| Alpha-1 antitrypsin (alpha1-AT) is a serine protease inhibitor (serpin) involved in a termination of an inflammation response. An inhibition of a target protease by a serpin alpha1-AT involves a large conformational change, pulling a bound protease from the apex to the opposite pole of the alpha 1-AT molecule, to form a thermodynamically more stable serpin-protease complex. During this large conformational change, the reactive center loop (RCL) of alpha1-AT is inserted between beta-strands 3A and 5A, becoming the fourth strand. It was proposed that the large conformational change during the protease inhibition must be facilitated by the metastability of the native alpha1-AT molecule. Therefore, identifying the sources of the native metastability was an intense focus of previous mutagenesis studies. Although the previous studies attempted to identify destabilizing regions within the native alpha1-AT structure by mutagenesis studies, this study has approached to the same problem from a different aspect -- through an investigation of the protein dynamics using hydrogen-deuterium exchange mass spectrometry (HX-MS). It was found that beta-sheet A in the native metastable form was rigid on the contrary to a suggestion made in previous studies. The same region was also found to be rigid in the intermediate prior to the polymerization indicating that a conformer with the opening of beta-sheet A is probably only transiently populated. This study further discovered that cooperativity exists within the native alpha 1-AT molecule indicating that the opening of beta-sheet A during the inhibition and the polymerization as suggested by previous studies is not possible without affecting other structural regions. In this study, roles of the protein dynamics in the function, folding/unfolding, and polymerization of wild-type alpha1-AT are discussed. |
