Alternative splicing in one class of receptors for this compound leads to ‘flip’ and ‘flop’ variants. Other ionotropic receptors for this compound include one that is pore-blocked by ketamine and has four subunits, two of which bind this compound and two of which bind glycine. The aforementioned AMPA and NMDA (15[1])receptors bind this (*) excitatory neurotransmitter, which is converted using pyridoxal phosphate to the inhibitory neurotransmitter GABA. (-5[1])After aspartate, this amino acid has the second lowest pKa of those naturally occurring and its R group is a three-carbon carboxylic acid. There is little evidence that headaches can be caused by (-5[1])the sodium salt of this amino acid, symbolised ‘E’. For 10 points, the mono-sodium variety of what amino acid is a common food additive that confers umami (10[1])flavour? ■END■ (10[1])