Which One of the Following Amino Acids Has Its R

Question

Which one of the following amino acids has its R group covalently linked to the amino group?
A)glycine
B)serine
C)cysteine
D)glutamic acid
E)proline

Accepted Answer

Proline is the only amino acid where the R-group is covalently linked to the amino group, forming a ring structure. This gives proline unique properties compared to other amino acids.