Multiple Choice
Which of the following statements about the 20 standard amino acids is correct?
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3. Amino Acids
Amino Acid Groups
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Which of the following amino acids contains a sulfur atom in its side chain?
A
Phenylalanine
B
Asparagine
C
Lysine
D
Methionine
Verified step by step guidance1
Step 1: Recall the structure of amino acids and their side chains. Amino acids are composed of a central carbon atom (α-carbon) bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group) that determines the properties of the amino acid.
Step 2: Understand that sulfur-containing amino acids have side chains that include sulfur atoms. The two common sulfur-containing amino acids are cysteine and methionine.
Step 3: Examine the side chains of the given amino acids: Phenylalanine has a benzyl group (aromatic ring), Asparagine has an amide group, and Lysine has a long aliphatic chain ending in an amino group. None of these contain sulfur.
Step 4: Focus on Methionine. Methionine's side chain contains a sulfur atom within a thioether group (-CH₂-S-CH₃), making it a sulfur-containing amino acid.
Step 5: Conclude that Methionine is the correct answer because it is the only amino acid in the list with a sulfur atom in its side chain.
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