Proteins are large biomolecules that occur in every living organism. All proteins are chemically similar since all are made up of many amino acid units linked together in a long chain. Amino acids are the building blocks from which all proteins are made. as their name implies, amino acids are difunctional; they contain both a basic amino group and an acidic carboxyl group.
Alanine, an amino acid
The strcutures of the 20 amino acid commonly found in proteins are shown in the following table. All 20 are alpha- amino acids; that is, the amino group in each is a substituent on the carbon atom alpha to, or next to, the carbonyl group.