Glutamate metabolism

Glutamic acid (symbol Glu or E) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is non-essential in humans, meaning the body can synthesize it. It is also an excitatory neurotransmitter, in fact the most abundant one, in the vertebrate nervous system. It serves as the precursor for the synthesis of the inhibitory gamma-aminobutyric acid (GABA) in GABA-ergic neurons. Glutamic acid (symbol Glu or E) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is non-essential in humans, meaning the body can synthesize it. It is also an excitatory neurotransmitter, in fact the most abundant one, in the vertebrate nervous system. It serves as the precursor for the synthesis of the inhibitory gamma-aminobutyric acid (GABA) in GABA-ergic neurons. It has a formula C5H9O4N. Its molecular structure could be idealized as HOOC-CH(NH2)-(CH2)2-COOH, with two carboxyl groups -COOH and one amino group -NH2. However, in the solid state and mildly acid water solutions, the molecule assumes an electrically neutral zwitterion structure −OOC-CH(NH+3)-(CH2)2-COOH. It is encoded by the codons GAA or GAG. The acid can lose one proton from its second carboxyl group to form the conjugate base, the singly-negative anion glutamate −OOC-CH(NH+3)-(CH2)2-COO−. This form of the compound is prevalent in neutral solutions. The glutamate neurotransmitter plays the principal role in neural activation. This anion is also responsible for the savory flavor (umami) of certain foods, and used in glutamate flavorings such as MSG. In Europe it is classified as food additive E620. In highly alkaline solutions the doubly negative anion −OOC-CH(NH2)-(CH2)2-COO− prevails. The radical corresponding to glutamate is called glutamyl. When glutamic acid is dissolved in water, the amino group (-NH2) may gain a proton (H+), and/or the carboxyl groups may lose protons, depending on the acidity of the medium. In sufficiently acidic environments, the amino group gains a proton and the molecule becomes a cation with a single positive charge, HOOC-CH(NH+3)-(CH2)2-COOH. At pH values between about 2.5 and 4.1, the carboxylic acid closer to the amine generally loses a proton, and the acid becomes the neutral zwitterion −OOC-CH(NH+3)-(CH2)2-COOH. This is also the form of the compound in the crystalline solid state. The change in protonation state is gradual; the two forms are in equal concentrations at pH 2.10. At even higher pH, the other carboxylic acid group loses its proton and the acid exists almost entirely as the glutamate anion −OOC-CH(NH+3)-(CH2)2-COO−, with a single negative charge overall. The change in protonation state occurs at pH 4.07. This form with both carboxylates lacking protons is dominant in the physiological pH range (7.35–7.45). At even higher pH, the amino group loses the extra proton and the prevalent species is the doubly-negative anion −OOC-CH(NH2)-(CH2)2-COO−. The change in protonation state occurs at pH 9.47. The carbon atom adjacent to the amino group is chiral (connected to four distinct groups), so glutamic acid can exist in two optical isomers, d(-) and l(+). The l form is the one most widely occurring in nature, but the d form occurs in some special contexts, such as the cell walls of the bacteria (which can manufacture it from the l form with the enzyme glutamate racemase) and the liver of mammals.