A 42-year-old man presents to the physician accompanied by his wife with concern about short-term memory for the past 2 weeks. The wife says that he tends to make up stories rather than telling her the truth. He underwent gastric bypass surgery 3 months ago. Vital signs are within normal limits. Physical examination shows symmetric peripheral neuropathy in the lower extremities with motor and sensory changes. He is diagnosed with a vitamin deficiency.
Which of the following enzymes in the Krebs cycle requires this vitamin to work?
The correct answer is:
α-ketoglutarate dehydrogenase
The patient has vitamin B1 (thiamine) deficiency presenting with signs and symptoms of dry beriberi including symmetrical peripheral neuropathy characterized by both sensory and motor impairments primarily in the distal extremities. Thiamine is primarily found in foods such as legumes, pork, brown rice, yeast, and cereals made from whole grains. Thiamine deficiency can also be seen as a nutritional complication of gastric surgery. TThe α-ketoglutarate dehydrogenase complex is a Krebs cycle enzyme that converts α-ketoglutarate into Succinyl-CoA, with thiamine as its co-factor.
Answer choice B: Citrate synthase, is incorrect. Although citrate synthase is one of enzymes of the Krebs cycle, this enzyme does not need thiamine to function.
Answer choice C: Methionine synthase, is incorrect. Methionine synthase converts homocysteine into methionine and is not part of the Krebs cycle.
Answer choice D: Pyruvate dehydrogenase, is incorrect. Pyruvate dehydrogenase converts pyruvate to Acetyl-CoA with the help of thiamine as co factor. However, this enzyme is not part of Kreb cycle.
Answer choice E: Succinylcholine CoA synthase, is incorrect. Succinylcholine CoA synthase catalyzes the reversible reaction of succinyl-CoA to succinate without the need for thiamine.
Key Learning Point
Dry beriberi features impaired reflexes and symmetrical motor and sensory deficits in the extremities. Another variation of dry beriberi is Wernicke encephalopathy. This condition presents in well-defined steps starting with nausea and vomiting, followed by horizontal nystagmus, ocular nerve palsy, fever, ataxia, and progressive mental impairment, eventually leading to the Korsakoff syndrome if not treated with vitamin B1 supplement.
The α-ketoglutarate dehydrogenase complex is a Kreb cycle enzyme that is responsible for catalyzing the reaction between α-ketoglutarate, thiamine (enzyme co-factor) and CoA, which forms the end product, succinyl CoA.
