How many standard amino acids are there ?
The proteinogenic amino acids are small subset of this group that possess central carbon atom ( α - or 2 -) bearing an amino group , a carboxyl group , a side chain and an α - hydrogen levo conformation , with the exception of glycine , which is achiral , and proline , whose amine group is a secondary amine and is consequently frequently referred to as an imino acid for traditional reasons , albeit not an imino .