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Subject - Biochemistry
Amino acids can be classified according to various structural and functional properties. The classification into proteinogenic and non-proteinogenic amino acids is of essential importance in the life sciences. The former are structurally characterized by having a C-α-atom which is bound to a carboxy group, an amino group and an organic side chain R. In total, 22 different proteinogenic amino acids are known, including selenocysteine and pyrrolysine. While selenocysteine (Sec) occurs in different eukaryotic enzymes, for example in bacterial cell walls while in the L form predominates.peroxidase, pyrrolysine (N6-[(2R,3R)-3-methyl-3,4-dihydro-2H-pyrrol-2-yl-carbonyl]-L-lysine) was only found in methanogenic bacteria as yet. 21 of the 22 proteinogenic amino acids are chiral with the exception of glycine where the side chain is substituted by an H atom. Accordingly, members of the former group can exist in two enantiomeric forms referred to as L- and D-isomers. The D-configuration, for example, is found in
Non-proteinogenic amino acids are either intermediates in metabolic pathways, citrulline or ornithine, for example, occur in the urea cycle, or are molecular components in coenzymes, such as β-alanine in coenzyme A. However, they are not building blocks in the biosynthesis of proteins.
Further categorizations exist within this classification:
a) classification according to physical-chemical properties, such as basic, acidic, non-polar, polar, non-charged,
b) classification into essential amino acids, for example for humans: Phe, Trp, Lys, Met, Leu, Ile, Val, Thr, His and Arg (for small children and seniors)
c) classification into non-essential amino acids, for example in cellular metabolic pathways (gluconeogenesis and ketogenesis).