Cysteine, sulfur, disulfide bonds & reducing agents: oxidation, reduction, nucleophilicity, etc.

Cysteine crosslinks (disulfide bonds), cysteine oxidation, & reducing agents in the body & the lab. 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!
blog form: bit.ly/cysteine...
It’s Day 11 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 Cysteine (Cys, C)
Cys’ side chain is a -CH₂-SH group. -SH is called a THIOL (it’s alcohol’s (-OH) sulfur CYSter). More on alcohols here: bit.ly/2QWKWXp 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
More on what these terms mean in a sec, but the key take-away is that, 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, which involve thing 1 (the reductant) giving e⁻ to another thing 2 (the oxidizer), reducing thing 2 & oxidizing thing 1 in the process. You can remember this with the mnemonic OIL RIG: Oxidation Is Loss (of e⁻), Reduction Is Gain (of e⁻)
But I haven’t really explained what electrons are, so let’s back up a sec and review. Atoms (like individual C’s, H’s, O’s, & N’s) are really tiny, but they’re made up of even tinier parts called “subatomic particles,” which include electrons, protons, and neutrons. Electrons are negatively-charged subatomic particles that whizz around in “electron clouds” around a dense central core called the atomic nucleus where positively-charged protons (with some gluing together help from neutral neutrons) are tasked with reigning them in.
Finished in comments

[@thebumblingbiochemist]

Different atoms have different numbers of electrons. They keep most of them to themselves, held in by the positively-charged protons in the nucleus, but the electrons furthest away from the nucleus (valence electrons) feel the pull less, so they can play the field more so the atom can get to its “ideal” numbers of valence electrons - for a lot of the molecules we talk about in biochemicals (like carbon (C), nitrogen (N), oxygen (O) & sulfur (S)) this number is 8 - leading to the the so-called “octet rule.” Hydrogen (H) is too small for that many electrons - it only has a single proton, so there’s no way it could keep a hold on 8! Instead, it shoots for 2. If you look to the right-most column of the periodic table, you see the ones that already have their ideal number, the “noble gases.” All the elements want to be like those. One way to get to the ideal is by forming “covalent bonds” which involve atoms sharing pairs of electrons. This creates strong bonds but involves “permanent” commitment from both partners. An alternative is gaining or losing electrons without necessarily forming new bonds. I know this sounds weird, but bear with me for a minute. For some atoms, losing electrons gets them closer to their ideal, and for others, gaining electrons gets them closer. So, in “rig-ged reactions” the molecules conspire so each gets what they want. When atoms lose electrons, we call it oxidation - the OIL in OIL RIG. And when they do, the electrons have to go somewhere - and the somewhere they go to is another atom. So that other atom gains electrons, and we call that reduction - the RIG.Together, we call these redox reactions and you can’t have 1 without the other. These reactions happen because one molecule, the reducing agent, or REDUCTANT, “wants” to get rid of electron(s) and the other molecule, the oxidizing agent, or OXIDANT “wants” to take them. But once the reductant gives the oxidant the electron, the reductant becomes oxidized and once the oxidant takes an electron it becomes reduced and reluctant to act as an oxidant! So it’s not like something’s always in the “I want to reduce” state or alway in the “I want to oxidize” state. Their “mood” depends on their “redox environment” - the amounts of reductants and oxidants around them. note: sometimes, especially where metals are involved, you see the transfer of full electrons. But, other times, it’s harder to see when redox occurs because, instead of full transfers, what happens is more that an atom gets more or less of a shared electron cloud. For example, oxygen is “electronegative” which basically means it’s an electron hog. So, if a carbon atom has to share a pair of electrons with oxygen, the shared electrons are going to spend more time near the oxygen. Therefore, that carbon has lost electron density (been oxidized). If, however, the carbon is bound to a hydrogen, it will get more of the electron density, so we can say it’s been reduced. more on this here: bit.ly/ridiculousredox Cysteines can act as reducing agents - until they link up as cystines. Forming those disulfide bonds requires that 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. Therefore, it’s important they get it right the first time, because they might not find the right partner if they have to do it again… The reduced state of Cys is nucleophilic - it seeks out the positive charge coming from a neighboring atom’s nucleus to bind to. So you want to make sure the neighbor it finds is the neighbor it’s supposed to bind! note: This nucleophilicity is enhanced when cysteine gets “deprotonated” (loses an H⁺) to give you an -S⁻. This S⁻ is called a “thiolate” and it’s really unhappy and thus reactive

[@thebumblingbiochemist]

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 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. tech note: in the lab, 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). more here: bit.ly/dttreducingagents Outside of the cell, however, conditions are harsher, and more oxidizing. So extracellular proteins often have disulfide bonds to sturdy them up and/or to keep multiple protein chains attached in multimers. To avoid getting reduced, these proteins go through alternative processing pathways where they’re made & shuttled out of the cell in protective membrane-bound vesicles. 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. More on it here: doi.org/10.1021/cr60033a001 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 - the fact 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 (the ratio 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.

[@thebumblingbiochemist]

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) I know this post has gotten long, but I have one more kinda related thing to share with you - and I’ll try to make it snappy! Analogously to what we saw with strands of hair, inside each band of a rubber band there are actually lots of strands and each of those strands is a chain of repeating similar units - we call chains like that polymers and the units monomers. You put together different types of monomers to get different types of polymers. The strands inside the strands of your hair are chains (polymers) of amino acid monomers, and the strands in natural rubber (NR) are chains (polymers) of isoprene (a carbon-hydrogen zig-zag thing). Rubber gets its pull-back force by connecting its chains through sulfur crosslinks and it gets its crosslinks through a process called vulcanization. There are a bunch of different methods depending on the properties desired (sturdiness, stretchiness, heat resistance, etc.). But sulfur vulcanization methods involve taking sulfur that’s linked to one thing & getting it to link to other things. With hair perming, you want it to be “permanent” so you want there to be lots of short crosslinks. But, for a rubber band, you want to allow for a bit more flexibility - so you might want sparser cross linking and/or longer bridges. Lots of cross linking will make the rubber hard & brittle; short crosslinks give you heat & weather resistance, & longer crosslinks make for a more durable product. more here: bit.ly/2FJUXPU I know, that wasn’t really about cysteine… but I always love when you can connect concepts! Our bodies definitely need cysteine, but it is considered “non-essential” in the dietary sense meaning we don’t need to get it through our food. Instead, our bodies can make it from the amino acid serine. Cysteine is also considered “glucogenic” because our bodies can make glucose (blood sugar) from it how does it measure up? coded for by: UGU, UGC
chemical formula: C₃H₇NO₂S molar mass: 121.15 g·mol⁻¹   more about redox: bit.ly/redoxbiochem   more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 bit.ly/2OllAB0 or search blog: thebumblingbiochemist.com

[@JoyAndStudy]

Thank you so much 🎉

[@lucygooseAZ]

New here. Trying to make sense of chloride channel ion transfer/activity as it relates to CFTR mutant folding in the endoplasmic reticulum. Do you have a lesson on this? Specifically G85E and G91R thank you!

[@thebumblingbiochemist]

I don't have a lesson on that sorry! But I do have a post on CF bit.ly/cysticfibrosisscience

[@thebumblingbiochemist]

I'm not familiar with those conditions, sorry!