Glutamic acid

Glutamic acid (abbreviated as Glu or E; encoded by the codons GAA or GAG) is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated -+NH3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated –COO- form under biological conditions), and a side chain carboxylic acid, classifying it as a polar negatively charged (at physiological pH), aliphatic amino acid. It is non-essential in humans, meaning the body can synthesize it.

In neuroscience, its carboxylate anion glutamate is an important excitatory neurotransmitter that plays the principal role in neural activation.

Chemistry

The side chain carboxylic acid functional group has a pK_a of 4.1 and therefore exists almost entirely in its negatively charged deprotonated carboxylate form at pH values greater than 4.1; therefore, it is negatively charged at physiological pH ranging from 7.35 to 7.45.

History

Although they occur naturally in many foods, the flavor contributions made by glutamic acid and other amino acids were only scientifically identified early in the twentieth century. The substance was discovered and identified in the year 1866, by the German chemist Karl Heinrich Ritthausen who treated wheat gluten (for which it was named) with sulfuric acid. In 1908 Japanese researcher Kikunae Ikeda of the Tokyo Imperial University identified brown crystals left behind after the evaporation of a large amount of kombu broth as glutamic acid. These crystals, when tasted, reproduced the ineffable but undeniable flavor he detected in many foods, most especially in seaweed. Professor Ikeda termed this flavor umami. He then patented a method of mass-producing a crystalline salt of glutamic acid, monosodium glutamate.

Glutamate flavoring

Glutamic acid and its ions and salts, called glutamates, are flavor-enhancing compounds which provide an umami (savory) taste to food.

Glutamic acid is a natural constituent of many fermented or aged foods, including soy sauce, fermented bean paste, and cheese, and is also a component of hydrolyzed protein such as yeast extract.

The sodium salt of glutamic acid, monosodium glutamate (MSG), is a widely used additive in the food industry.

Glutamic acid versus glutamate

When glutamic acid or one of its salts is dissolved in aqueous solutions, a pH-dependent instantaneous chemical equilibrium of the amino acid's ionized forms, including zwitterionic forms, will result. These ions are called glutamates. Salts exist only in a dry and crystallized form. The form ultimately responsible for the taste is the glutamate ion, and the form of glutamic acid at the time of the addition is not important. However, crystalline glutamic acid salts such as monosodium glutamate dissolve much better and faster than crystalline glutamic acid, a property important for use as a flavor enhancer.

Glutamic acid (data page)

References