A family suffering a hereditary form of severe emphysema, with some family members also showing hepatic disease. Liver biopsy in one of the family members demonstrates increased reactivity with antibodies directed against a-1-antitrypsin. Serum levels of the enzyme are low, leading to increased activity of a number of serine proteases. Increased activity of which of the following serine proteases has been most closely tied to the emphysema that develops in these patients?