Definition: In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Input: The SARS coronavirus main peptidase (SARS-CoV M pro ) plays an essential role in the life-cycle of the virus and is a primary target for the development of anti-SARS agents. Here, we report the crystal structure of M pro at a resolution of 1.82 Å, in space group P2 1 at pH 6.0. In contrast to the previously reported structure of M pro in the same space group at the same pH, the active sites and the S1 specificity pockets of both protomers in the structure of M pro reported here are in the catalytically competent conformation, suggesting their conformational flexibility. We report two crystal structures of M pro having an additional Ala at the N terminus of each protomer (M +A(-1) pro ), both at a resolution of 2.00 Å, in space group P4 3 2 1 2: one unbound and one bound by a substrate-like aza-peptide epoxide (APE). In the unbound form, the active sites and the S1 specificity pockets of both protomers of M +A(-1) pro are observed in a collapsed (catalytically incompetent) conformation; whereas they are in an open (catalytically competent) conformation in the APE-bound form. The observed conformational flexibility of the active sites and the S1 specificity pockets suggests that these parts of M pro exist in dynamic equilibrium. The structural data further suggest that the binding of APE to M pro follows an induced-fit model. The substrate likely also binds in an induced-fit manner in a process that may help drive the catalytic cycle.

Crystal Structures Reveal an Induced-fit Binding of a Substrate-like Aza-peptide Epoxide to SARS Coronavirus Main Peptidase