Serine　proteases　play　important　roles　in　numerous　physiological　and　pathophysiological　processes.　Moreover,　serine　proteases　arc　classical　subjects　for　studies　of　catalytic　and　inhibitory　mechanisms　of　enzymes.　Here,　we　determined　the　crystal　structures　of　a　serine　protease,　murine　plasma　kallikrein　(mPK),　and　its　complex　with　a　peptidic　inhibitor.　Although　mPK　in　the　complex　adopts　a　canonical　protease　structure,　the　apo-mPK　exhibits　a　previously　unobserved　structural　feature:　the　entrance　of　the　intact　Si　pocket　is　blocked　by　Clu217.　In　addition,　molecular　dynamics　simulations　and　functional　assays　support　the　flexibility　of　Glu217　and　suggest　that　this　flexibility　play　s　a　role　in　regulating　the　activity　of　serine　proteases.