Correct Answer: B. Methionine
Vitamin B12 (cobalamin) is an essential cofactor for methionine synthase, the enzyme that catalyzes the conversion of homocysteine to methionine using 5-methyltetrahydrofolate as the methyl donor. In B12 deficiency, this reaction is blocked, leading to accumulation of homocysteine (elevated homocysteine is the biochemical hallmark) and depletion of methionine. Methionine is the precursor for S-adenosylmethionine (SAM), the universal methyl donor in the body. SAM is critical for myelin synthesis and DNA methylation—both essential for neurological function. The neurological manifestations (positive Romberg sign, peripheral neuropathy, subacute combined degeneration) occur because methionine deficiency impairs myelin formation and neuronal methylation reactions. The anemia results from impaired DNA synthesis due to the "methyl trap" hypothesis: without B12, folate becomes trapped as 5-methyltetrahydrofolate and cannot support nucleotide synthesis. Thus, the primary amino acid deficiency is methionine, not homocysteine (which accumulates) or other amino acids. This is the discriminating biochemical link between B12 deficiency and neurological disease in Indian clinical practice.
Why the other options are wrong
A. Glutamate — Glutamate is a non-essential amino acid synthesized from α-ketoglutarate and is not directly dependent on B12 metabolism. While glutamate is involved in neurotransmission, its deficiency does not explain the elevated homocysteine or the specific pattern of subacute combined degeneration seen in B12 deficiency. This is a distractor based on the neurological presentation. C. Tyrosine — Tyrosine is a non-essential amino acid derived from phenylalanine and is unrelated to the B12–homocysteine–methionine metabolic axis. Although tyrosine is a precursor for catecholamines and thyroid hormones, its deficiency would not produce elevated homocysteine or the characteristic neurological findings of B12 deficiency. This option exploits confusion about amino acid metabolism. D. Cysteine — Cysteine is synthesized from homocysteine via the transsulfuration pathway (not directly B12-dependent) and is not the primary deficient amino acid in B12 deficiency. While homocysteine is elevated, cysteine levels may be normal or even increased because the transsulfuration pathway is intact. The question asks for the amino acid that is deficient, not the one that accumulates.
High-Yield Facts
- Methionine synthase requires B12 as a cofactor; B12 deficiency blocks homocysteine → methionine conversion.
- Elevated homocysteine in B12 deficiency is a biochemical marker; it indicates methionine depletion and impaired methylation.
- S-adenosylmethionine (SAM), derived from methionine, is the universal methyl donor for myelin and DNA synthesis; its deficiency causes neurological disease.
- Subacute combined degeneration (dorsal columns, lateral corticospinal tracts) results from impaired myelin synthesis due to methionine deficiency, not from homocysteine toxicity alone.
- Romberg sign and peripheral neuropathy in B12 deficiency reflect demyelination caused by inadequate SAM-dependent methylation reactions in the nervous system.
Mnemonics
B12 → Methionine → Myelin B12 activates methionine synthase → methionine production → SAM formation → myelin methylation. No B12 = no methionine = no myelin = neurological disease. Use this when you see B12 deficiency + neurological signs. HCY ↑ = Methionine ↓ In B12 deficiency, homocysteine accumulates (HCY ↑) because it cannot be converted to methionine; therefore methionine is depleted (Methionine ↓). Elevated homocysteine is the biochemical proof of methionine deficiency.
NBE Trap
NBE pairs elevated homocysteine with cysteine (option D) to trap students who think "homocysteine is high, so cysteine must be low." In reality, cysteine synthesis from homocysteine is intact in B12 deficiency; the block is at methionine synthase, not the transsulfuration pathway. The question tests understanding of the specific enzymatic block, not just amino acid accumulation patterns.</trap> <parameter name="textbookRef">Harper Biochemistry Ch. 29 (Amino Acid Metabolism); KD Tripathi Pharmacology Ch. 45 (Vitamins); Robbins Pathology Ch. 7 (Nutritional Deficiencies)
Clinical Pearl
In Indian clinical practice, B12 deficiency is common in vegetarians and patients with pernicious anemia or malabsorption (post-gastrectomy, tropical sprue). The neurological triad—anemia, elevated homocysteine, and Romberg sign—is pathognomonic for B12 deficiency. Measuring serum methionine (low) and homocysteine (high) confirms the diagnosis and guides supplementation with B12 injections (the standard DOC in India) to restore methionine synthesis and prevent irreversible neurological damage.</clinicalPearl> </invoke>