Native State Dynamics of Human Neuroserpin Investigated with Hydrogen/Deuterium Exchange and Molecular Dynamics Simulations

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Wild-type human neuroserpin, a member of the serine protease inhibitor superfamily, is expressed in neurons of the central and peripheral nervous system, as well as in the adult brain. Polymerization of certain mutants of neuroserpin is associated with dementia caused by familial encephalopathy. We have performed hydrogen/deuterium exchange-mass spectrometry in order to monitor the structural stability and flexibility of different regions of the neuroserpin structure. We find that beta-sheet A, a critical region thought to be involved in polymerization, is less stable and more labile in neuroserpin than in other serpins such as alpha-1 antitrypsin and antithrombin. This may explain why wild-type neuroserpin is more susceptible to polymerization than other serpins. Molecular dynamics simulations also indicate that Wild Type neuroserpin shows increases flexibility on the nanosecond timescale as compared with alpha-1 antitrypsin. In the simulations, a novel 2 stranded beta-sheet was formed between the N terminal portion of the reactive center loop and the loop connecting strand 3A to beta-sheet C. This phenomenon occurred repeatedly in multiple independent simulations. If such an interaction in fact occurs in solution, it could contribute to the relatively poor inhibitory efficiency of neuroserpin compared to other serpins by retarding the insertion of the reactive center loop into sheet A after proteolytic cleavage. Simulations of a pathological mutant of neuroserpin showed distortions near the top of the central beta-sheet A, a critical site for polymer formation. This distortion may help explain why the mutant is more prone to polymerize than wild type.