic acids, mechanistically interpreting mutations with strong phenotypes, and docking inhibitors into the RT and RNase H active sites. The HBV P fold, including the orientation of the TP domain, was conserved among hepadnaviruses infecting rodent to fish hosts and a nackednavirus, but not in other non‐retroviral RTs. Therefore, this protein fold has persisted since the hepadnaviruses diverged from nackednaviruses >400 million years ago. This model will advance mechanistic analyses into the poorly understood enzymology of HBV reverse transcription and will enable drug development against non‐active site targets for the first time.

A novel, family GH10 enzyme, Xyn10B from Acidothermus cellulolyticus 11B was cloned and expressed in Escherichia coli. This enzyme was purified to homogeneity by binding to regenerated amorphous cellulose. It had higher binding on Avicel as compared to insoluble xylan due to the presence of cellulose-binding domains, CBM3 and CBM2. This enzyme was optimally active at 70 °C and pH 6.0. It was stable up to 70 °C while the CD spectroscopy analysis showed thermal unfolding at 80 °C. Xyn10B was found to be a trifunctional enzyme having endo-xylanase, arabinofuranosidase and acetyl xylan esterase activities. Its activities against beechwood xylan, p-Nitrophenyl arabinofuranoside and p-Nitrophenyl acetate were found to be 126,480, 10,350 and 17,250 U μmol, respectively. Xyn10B was highly active producing xylobiose and xylose as the major end products, as well as debranching the substrates by removing arabinose and acetyl side chains. Due to its specific characteristics, this enzyme seems to be of importance for industrial applications such as pretreatment of poultry cereals, bio-bleaching of wood pulp and degradation of plant biomass.

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