Dual Modes of Modification of Hepatitis A Virus 3C Protease by a Serine-derived β-Lactone: Selective Crystallization and Formation of a Functional Catalytic Triad in the Active Site

Hepatitis A virus (HAV) 3C proteinase is a member of the picornain cysteine proteases responsible for the processing of the viral polyprotein, a function essential for viral maturation and infectivity. This and its structural similarity to other 3C and 3C-like proteases make it an attractive target for the development of antiviral drugs. Previous solution NMR studies have shown that a Cys24Ser (C24S) variant of HAV 3C protein, which displays catalytic properties indistinguishable from the native enzyme, is irreversibly inactivated by N-benzyloxycarbonyl-l-serine-β-lactone (1a) through alkylation of the sulfur atom at the active site Cys172. However, crystallization of an enzyme–inhibitor adduct from the reaction mixture followed by X-ray structural analysis shows only covalent modification of the ε2-nitrogen of the surface His102 by the β-lactone with no reaction at Cys172. Re-examination of the heteronuclear multiple quantum coherence (HMQC) NMR spectra of the enzyme–inhibitor mixture indicates that dual modes of single covalent modification occur with a ≥3:1 ratio of S-alkylation of Cys172 to N-alkylation of His102. The latter product crystallizes readily, probably due to the interaction between the phenyl ring of the N-benzyloxycarbonyl (N-Cbz) moiety and a hydrophobic pocket of a neighboring protein molecule in the crystal. Furthermore, significant structural changes are observed in the active site of the 3C protease, which lead to the formation of a functional catalytic triad with Asp84 accepting one hydrogen bond from His44. Although the 3C protease modified at Cys172 is catalytically inactive, the singly modified His102 N(ε2)-alkylated protein displays a significant level of enzymatic activity, which can be further modified/inhibited by N-iodoacetyl-valine-phenylalanine-amide (IVF) (in solution and in crystal) or excessive amount of the same β-lactone inhibitor (in solution). The success of soaking IVF into HAV 3C–1a crystals demonstrates the usefulness of this new crystal form in the study of enzyme–inhibitor interactions in the proteolytic active site.