Abstract
Serpins form a large class of protease inhibitors involved in regulation of a wide spectrum of physiological processes. Recently identified prokaryotic members of this protein family may provide a key to the evolutionary origins of the unique serpin fold and the associated inhibitory mechanism. We performed a biochemical characterization of a serpin from Bifidobacterium longum, an anaerobic Gram-positive bacterium that naturally colonizes human gastrointestinal tract. The B. longum serpin was shown to efficiently inhibit eukaryotic elastase-like proteases with a stoichiometry of inhibition close to 1. Porcine pancreatic elastase and human neutrophil elastase were inhibited with the second order association constants of 4.7 x 10(4) m(-1) s(-1) and 2.1 x 10(4) m(-1) s(-1), respectively. The B. longum serpin is expected to be active in the gastrointestinal tract, because incubation of the purified recombinant serpin with mouse feces produces a stable covalent serpin-protease adduct readily detectable by SDS-PAGE. Bifidobacteria may encounter both pancreatic elastase and neutrophil elastase in their natural habitat and protection against exogenous proteolysis may play an important role in the interaction between these commensal bacteria and their host.
Highlights
The beta sheet A to form a six-stranded anti-parallel  sheet at the core of the cleaved serpin structure [4]
This applies to the B. longum serpin, which is an orphan in the current serpin classification
The ability of prokaryotic serpins to undergo reactive center loop insertion was predicted from the sequence analysis of their RCL hinge regions [9, 11], but until now biochemical data were available for only one prokaryotic serpin, thermopin [12]
Summary
The beta sheet A to form a six-stranded anti-parallel  sheet at the core of the cleaved serpin structure [4]. The sequence-based analysis suggests that prokaryotic members of the serpin superfamily are functional protease inhibitors, and a serpin from Thermobifida fusca, thermopin, was shown to inhibit chymotrypsin albeit by means of an unconventional cleavage site [12].
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