Amino acids are mostly synthesised from

Amino acids are mostly synthesized from α-ketoglutaric acid.
α-Ketoglutarate is one of the most important nitrogen transporters in metabolic pathways. The amino groups of amino acids are attached to it (by transamination) and carried to the liver where the urea cycle takes place.
α-Ketoglutarate is transaminated, along with glutamine, to form the excitatory neurotransmitter glutamate. Glutamate can then be decarboxylated into the inhibitory neurotransmitter gamma-aminobutyric acid.