Glutamate, ionic “bonds,” salts, chemophobia, acidic amino acids, transamination, MSG myth, etc.

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MSG stands for MonoSodium Glutamate and it’s just the sodium salt of glutamate. It gets added to food sometimes because glutamate is responsible for that rich “5th flavor,” umami. “Salt” is just another name we give to a neutral ionic compound - ions, by definition, are charged, thanks to their imbalance of protons & electrons. So in order to get a salt you have to have a positive thing(s) (cation) and a negative thing (anion) - but you can mix and match. Stick a sodium ion (Na⁺) with a chloride ion (Cl⁻) and you get NaCl, or as you might know it better, table salt. If, instead of a chloride ion you stick the sodium with a glutamate anion, you get monosodium glutamate (MSG). Add in some disputed science and xenophobia and you get the myth that MSG causes a variety of strange symptoms. ⠀
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If you want to break down proteins for parts, you have to remove the Ns from their amino acids - when N is attached to hydrogen(s) we call it an “amine” so we can call N removal “deamination.” And glutamate plays a key role in making sure this is done safely (if you just break off N as ammonia (NH₄⁺) without doing anything with that ammonia it can cause problems).⠀
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Glutamate helps with deamination through a 2-step process called “transdeamination.” First, in the “transamination” step, an amine group is transferred from an amino acid that’s getting broken down to a-ketoglutarate with the help of a protein enzyme (reaction mediator) called a transaminase or aminotransferase. α-ketoglutarate, AKA 2-oxoglutarate (2-OG), is “just” glutamate with a carbonyl (C=O) group instead of an amine group. So if you swap the carbonyl for an amino by TRANSfering the amino group from amino acid to α-ketoglutarate, you’re left with glutamate and an “α-keto acid.” The exact α-keto acid you’re left with depends on what you stole from.⠀
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At this point you might be wondering - why the heck did we do that? what did we accomplish? We removed a nitrogen group from one amino acid, successfully breaking that down, but now we made a new amino acid?! The key thing is that, regardless of what amino acid you started with, you’ve made the *same* new amino acid - glutamate. So you’ve collected the amine group from various sources into the same common carrying molecule - glutamate - so when you get to the actual removal, there’s only a single type of molecule that the remover has to be able to work with.
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In the second step, “oxidative deamination,” the amine group can get cut off as ammonia - wait, isn’t this what we were trying to avoid all along?! The key is that the ammonia is now released in a controlled fashion into the awaiting arms of the urea cycle which is concentrated in the liver. Similarly to how recycling gets collected and taken to a central processing factory, the liver is the only organ with a complete “urea cycle” which can combine the ammonia with bicarbonate HCO₃⁻ and then turn it into urea, which is a less toxic nitrogenous waste product you can just pee out. ⠀
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Oxidative deamination is catalyzed (sped up) by an enzyme called glutamate dehydrogenase. The reaction is reversible and the NADP form is used in the making glutamate direction whereas the NAD form is used in the breaking glutamate direction. A cool thing about glutamate dehydrogenase is that, although it does not require ATP, it is regulated by it - when ATP levels are high, the cell knows it has plenty of energy so it can focus on building instead of breaking, so the glutamate-making direction is favored. But when ATP levels are low, breakdown is favored.⠀
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You can also use glutamate to make glutamine (the amide form of glutamate, which has an -NH₂ group instead of an -O/OH attached to the carbonyl in its cap). That *does* require ATP money and is helped along by the enzyme glutamine synthetase. In addition to giving you a source of glutamine for protein-making and stuff, this can serve as an alternative to the urea route - the glutamate you get from transamination can be made into glutamine and taken to the kidneys where it gets deamidated by glutaminase to give you back glutamate, which can then be deaminated by glutamate dehydrogenase which, releases NH₃ which, instead of getting used for urea making (we’re in the kidney now, remember) gets used to sop up extra protons to give you NH₄⁺ and maintain pH balance. ⠀

[dennisgarber]

I guess, I would be more interested in addiction to msg. My guess is that it would be about the same as table salt?