Question

What is common among the following amino acids: Phenylalanine, leucine, methionine and valine?

• A. They are the only amino acids not found in milk
• B. They are essential amino acids
• C. They are not required by humans
• D. Females do not require these amino acids

Answer 1

Answer: (B)

They are essential amino acids

Phenylalanine is an essential α-amino acid with the formula C${}_9$H${}_{1}1$NO${}_2$. It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino acid is classified as neutral, and nonpolar because of the inert and hydrophobic nature of the benzyl side chain.

Leucine is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH${}_3$${}^{+}$ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain isobutyl group, making it a non-polar aliphatic amino acid.

Methionine is an essential amino acid in humans. As the substrate for other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical role in the metabolism and health of many species, including humans. It is encoded by the codon AUG.

Valine is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH${}_3$${}^{+}$ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain isopropyl group, making it a non-polar aliphatic amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Human dietary sources are foods that contain protein, such as meats, dairy products, soy products, beans and legumes.

So the correct option is 'they are essential amino acids'.