The Sistine Chapel wasn’t built in a day, but thanks to the amino acid cysteine (Cys, C) your hair can be curled in one! Although the history of cysteine research is a bit hairy – It was hard to even find it in proteins at all. You might have heard of the curse of Tutankhaman  – well, biochemists spoke of a kind of curse of cysteine research – from disappearing samples to disappearing researchers, early cysteine investigations were beset with problems. But eventually they were able to figure it out – and now we know so much about it we can take advantage of its unique property to form special links to curl our hair or straighten out its kinks! 

It’s Day 11 of #20DaysOfAminoAcids – the bumbling biochemist’s version of an advent calendar (past the halfway point!) Amino acids are the building blocks of proteins. There are 20 (common) ones, each with a generic backbone 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.  

Cys’ side chain is a -CH₂-SH group. -SH is called a THIOL (it’s alcohol’s (-OH) sulfur CYSter).  More on alcohols here: but it just refers to molecules having hydroxyl (-OH) group(s). So not all alcohols are “alcoholic” in the social outing sense. Anything with an -OH is an “alcohol” in the chemistry sense, so things like sugar are super alcoholic. 

Cysteine isn’t gonna get you drunk, but the thiol can lead to some protein personality changes! 2 Cys (either within the same protein or between 2 proteins) can link together (protein)-SH + HS-(protein) 👉 (protein)-S-S-(protein) to give you a  DISULFIDE BOND or “cross-link” between 2 Cys residues in the same or different proteins

-SH + -SH ⇌ -S-S-

Unlike other charm-charm interactions, which are just *attractions* based on charge (or partial charge) differences, this is a covalent bond, so it’s strong (and good for sturdying-up secreted proteins that have to live outside the comfort of the cell). For example, pairs of disulfide bonds are used to keep the 2 separate peptide chains of the hormone insulin connected as it travels throughout your bloodstream to tell cells to let in and use glucose (blood sugar). 

BUT these Cys-Cys bonds aren’t quite as strong as the covalent bonds in the protein’s backbone (linking the chain links) so you can split them back up without splitting up the chain links. This splitting & unsplitting involves REDuction & OXidation (REDOX) reactions. More here ( but basically they involve thing 1 (the oxidant) giving e⁻ to another thing 2 (the reductant), reducing thing 2 & oxidizing thing 1 in the process

remember OIL RIG: Oxidation Is Loss (of e⁻), Reduction Is Gain (of e⁻)

When cysteine’s link up as cystines, they share their “extra” e⁻ so they’re “losing” e⁻ (being OXIDIZED) (lose the “e” in the name (and also “e⁻”)). And when they split up, they get the e⁻ they’d been sharing “back”, so they’re REDUCED

Which state Cys is in depends on the “redox environment” (whether there are more reductants or oxidants around) and keeping the right redox conditions is really important for protein shape & function because, since these bonds are stronger than the normal intermolecular bonds, they’re harder to break up, so it’s important you get it right the first time. Because you might not find the right partner if you have to do it again…

If a cysteine residue gets reduced, it gains electrons – and the negative charge they bring – so it becomes nucleophilic – seeks out the positive charge coming from a neighboring atom’s nucleus. So you want to make sure the neighbor it finds is the neighbor it’s supposed to bind! Inside your cell, conditions are kept “reducing” thanks to glutathione “buffering” so proteins that do have vulnerable disulfide bonds that need to be kept go through alternative processing pathways where they’re made & shuttled out of the cell (into the oxidizing extracellular environment) in protective membrane-bound vesicles.

An oxidizing state in your cells can thus be “dangerous” because inappropriate bridging can occur if they get oxidized by reactive oxygen species (ROS). Low, CONTROLLED, levels of ROS are important for things like signaling but high levels can be damaging because, as their name suggests, they’re reactive and they can react with things like DNA & proteins, messing them up. So your cells Cells control the redox environment by stocking up on reducing agents like glutathione. Glutathione is made from 3 protein letters (amino acids) – Glu, Cys, & Gly & acts as a “Redox buffer.” It goes goes back and forth between reduced (GSH) & oxidized forms(GSSG) forms, where 2 glutathione are linked by a disulfide bond.

When we purify proteins, we usually try to keep them in an environment that’s close to what they’re naturally found in. Evolution’s had years and years of natural selection to adapt these proteins to be happiest and most active in that environment. If you’re dealing with an intracellular protein that’s used to that reducing environment and you change things up to an oxidizing environment, proteins can panic like it’s the end of the world & hook up with the nearest available thing instead of waiting for their “soulmate.” 

When we’re studying proteins that are normally inside of the general “inside” of cells, the environment we’re trying to mimic is that of the cytoplasm. So we want to keep a “reducing environment” – so we also usually add a reducing agent, which can sop up extra electrons. Instead of using glutathione, which oxidizes too easily, a few common reducing agents we use in the lab are DTT (DiThioThreiotol), TCEP ((Tris(2-Carboxyethyl) phosphine hydrochloride)), & BME (β-mercaptoethanol, aka 2-mercaptoenthanol). 

We want to avoid new bridging in protein preps, but breaking and reforming disulfide bonds is actually how perms work which segues us into our hairy tail.

Cysteine was the first amino acid to be discovered – but not the first to be shown to be a protein letter (that honor goes to glycine (or leucine depending where you look). Cystine was first described in 1810 by Wollaston who found it in a urinary calculus (a bladder stone) – he named it cystic oxide but didn’t know what it was. Over the next couple decades there were some brief mentions of it – including from a guy named Lassaigne who said he found it in a calculus from a dog but he didn’t describe it well and based on what he did describe, it probably wasn’t even cysteine.

Of course, at that time, the name was still “cystic oxide” – but then in 1832 a guy named Berzelius comes along and he’s like “dude – soooo many organic compounds contain oxygen – adding the name “oxide” isn’t helpful” – so he shortens the name to cystine. William Prout went on the chemically characterize it, but he didn’t detect sulfur – that cystine contained sulfur wasn’t discovered until 1837 by Baudrimont & Malaguti

Even once they found the sulfur, they had a really hard time figuring out the empirical formula (how many of each type of atom per molecule) – because cystIne was tricking them with that whole linking up thing. And it was hard to even find it in proteins at all. You might have heard of the curse of Tutankhamen  – well, biochemists spoke of a kind of curse of cysteine research – from disappearing samples to disappearing researchers, early cysteine investigations were beset with problems For example, In early protein letter reconnaissance missions, sulfuric acid was used to chop up proteins into their individual letters through acid hydrolysis. Before analyzing the letters, the scientists would remove the acid as calcium sulfate, and cystine was likely lost in the precipitate. And even if there was some cysteine in there, there likely wasn’t much because they were mainly using things like the milk protein casein instead of more cysteine-rich proteins like horn or hair.

Hair – yup there’s lots of cysteine – and cystine – there!. Inside each strand of hair there are actually lots of strands and each of those strands of the protein KERATIN. And it has lots of cysteines. The protein KERATIN gives your hair & nails strength  & it get’s *its* strength from disulfide crosslinks holding strands of keratin together, Each strand has lots of Cys, so lots of ways to link & the “choice” of way determines the curviness of our hair. Keratin’s stretchy, so you can physically force your hair to curl, but it will de-bounce back unless you use some “chemical force.” in a perm, you physically force hair into the shape you want (e.g. curl it around curlers), then apply a reducing agent to reduce the crosslinks along with heat to remove the non-covalent bonds ⏩ the strands can now stabilize in new positions ⏩ add an oxidizing agent to make new cross-links in the new position. A similar function is carried out in our body by an enzyme called protein disulfide iosmerase (PDI) which catalyzes the shuffling around of cross-links to correct improper bonds that would stabilize a misfolded form

If you want to break bridges, you can add a reducing agent. And if you want to build bridges, you add an oxidizing agent. Ones commonly used for hair perms are ammonium thioglycolate as a reductant to break the bonds & hydrogen peroxide as the oxidant to reform them – when you’re ready! (e.g. after you’ve wrapped hair around curlers)

Sorry for the lame-o post – I was doing some really cool experiments, so mishmashed some past posts and didn’t have time to polish it up. But you can find a lot more about various parts of it at the link. Thanks for understanding – I’m a full-time grad student & my research comes first. 

how does it measure up?
coded for by: UGU, UGC
chemical formula: C3H7NO2S
molar mass: 121.15 g·mol−1

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉

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