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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Human immunodeficiency virus (HIV) and Hepatitis B virus (HBV) ribonucleases H (RNase H) are type 1 RNases H that are promising drug targets because inhibiting their activity blocks viral replication. RNases H cleave RNA in RNA/DNA hybrids. Eukaryotic RNase H1 is an essential protein and probable off‐target enzyme for viral RNase H inhibitors. α‐hydroxytropolones (αHTs) comprise an anti‐RNase H inhibitor class that can inhibit the HIV, HBV, and human RNases H1. These compounds work by binding the RNase H active site by chelating the catalytic divalent metal cofactors. We hypothesized that a better understanding of RNase H1 inhibition will help development of compounds selective for the viral RNases H. To this end, we expressed and purified recombinant human RNase H1 and determined its inhibition mechanism(s) in steady‐state kinetics by two αHTs, 110 and 404 (Fig. 1). Inhibition was not competitive with a 12‐mer RNA/DNA substrate, but the turnover rate was reduced despite inhibitor binding to the active site (Fig. 2). 110 and 404displayed inhibition constants of 9 μM and 3 μM in saturating substrate concentrations, respectively, and these values were elevated 2‐3‐fold in very low substrate. Saturating 110and 404concentrations modestly reduced the apparent substrate binding constant (KM) from 90 nM to ~30 nM, while reducing the turnover rate (kcat = 0.17 s‐1) ~20‐fold. We found that 110enhanced affinity of RNase H1 for substrate by 4‐fold using a fluorescence polarization (FP) substrate binding assay with Ca2+ instead of Mg2+ to prevent RNA cleavage. 404, on the other hand, competed with substrate in binding assays, raising the substrate's KD~7‐fold from 24 nM without compound to ~150 nM. Induced fit docking studies in the Schrödinger suite suggest 110 binds to the active site metals as expected, while the substrate is still capable of binding via RNase H1’s high‐affinity auxiliary RNA/DNA hybrid binding domain (HBD) and substrate binding groove within the RNase H domain. 110 made favorable contacts with both enzyme and substrate, stabilizing an ESI complex. 404, on the other hand, occupies much of the substrate binding groove as well as the active site due to its larger structure. This would explain why 404 competes with substrate binding, while 110enhances substrate binding. The reason the KM decreased with 404 despite its competitive behavior in substrate binding assays is not clear. However, we hypothesize that 404locally competes with the substrate for the RNase H1 active site and the substrate binding groove within the RNase H domain without interfering with the HBD:substrate interface. This could lower the overall ES affinity. However, we speculate that if substrate release is slow relative to RNA hydrolysis and product release, the compound could behave uncompetitively in kinetics assays by inhibiting the breakdown of the ES complex through catalysis, only permitting enzyme‐substrate dissociation via the putatively slow substrate release pathway. Thus, these results illustrate that non‐competitive steady‐st...

Hepatitis B virus (HBV) replicates by protein-primed reverse transcription. It chronically infects >250 million people, and the dominant anti-HBV drugs are nucleos(t)ide analogs targeting the viral polymerase (P). P has four domains, the terminal protein (TP) that primes DNA synthesis, a spacer, the reverse transcriptase (RT), and the ribonuclease H (RNaseH). Despite being a major drug target and catalyzing a reverse transcription pathway very different from the retroviral pathway, HBV P has resisted structural analysis for decades. Here, we exploited advances in protein structure prediction to model the structure of P. The predicted HBV RT and RNaseH domains aligned to the HIV RT-RNaseH at 3.75 A RMSD, had a nucleic acid binding groove spanning the two active sites, had DNA polymerase active site motifs in their expected positions, and accommodated two Mg++ ions in both active sites. Surprisingly, the TP domain wrapped around the RT domain, with the priming tyrosine poised over the RT active site. This model was validated using published mutational analyses, and by docking RT and RNaseH inhibitors, RNA:DNA heteroduplexes, and the HBV e RNA stem-loop that templates DNA priming into the model. The HBV P fold, including the orientation of the TP domain, was conserved among hepadnaviruses from rodents to fish and in P from a fish 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 guide drug development and mechanistic studies into P function.

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