Secondary amino acids, also known as imino acids, are a subgroup of amino acids that possess an additional amino group attached to a carbon atom within their side chain. Unlike the standard amino acids, which have the amino group attached to the alpha carbon (the carbon adjacent to the carboxyl group), secondary amino acids have the amino group connected to another carbon atom in the side chain. The most well-known secondary amino acid is proline. Proline is unique among amino acids due to its cyclic structure, which forms a five-membered ring. The amino group in proline is attached to the alpha carbon and then forms a bond with the carbon atom in the side chain, closing the ring structure. This distinctive structure causes proline to have distinct properties and plays a crucial role in protein folding and structure. Proline's presence in a protein chain can induce kinks or bends, influencing the overall conformation of the protein. This effect arises from the restricted rotation around the proline's cyclic structure. As a result, proline often occurs in regions of proteins where a sharp turn or loop is required, such as in protein motifs like helix-turn-helix or beta-turns. In addition to proline, there are a few other secondary amino acids, although they are less common. These include hydroxyproline, which is a modified form of proline with a hydroxyl group (-OH) added, and pipecolic acid, which has a six-membered ring structure. These secondary amino acids can be found in certain proteins, particularly those involved in collagen synthesis and plant defense mechanisms. Overall, secondary amino acids, primarily represented by proline, exhibit distinct structural properties and play crucial roles in protein folding, conformation, and function.
