Crystal structures of Klebsiella pneumoniae pullulanase (KPP) in complex with α‐cyclodextrin (α‐CD), β‐cyclodextrin (β‐CD) and γ‐cyclodextrin (γ‐CD) were refined at around 1.98–2.59 Å resolution from data collected at SPring‐8. In the structures of the complexes obtained with 1 mM α‐CD or γ‐CD, one molecule of CD was found at carbohydrate‐binding module 41 only (CBM41). In the structures of the complexes obtained with 1 mM β‐CD or with 10 mM α‐CD or γ‐CD, two molecules of CD were found at CBM41 and in the active‐site cleft, where the hydrophobic residue of Phe746 occupies the inside cavity of the CD rings. In contrast to α‐CD and γ‐CD, one β‐CD molecule was found at the active site only in the presence of 0.1 mM β‐CD. These results were coincident with the solution experiments, which showed that β‐CD inhibits this enzyme more than a thousand times more potently than α‐CD and γ‐CD. The strong inhibition of β‐CD is caused by the optimized interaction between β‐CD and the side chain of Phe746. The increased Ki values of the F746A mutant for β‐CD supported the importance of Phe746 in the strong interaction of pullulanase with β‐CD.

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Although endogenous animal cellulases have great potential for industrial applications such as bioethanol production, few investigations have focused on these enzymes. In this study, the glycoside hydrolase family 45 (GH45) subfamily B endoglucanase EG27II from the snail Ampullaria crossean was expressed using a Pichia pastoris expression system and the crystal structure of the apo form was determined at 1.00 Å resolution; this is the highest resolution structure of an animal endoglucanase. The results showed that EG27II has a double‐ψ six‐stranded β‐barrel and that the structure of EG27II more closely resembles those of subfamily C enzymes than those of subfamily A enzymes. The structure of EG27II complexed with cellobiose was also determined under cryoconditions and at room temperature at three pH values, pH 4.0, 5.5 and 8.0, and no structural changes were found to be associated with the change in pH. The structural comparison and catalytic activity measurements showed that Asp137 and Asn112 function as the catalytic acid and base, respectively, and that Asp27 is also an important residue for catalysis. These high‐resolution structures of EG27II provide a large amount of information for structure‐based enzyme modification and cell‐surface engineering, which will advance biofuel production using animal‐derived cellulases.

Intracellular iron concentration is tightly controlled for cell viability. It is known to affect the growth of several viruses, but the molecular mechanisms are not well understood. We found that iron chelators inhibit growth of human parainfluenza virus type 2 (hPIV-2). Furthermore, infection with hPIV-2 alters ferritin localization from granules to a homogenous distribution within cytoplasm of iron-stimulated cells. The V protein of hPIV-2 interacts with ferritin heavy chain 1 (FTH1), a ferritin subunit. It also binds to nuclear receptor coactivator 4 (NCOA4) that mediates autophagic degradation of ferritin, so-called “ferritinophagy”. V protein consequently interferes with interaction between FTH1 and NCOA4. hPIV-2 growth is inhibited in FTH1 knockdown cell line where severe hPIV-2-induced apoptosis is shown. In contrast, NCOA4 knockdown results in promotion of hPIV-2 growth and limited apoptosis. Our data collectively suggest that hPIV-2 V protein inhibits FTH1-NCOA4 interaction and subsequent ferritinophagy. This iron homeostasis modulation allows infected cells to avoid apoptotic cell death, resulting in effective growth of hPIV-2. Importance hPIV-2 V protein interferes with interaction between FTH1 and NCOA4, and inhibits NCOA4-mediated ferritin degradation, leading to inhibition of iron release to cytoplasm. This iron homeostasis modulation allows infected cells to avoid apoptotic cell death, resulting in effective growth of hPIV-2.