Threonine biochemistry: what you should now (and what's cool to know about how it was discovered...)

Three cheers for THREONINE! Once scientists identified the first amino acids, the race was on - Pokemon Go amino acid style - gotta find them all! And the last to be found and characterized was Threonine (Thr, T). It’s a pretty thrilling tale that will hopefully never get stale! It was last to be discovered but I couldn’t wait to tell you about it, especially when I found out that grad students were literally essential to figuring that out threonine is essential to the human diet…
blog form with figures: bit.ly/threonin...
It’s Day 6 of #20DaysOfAminoAcids - the bumbling biochemist’s version of an advent calendar. Amino acids are the building blocks of proteins. There are 20 (common) genetically-specified ones, each with a generic backbone with to allow for linking up through peptide bonds to form chains (polypeptides) that fold up into functional proteins, as well as unique side chains (aka “R groups” that stick off like charms from a charm bracelet). Each day I’m going to bring you the story of one of these “charms” - what we know about it and how we know about it, where it comes from, where it goes, and outstanding questions nobody knows.
More on amino acids in general here bit.ly/aminoaci... but the basic overview is:
amino acids have generic “amino” (NH₃⁺/NH₂) & “carboxyl” (COOH/COO⁻) groups that let them link up together through peptide bonds (N links to C, H₂O lost, and the remaining “residual” parts are called residues). The reason for the “2 options” in parentheses is that these groups’ protonation state (how many protons (H⁺ ) they have) depends on the pH (which is a measure of how many free H⁺ are around to take).
Those generic parts are attached to a central “alpha carbon” (Ca), which is also attached to one of 20 unique side chains (“R groups”) which have different properties (big, small, hydrophilic (water-loving), hydrophobic (water-avoided), etc.) & proteins have different combos of them, so the proteins have different properties. And we can get a better appreciation and understanding of proteins if we look at those letters. So, today let’s look at threonine!
Threonine (or as its friends call it Thr, or just T) has three “groups” in its side chain, and one of them has an O. But that’s not why it got that name - the name has a less mnemonically-satisfying origin - structural relationship to the 4-carbon carbohydrate “threnodic acid,” which gets its name from coming from the sugar threose, which gets its name from scrambling up the letters of a sterically-scrambled (atoms stick off other atoms in different directions) version of this sugar, erythrose, which gets its name because it turns red in basic solutions. If you’re interested in etymologies, I found this awesome website, chemtymology.co.uk where I learned this
Speaking of origins, I want to tell you about how threonine was discovered, but first I want to tell you all about threonine biochemistry-wise, and how it provides opportunities for proteins to get “modified” after they’ve been made. I know, I know, I’ve been rambling on and on about how important the primary sequence (order of amino acids) is to proteins because their unique parts affect how they fold up. But now I’m telling you that parts of the unique parts can be changed?! Don’t worry - I haven’t been lying to you about the primary importance of primary structure - only specific amino acids can be altered, and threonine’s one of the them, so the amino acid sequence matters even here!
So what’s so special about threonine? Threonine looks like valine which, as you might remember, has that “V” of a side chain where each “point” on the V is a carbon/hydrogen group - so 1 methylene (CH₂) branching off into 2 methyl (CH₃) groups. Threonine also has a V, but there’s a BIG difference. Instead of 2 methyls, one of the branches is a hydroxyl (-OH) group. This makes Thr an “alcohol.” When you hear the word “alcohol” you probably think of wine, or beer, etc. but that’s just 1 kind of alcohol (ethanol) - the term “alcohol” just refers to something that has one or more hydroxyl (-OH)) groups.
Why does this matter? Molecules are formed by atoms linking up through strong covalent bonds, which are formed by atoms sharing electrons. Electrons are negatively-charged subatomic particles that whizz around each atoms’ dense central core called the nucleus, which contains positively-charged protons (and neutral neutrons). Some atomic nuclei keep a “tighter leash” on their electrons (including the ones which they share), which can uneven the charge balance, in a phenomenon we call POLARITY. Since opposite charges attract, this can lead to partially charged regions being attracted to other partly or fully charged things with the opposite charge.
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