A 30-year-old dental hygienist presents with a 2-month history of progressive numbness in her feet and difficulty walking. She admits to daily recreational use of nitrous oxide (“laughing gas”) for the past 6 months. On neurologic examination, she has decreased vibration and position sense in her lower extremities and a broad-based gait. Laboratory studies reveal hemoglobin 10.5 g/dL, mean corpuscular volume 110 fL, and hypersegmented neutrophils on peripheral smear. Serum homocysteine and methylmalonic acid levels are elevated. Which of the following enzymes is most likely inactivated by nitrous oxide exposure in this patient?