Q. A young man is brought to the emergency department with headache and cyanosis of nail beds and lips accompanied by dizziness without syncope. During intake it is revealed he is currently being treated for HIV. He is placed on supplemental O2, and blood is drawn for arterial blood gas analysis. While drawing the blood, the on-call Resident notes the arterial blood has a dark brown color. Blood gas analysis reveals a pH of 7.39, PPO2 of 96 mm Hg, PPCO2 of 35 mm Hg, and O2sat of 82% on room air. In a healthy adult, which enzyme would primarily prevent this condition from occurring?