Creatine is synthesized from three amino acids. The first committed reaction transfers an amidino group from arginine to glycine, and a later step adds a methyl group. Which methyl donor completes creatine synthesis?

• A Methylcobalamin
• B Tetrahydrofolate-derived N5-methyltetrahydrofolate
• C Betaine directly donating to guanidoacetate
• D S-adenosylmethionine ✓

Explanation

Glycine amidinotransferase forms guanidoacetate from arginine and glycine in the kidney, and guanidoacetate N-methyltransferase then methylates it using S-adenosylmethionine, chiefly in the liver, to yield creatine. This makes creatine synthesis a significant consumer of SAM methyl groups. Methyltetrahydrofolate remethylates homocysteine to methionine via methionine synthase and does not directly methylate guanidoacetate, while methylcobalamin serves as a cofactor in that remethylation step instead.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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