NANP Board Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

Question: 1 / 400

Which amino acid serves as a methyl donor in many biochemical pathways, including the conversion of homocysteine to methionine?

Asparagine

Cysteine

The correct answer is cysteine, as it plays a vital role in biological processes as a precursor for many important biomolecules. In the context of biochemical pathways, cysteine is involved in the methylation reactions, particularly in the conversion of homocysteine to methionine. This conversion is facilitated by the methyl donor S-adenosylmethionine (SAM), which is derived from methionine, and cysteine is integral to the cycle that regenerates methionine from homocysteine.

Understanding the role of methyl donors is crucial in biochemical pathways, as they contribute to the regulation of gene expression and protein function. Cysteine's importance is further supported by its involvement in the synthesis of glutathione, an antioxidant that helps protect cells from oxidative stress.

Other amino acids listed do not directly serve as methyl donors in this specific pathway, highlighting the unique role of cysteine in methylation processes within metabolic pathways.

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Phenylalanine

Tyrosine

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