Proline differs from the other standard amino acids in that only the former:

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11ecba2d_0e68_171e_a3bf_199d69021b67_MD0008_00 The core structure of amino acids, called the backbone, consists of an amino group linked to an alkyl carbon (the α-carbon), which is linked to a carboxyl group. The α-carbon is also linked to an R group, which varies from one amino acid to another and determines the identity of each amino acid. Twenty standard amino acids, each with a unique R group, are found in proteins in all domains of life. Proline is unique among the standard amino acids because its R group forms a ring that includes the backbone amino group. Because of this, the amino group of the proline backbone has two bonds to carbon, making it a secondary amine. In contrast, all other standard amino acids have primary amines in their backbones. (Choice A) The acidic amino acids (aspartate and glutamate) have negatively charged side chains. Proline does not. (Choice B) Peptide bonds are amide bonds. Due to resonance, all amide bonds (including peptide bonds) have partial double-bond character, and therefore all are planar, and not just the bond in proline. (Choice D) The R group of proline is bonded to the backbone at two points instead of one, making it relatively rigid and unlikely to be found in the flexible regions of a protein. Note that β-turns, which commonly contain proline, are highly structured and inflexible. In addition, many amino acids are found in flexible-loop regions, so even if it were common, proline's inclusion in flexible loops would not be a unique property of proline. Educational objective: All amino acids have a backbone consisting of an amino group, an α-carbon, and a carboxyl group. Each amino acid also has an R group that determines its identity. Proline is unique among the standard amino acids because its R group bonds to the backbone amino group, making it a secondary rather than a primary amine.

Proline Differs from the Other Standard Amino Acids in That