The chemical formula of amino acids can be written as R-CH(NH2)-COOH, where R represents the side chain group. They can be broadly classified into proteinogenic amino acids and non-proteinogenic amino acids. Proteinogenic amino acids, also known as standard amino acids, are directly involved in the synthesis of protein molecules. Based on the different positions of the amino group attached to the carbon chain, amino acids can be categorized into α-, β-, γ-, etc. However, all naturally occurring amino acids that constitute proteins are α-amino acids, totaling 22 types, including 20 common amino acids and 2 rare amino acids. Non-proteinogenic amino acids cannot directly participate in protein synthesis and require modification to be incorporated into proteins, such as citrulline, ornithine, and canavanine.