NANP Board Practice Exam

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

1. Asparagine

2. Cysteine

3. Phenylalanine

4. Tyrosine

The correct answer is: Asparagine

The correct answer is that methionine serves as a methyl donor in many biochemical pathways, particularly in the conversion of homocysteine to methionine. This process is crucial for maintaining proper levels of homocysteine in the body, as elevated levels are linked to cardiovascular issues. Methionine is a sulfur-containing amino acid that provides a methyl group (–CH3) through its active form, S-adenosylmethionine (SAMe). SAMe is a key compound that participates in numerous methylation reactions, which are essential for DNA methylation, gene expression regulation, and the synthesis of neurotransmitters. The other amino acids listed do not function primarily as methyl donors in the same way methionine does. While asparagine, cysteine, phenylalanine, and tyrosine are important in various metabolic processes, they do not play a direct role in the conversion of homocysteine to methionine through methyl donation. Understanding this functional role of methionine is important for recognizing how methylation affects health and metabolism.