-site cysteine (Cys145) sulfur forms a covalent bond to the methylene carbon (1.8 Å C–S bond length). The ketone carbonyl is positioned within the oxyanion hole and as such engaging in hydrogen bonds with backbone NH groups of Gly143 and Cys145. A detailed analysis of an extended hydrogen bond network from catalytic His41 reveals that the side-chain imidazole serves as a hydrogen bond donor to the interior of the protease. Specifically,