The molecule that makes food taste savoury also fires most of your neurons
Glutamate gives parmesan, soy sauce and seaweed broth their deep savoury taste, the flavour a Japanese chemist named umami in 1908. The very same molecule is the brain's main accelerator, the most abundant excitatory neurotransmitter in vertebrates, and it is central to how we learn and remember.
Glutamic acid is one of the amino acids used to build proteins. The German chemist Karl Heinrich Ritthausen first isolated it in 1866 by treating wheat gluten with sulfuric acid, hence the name; glutamic acid makes up 30 to 35 percent of gluten. Its single-letter code is E, chosen simply because it follows D for aspartate, which is one carbon unit smaller.
In 1908 Kikunae Ikeda at Tokyo Imperial University boiled down a large quantity of kombu broth and found brown crystals that, when tasted, reproduced the savoury note he had noticed in many foods. He called the taste umami and patented a way to mass-produce the sodium salt, monosodium glutamate. Protein-bound glutamate is tasteless; only the free form, abundant in cheese and soy sauce, registers on the tongue. By 2006 about 1.5 million tons were made each year, more than any other amino acid, largely by the bacterium Corynebacterium glutamicum fermenting sugars and ammonia. Most dietary glutamate never gets far: about 95 percent is consumed by intestinal cells on first pass.
In the nervous system, nerve impulses release glutamate from storage vesicles to activate receptors such as NMDA and AMPA on the neighbouring cell. Long-term potentiation, the strengthening of connections thought to underlie memory, happens at glutamate synapses in the hippocampus and cortex. Glutamate is also the raw material for GABA, the brain's main brake. Drugs such as PCP and ketamine block NMDA receptors, which helps explain their dissociative effects.
It is also a metabolic workhorse. Many amino acids hand their nitrogen to a citric-acid-cycle molecule to form glutamate, which then releases it as ammonia to be turned into urea by the liver and excreted. That abundance has a dark side: aggressive brain tumours called gliomas can use glutamate as fuel, especially when they carry mutations in the IDH1 gene.
Source: Glutamic acid
