Inhibitor binding induces active site stabilization of the HCVNS3 protein serine protease domain

Few structures of viral serine proteases, those encoded by the Sindbis and Semliki Forest viruses, hepatitis C virus (HCV) and cytomegalovirus, have b een reported. in the life cycle of HCV a crucial role is played by a chymot rypsin-like serine protease encoded at the N-terminus of the viral NS3 prot ein, the solution structure of which we present here complexed with a coval ently bound reversible inhibitor. Unexpectedly, the residue in the P2 posit ion of the inhibitor induces an effective stabilization of the catalytic Hi s-Asp hydrogen bond, by shielding that region of the protease from the solv ent. This interaction appears crucial in the activation of the enzyme catal ytic machinery and represents an unprecedented observation for this family of enzymes. Our data suggest that natural substrates of this serine proteas e could contribute to the enzyme activation by a similar induced-fit mechan ism. The high degree of similarity at the His-Asp catalytic site region bet ween HCV NS3 and other viral serine proteases suggests that this behaviour could be a more general feature for this category of viral enzymes.