A 53-year-old man, who has been smoking for the past 35 years at a 2-pack-a-day rate, visits his physician for a cough that will not go away and for difficulty in breathing. A chest X-ray rules out cancer but does display an increased anterior-posterior (AP) diameter, flattened diaphragms, and "air trapping." The patient is told that his condition will not improve and that he needs to stop smoking to stop the progression of the disease. At the molecular level, this disease is caused by which one of the following?