Overview

Glutamic acid is one of the 20 standard amino acids found in proteins and is classified as non-essential for humans because the body can synthesise it. In physiological conditions the side-chain carboxyl group is usually deprotonated and the molecule is referred to as glutamate. Salts and esters are known collectively as glutamates. The biologically predominant stereoisomer is L-glutamic acid, which is the form incorporated into proteins and used in metabolism and signalling.

Chemical characteristics

Glutamic acid has an alpha-amino group, an alpha-carboxyl group and a second carboxyl group on its side chain, making it a dicarboxylic amino acid. This gives glutamate an anionic character at physiological pH and allows it to participate in ionic interactions, metal binding and active sites of enzymes. Two stereoisomers exist (L and D), but L-glutamate dominates in biochemistry and nutrition.

Metabolic roles

Glutamate is a central metabolic intermediate in amino acid metabolism and nitrogen handling. It acts as an amino group donor in transamination reactions and is interconvertible with glutamine via the enzymes glutamine synthetase and glutaminase. These interconversions form part of nitrogen recycling in tissues and the brain, commonly described as the glutamate–glutamine cycle.

Neurotransmitter function

Free glutamate is the principal excitatory neurotransmitter in the vertebrate central nervous system. It acts at several ionotropic receptors (commonly named AMPA, NMDA and kainate receptors) and at metabotropic glutamate receptors to mediate fast synaptic transmission and modulatory signalling. Glutamatergic transmission is essential for synaptic plasticity phenomena such as long-term potentiation, which are linked to learning and memory. Excessive extracellular glutamate can cause overactivation of receptors and neuronal injury in pathological states; this process is often referred to as excitotoxicity.

Dietary sources and umami taste

Glutamic acid occurs naturally in many protein-rich foods. Typical dietary sources include:

  • meats and poultry;
  • dairy products such as aged cheeses;
  • soy products, ripe tomatoes, mushrooms and certain seaweeds.

Free glutamate is also responsible for the umami taste, recognised as one of the basic taste qualities. The savoury effect of free glutamate led to the development and commercial use of glutamate salts as flavour enhancers.

Food additives, production and industrial uses

Glutamate salts such as monosodium glutamate are manufactured at scale, commonly by fermentation of carbohydrate feedstocks. The best-known additive is monosodium glutamate. Regulatory systems identify several glutamate preparations by codes such as E620–E625. Glutamates are widely used in processed and convenience foods as flavour enhancers and are also common in many traditional cuisines; see general ingredient and food-industry summaries at convenience food and food additive resources.

Safety, regulation and distinctions

Food safety authorities have reviewed glutamate additives and generally consider them safe for the general population when used at customary levels. A minority of consumers report transient sensitivity to large doses of added monosodium glutamate, but controlled studies have not demonstrated consistent, reproducible adverse effects at ordinary dietary exposures. It is important to distinguish dietary free glutamate and glutamate constituting part of intact proteins; the latter is released by digestion and handled metabolically in typical ways. For biochemical classification and broader context see overview entries at classification and neurotransmitter information at neurotransmitter.