d
\(COOH\) and \(NH _3\) (amine group). So every amino acid has a central carbon with a \(H, R\) group (varies for each amino acid), a carboxylic group and an amine group. These groups are joined to a single (aliphatic) carbon. The carbon directly attached to a carboxyl group is the alpha position, so the amino acids in proteins are all alpha-amino acids.