AA meaning in Chemistry ?

Amino acids are organic compounds that contain amino ( − NH 3 + {\displaystyle -{\text{NH}}_{3}^{+}} ) and carboxylate ( − CO 2 − {\displaystyle -{\text{CO}}_{2}^{-}} ) functional groups, along with a side chain (R group) specific to each amino acid. The elements present in every amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N); in addition sulfur (S) is present in the side chains of cysteine and methionine, and selenium (Se) in the less common amino acid selenocysteine. More than 500 naturally occurring amino acids are known to constitute monomer units of peptides, including proteins, as of 2020 (though only 20 appear in the genetic code, plus selenocysteine, which is encoded in a special way.)

Amino acids are formally named by the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature in terms of the fictitious "neutral" structure shown in the illustration. For example, the systematic name of alanine is 2-aminopropanoic acid, based on the formula CH 3 -CH(NH) 2 -COOH {\displaystyle {\text{CH}}_{3}{\text{-CH(NH)}}_{2}{\text{-COOH}}} . The Commission justified this approach as follows:

The systematic names and formulas given refer to hypothetical forms in which amino groups are unprotonated and carboxyl groups are undissociated. This convention is useful to avoid various nomenclatural problems but should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules.

The last part of the second sentence ("[This] should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules") is vital.

They can be classified according to the locations of the core structural functional groups, as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, ionization, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.