A patient with vitamin B₁₂ deficiency shows elevated plasma homocysteine. The most direct explanation is:
AB₁₂ is needed for the transsulfuration pathway, so homocysteine cannot be converted to cysteine
BB₁₂ is a cofactor for methionine synthase, which remethylates homocysteine back to methionine; without B₁₂, this route is blocked and homocysteine accumulates
CB₁₂ deficiency reduces SAM synthesis, forcing more homocysteine to accumulate upstream
DB₁₂ is required for the initial activation of methionine to SAM by methionine adenosyltransferase
Methionine synthase — which uses N⁵-methyl-THF to remethylate homocysteine back to methionine — requires vitamin B₁₂ as a cofactor. When B₁₂ is deficient, this reaction stalls, blocking the remethylation route. Homocysteine accumulates because it cannot be recycled. Option A is wrong: the transsulfuration pathway requires B₆ (for cystathionine β-synthase), not B₁₂. Options C and D describe steps that do not require B₁₂.
Question 2 Multiple Choice
SAM is described as the 'universal methyl donor' because:
AIt is the only molecule capable of transferring methyl groups in biological systems
BIt donates methyl groups to a vast range of acceptors — DNA, neurotransmitters, lipids, metabolites — making it a central hub of biosynthetic methylation across biology
CIt is universally present in all known organisms and always functions in the same pathway
DIt transfers methyl groups only to universal biosynthetic precursors rather than to specific end-products
SAM's 'universal' role refers to the extraordinary breadth of methylation reactions it supports: DNA methylation (epigenetic regulation), conversion of norepinephrine to epinephrine, creatine biosynthesis, phosphatidylcholine production, and dozens more. This breadth is possible because SAM carries a high-energy sulfonium-bound methyl group that is reactive toward many different nucleophilic acceptors. Option A is incorrect — other methyl donors exist, but SAM is by far the most important across biology.
Question 3 True / False
Cysteine is an essential amino acid because humans can seldom synthesize it under any circumstances.
TTrue
FFalse
Answer: False
Cysteine is *conditionally* essential — the body can synthesize it via the transsulfuration pathway (serine + homocysteine → cystathionine → cysteine), but only when methionine supply is adequate (to provide homocysteine) and vitamin B₆ is sufficient (for cystathionine β-synthase). If methionine intake is low or B₆ is deficient, cysteine synthesis fails and dietary cysteine becomes necessary. This distinguishes it from truly essential amino acids like lysine, which humans cannot synthesize at all.
Question 4 True / False
After SAM donates its methyl group, the resulting S-adenosylhomocysteine (SAH) is eventually converted to homocysteine, which sits at a metabolic branch point between remethylation and transsulfuration.
TTrue
FFalse
Answer: True
This accurately traces the methionine cycle: SAM → (methyl transfer) → SAH → (hydrolysis by SAH hydrolase) → homocysteine + adenosine. Homocysteine then either enters remethylation (back to methionine via methionine synthase using B₁₂/N⁵-methyl-THF, or via betaine-homocysteine methyltransferase) or transsulfuration (forward to cystathionine and then cysteine, requiring B₆). This branch point is clinically critical: failures at either branch — from B₆, B₁₂, or folate deficiency — cause homocysteine accumulation.
Question 5 Short Answer
Why does folate deficiency raise plasma homocysteine, even though folate is not directly part of the methionine cycle itself?
Think about your answer, then reveal below.
Model answer: Folate (as N⁵-methyl-THF) is the methyl donor that methionine synthase uses to remethylate homocysteine back to methionine. The enzyme transfers the methyl group from N⁵-methyl-THF onto homocysteine (with B₁₂ as cofactor), regenerating methionine. Without adequate folate, N⁵-methyl-THF is unavailable, so methionine synthase cannot complete the remethylation. Homocysteine accumulates because the primary recycling route is blocked. This explains why folate deficiency — like B₁₂ and B₆ deficiencies — is associated with hyperhomocysteinemia and the cardiovascular and neural tube risks linked to elevated homocysteine.
Folate connects the sulfur amino acid pathway to one-carbon metabolism (the topic this builds toward). N⁵-methyl-THF is generated by the folate cycle and used in the methionine cycle; the two cycles are therefore coupled. Deficiency in either folate or B₁₂ blocks the same enzymatic step (methionine synthase), which is why both produce similar clinical findings including elevated homocysteine and megaloblastic anemia.