The key difference between BCAA and amino acids is that the BCAA is branched chain amino acid that has aliphatic side chains with a branch whereas the amino acids are organic compounds containing amine groups, carboxylic groups, and a specific side chain.
Amino acids are the building blocks of proteins. The side group (R) of the amino acid determines the properties of the amino acid. Therefore, this side group is specific for amino acids. In other words, different amino acids have different side groups. However, BCAAs have an additional uniqueness; a central carbon atom bound to three or more carbon atoms.
