A 42-year-old male from Delhi with a 10-year history of poorly controlled diabetes mellitus type 2 presents to the emergency department with altered mental status, rapid shallow breathing (RR 28/min), and fruity-smelling breath. Arterial blood gas shows pH 7.18, HCO₃⁻ 8 mEq/L, and serum glucose 580 mg/dL. Serum ketones are markedly elevated. After initial insulin and fluid resuscitation, his metabolic acidosis begins to correct, but 48 hours later he develops recurrent metabolic acidosis with elevated lactate (6.8 mmol/L) and normal ketone levels. Which TCA cycle enzyme is most likely impaired due to thiamine depletion from aggressive insulin therapy?