Did you get the catecholamine call? It’s ADRENALINE!!!!!!!!!! Fight, flight, or flood your friends ‘ ears Friday about finally getting your freakishly frustrating experiment to work – none of it would be possible without TYROSINE! Without this amino acid (protein letter) you couldn’t make L-DOPA, and without L-DOPA there’d be no dopamine, and without dopamine you could have neither noradrenaline nor adrenaline!(Adrenaline is the same as epinephrin and noradrenaline = norepinephrine, BUT adrenaline is different from noradrenaline (it has an extra methyl (CH3) group). They also can play different roles, but both serve as critical chemical messengers called CATECHOLAMINES. I hope you’ll read on because it’s a pretty *dopa* story! So please don’t moan if I tell a tale of hormones and neurotransmitters.
Today’s post is kinda multi-part. After a brief intro on Tyrosine (Tyr, Y) I’m going to talk about hormones & neurotransmitters in general, and then I will tell you more about the actions of catecholamines specifically. And then I will tell you more about where those catecholamines come from – How does tyrosine become adrenaline? True nerds will stick around till the end, but I won’t judge you if you don’t! At least I had fun researching it 🙂
It’s Day 8 of #20DaysOfAminoAcids – the bumbling biochemist’s version of an advent calendar. 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.
More on amino acids in general here http://bit.ly/2P0pJrB
I hope you’re not getting Tyr-ed of these – but if you are, today’s post might help give you some needed adrenaline. Tyrosine (Tyr, Y) is “just” the amino acid we looked at yesterday, phenylalanine (Phe, F), with a hydroxyl (-OH) group plopped on (straight across from where the ring attaches to the methylene linker attaching it to the generic backbone). This may seem like a minor change, but it’s really important. And it opens up a LOT of anabolic (molecule-building) opportunities – as we’ll see Tyr is a precursor for catecholamine hormones (dopamine, adrenaline, noradrenaline), the pigment molecule melanin, and more!
Adding the OH does NOT affect tyrosine’s ability to delocalize some of its electrons around the ring, so we still classify tyrosine as AROMATIC. But the OH *does* introduce a bit of of polarity into it – atoms link together by sharing pairs of electrons in covalent bonds. If the atoms share fair, and there are the same # of protons (+ charged) and electrons (- charged), the charge cancels out everywhere and you get a “nonpolar” molecule. But if one of the atoms is an electron hog (we call these molecules electronegative) it steals more than it’s entitled to, so it becomes partly negative, the thing it’s stealing from becomes partly positive, and you’ve got yourself a “polar molecule”
Why am I making a big fuss about this? Water’s highly polar and it likes to interact with (and thus dissolve) other polar things. O is one of those electron hogs, so adding it on the Phe makes Tyr more polar (and when it gets a second -OH to become a catecholamine it’ll be even more so). And this is important because we need tyrosine & its metabolic products to be soluble so they can travel between neurons & throughout our blood in their chemical messenger duties!
Your body has billions of cells and they need ways to talk to one another. Nerves are great if you want to send a signal to one specific place – like to tell the end of a single one of Stephen Colbert’s eyebrows to move up. 🤨 You don’t want to send that “move” message to the whole eyebrow, or the other eyebrow, or definitely not the entire body. To get this level of specificity you have to have a direct connection.
Nerve cells called neurons communicate with other neurons and/or muscle, gland, etc. cells by passing messages really short distances. One neuron releases a chemical called a neurotransmitter (there are lots of different ones, including noradrenaline) into the gap between the 2 of them (synapse) and then receptors on the second one bind that neurotransmitter and relay a message into the cell.
But what if you want to send a message to your entire body? If you see a bear – or a beautiful peak on your chromatograph indicating you’ve gotten a yowza yield from your protein purification prep – you don’t just want to raise an eyebrow. You want your body to stop doing “housekeeping” work like digesting and keeping your fingertips warm and instead focus on doing what you need to do to run away – or run off to fill your friends in on your exciting news!
Unlike the nervous system, which requires all the “wiring” to be laid out, like having one of those “direct” phone lines, which are great if you only want one person to get your message (and no wiretaps allowed!), the ENDOCRINE SYSTEM is more like radio. It sends a message out to everyone, but you can only get it if you have your radio tuned in to the right wavelength. Instead of sending out waves, the endocrine system sends out chemical messenger molecules called hormones from “broadcast stations” called glands and only cells expressing the matching receptors can hear the message and respond (with various responses depending on the type of cell getting the broadcast).
Hormones get secreted into the bloodstream so they pass by all your cells but only cells with the right receptors can respond (like having your radio tuned to the right wavelength). These receptors are proteins that are embedded in your cells’ membrane. Like all proteins, the instructions for making them are written in your DNA “blueprint” – so each cell has the instructions and thus could make it if they wanted to but, thanks to lots of regulation, only certain cells do. And they can make more or less at different times to “ramp up” or “calm down” sensitivity & responses to signals.
Problem is, the signal still has to get to them, and signals that can easily go through the blood have a hard time getting into the cells that need them (even if they have the right receptor) and signals that can slip right into their target cells have a hard time getting there. Why? SOLUBILITY!
It’s important that these molecules are able to travel through the blood, so they need to be soluble. In order for something to be soluble, the solvent molecules (dissolver like water) have to want to bind to the solute (the molecule you want dissolved) more than they want to bind to other solute molecules. And the solute molecules have to want to bind those solvent molecules more than they want to bind each other and whatever else is floating around in there.
The reason people are always looking for evidence of water on other planets is that all life we know of is water-based because water’s an awesome solvent – for a lot of things – but not all things.
Many hormones *are* water-soluble because they’re HYDROPHILIC (water-loving) – hydrophilic hormones include peptide, protein, glycoprotein, & catecholamine hormones (this is the class adrenaline & noradrenaline are in). So they travel fine through your bloodstream, to the cells they want to target. But in order to get from the blood into your cells, they have to get through your cells’ lipid membrane.
Oil and water don’t mix so the lipid (basically oil coat) your water-y cells are surrounded in keeps the inside of the cells (cytoplasm) separate from the outside of your cells. To get things through, your membranes has things like pores provide channels and pumps use energy to actively transport them (sometimes in exchange for pumping something out (we call these co-transporters) that let through specific things.
But hormones usually take another option -> don’t let the molecule into the cell but make sure the message gets passed along – some molecules bind surface receptors -> these receptors are embedded in the membrane so a change to the outside part (like a hormone binding) can cause the receptor to shape-shift, causing something to happen on the inside side.
This can set off a signaling cascade that amplifies the signal & gets it where it needs to go. Often these cascades involve kinases which relay the message through phosphate-adding. The receptors themselves can be kinases (as is the case for insulin) or they can signal intracellular kinases (as is the case for growth hormones). Messages can also be passed into the cell by “2nd-messenger” systems like cyclic adenosine monophosphate (cAMP) or lipid messengers -> inositol triphosphate (IP3) and diacyl glycerol (DAG). You just need something where hormone binding on the outside can be relayed to the inside of the cell.
Other type of hormones, steroid hormones (like cortisol, testosterone, estrogen) and thyroid hormones like thyroxine *can* get through the membrane because they’re hydrophobic/lipophilic. Once inside, they can also get through the membrane surrounding the nucleus where the DNA is kept. Here they bind to an intracellular receptor -> hormone-receptor complex binds hormone response elements in DNA -> can directly affect the expression of genes (e.g. make more of this, less of that) But they have a hard time traveling around in the blood, because they hate water – so they travel through the blood by piggybacking on on transport proteins that shield them from their nemesis.
If a cell has the receptors and gets the signal, the actual response you get will depend on the type of cell getting the signal – cells specialize in different things and get different input combos (there are lots of other hormones and stuff whose messages also have to get taken into consideration) so your skin cells will respond differently than your muscle cells, and the muscle cells in your blood vessel walls will respond differently from those in your calves. Plus, as is the case with catecholamines, you can have multiple types of receptors that can bind the same molecule but elicit different responses. So there’s a lot of possibilities coming from these little molecules!
Here’s a little bit about what adrenaline & noradrenaline do, and then I’ll tell you more about how they’re made.
Adrenaline & noradrenaline can act both as “neurotransmitters” AND “hormones” – the “job title” just refers to whether a chemical messenger is released from a nerve cell or from a gland – it has nothing to do with the actual messenger molecule that’s released. So when noradrenaline is released from nerves in your sympathetic nervous system we call it a neurotransmitter. But when it’s released from your adrenal gland we call it a hormone.
Adrenaline is primarily “only” a hormone because it’s almost all released from your adrenal gland, with just a little bit being made & used by a group of neurons in your brainstem, but noradrenaline is the main neurotransmitter used by the sympathetic nerves in your cardiovascular system. Even under un-stressful conditions, it plays an important role in making sure your heart and blood vessels don’t relax too much (which they would without noradrenaline because the counterbalancing parasympathetic nervous system tells them to relax)
There are different types of adrenergic receptors (aka adrenoreceptors) and different cells express different combos and they can elicit slightly different responses – they’re all “G-protein coupled receptors” (GPCRs) which means that when the catecholamine binds, a little “G protein” attached to the receptor on the inside-the-cell part swaps a GTP (like ATP but a different RNA letter) for the GDP it did have and then interacts with other things to relay the message.
Different receptors, even if they all bind the same things, can thus set off different signaling pathways. End result? Get your heart to beat faster, your liver to stop storing glucose as the storage molecule glycogen, and instead break down glycogen (glycogenolysis) and release the resulting glucose into the bloodstream. Speaking of bloodstream, different receptors can lead to blood vessels constricting or widening. You want some of your blood vessels to constrict (vasoconstriction), which, like pinching a hose, raises blood pressure and restricts blood flow to less important regions like your fingertips. And then, to help you shunt the blood to where you want it, like your calve muscles which need blood to run, you want those places to widen instead of tighten their blood vessels. So cells in those places express receptors that get them to “loosen up” (vasodilate) when they get the catecholamine call.
So there’s a lot of concerted action that’s coming from that one molecule and leading to the “fight or flight” response. Let’s give that molecule some more attention!
First off, What’s with all the naming confusion? adrenaline and epinephrine mean the same thing – both chemically and etymologically – the words just come from different languages and were named by different scientists. There are different accounts of early discoverers – William Bates in 1886 found something produced by the adrenal gland and then in 1895, Polish physiologist Napoleon Cybulski found it. In 1901, Japanese chemist Jōkichi Takamine purified it from adrenal glands, named it adrenaline (Latin “ad” (near) + “renal” (kidneys)) & patented it – and then Europe went with it. But another scientist, John Abel, had already prepared an adrenal extract and called it epinephrine (Greek “epi” (on top of) + “nephros” (kidneys)). It turns out that this didn’t even contain active adrenaline, but the US went with it. So now it’s a big confusing mess of names! But just remember that while adrenaline and epinephrin are the same adrenaline and noradrenaline are different!
Speaking of which, let’s look more at how they’re chemically different but start off the same!
They’re gonna have to “pass through” tyrosine, but that tyrosine can come from your food or from making it from phenylalanine (but you can’t make phenylalanine so Phe is “essential” while Tyr is “nonessential” in the dietary sense).
About half of the Phe you eat is actually used to make Tyr. An enzyme called PHENYLALANINE HYDROXYLASE (PAH) adds a hydroxyl (-OH) group to Phe at the 4th position in the ring (We call this “para” position (where things sticking out from ring are straight across from each other. If the things are right next to each other (adjacent) we call it ortho, and one apart gives you meta). This changes the R group to 4-hydroxylbenzyl. In other words, congrats, you’ve just made Tyr! As we saw yesterday, mutations in PAH can cause the disease phenylketonuria (PKU) which is why you can see those warnings on nutritional labels http://bit.ly/2qzMRFi
The first step on the path from Tyrosine to adrenaline is adding ANOTHER -OH to the ring to give you L-DOPA. (The L refers to the stereochemistry (which way the atoms stick off from each other – L-DOPA comes from L-tyrosine, which is the “normal” version of tyrosine).This second OH make it even more soluble, and it gives it a dope name – we call benzene-based (6-sided aromatic ring) molecules with 2 -OH groups “catechols” and since it also has an amine (NH2/NH3) group, we call L-DOPA and its derivatives CATECHOLAMINES.
Since (-OH) is called a hydroxyl group, we call this gifting of a hydroxyl group hydroxylation. Although is it really proper gifting if you steal from molecular oxygen (O2) to give to the tyr-ed? I’ll let you grapple with that moral quandary and instead get back to the mechanism at hand. This specific hydroxylation is helped out by an enzyme called tyrosine hydroxylase (TH), which is a homotetramer (4 copies of the same protein chain working as a team), which is itself helped out by a cofactor (non-protein helper molecule that the protein enzyme binding to to give it “superpowers”) called tetrahydrobiopterin (THB)
L-DOPA is turned into dopamine by removing the carboxyl group – the thing that made an amino acid an acid. But you still have the amino part, and you have the catechol part. So dopamine is still characterized as a catecholamine. This decarboxylation is catalyzed by an enzyme called DOPA decarboxylase (DDC). It also decarboxylates other aromatic amino acids, so another name for it is aromatic amino acid decarboxylase (AADC). This uses a vitamin B6-derived cofactor called pyridoxal phosphate (PLP)
Now that you have dopamine, what to do? Some cells in the substantial nigra (part of the basal ganglia in your midbrain) stop here and use dopamine as a neurotransmitter as is, where it plays important roles in reward responses. But, otherwise, the next step is turning it into noradrenaline (aka norepinephrine). This is accomplished by adding another OH – but this time to one of the non-ring carbons. And this is catalyzed by dopamine β-hydroxylase (DBH). It’s also a tetramer, but a heteromeric one (not all its subunits are the same). And It, too, uses a cofactor, but it uses ascorbate so it’s vitamin-C dependent.
Noradrenaline can be used as-is. Or it can be further processed to form adrenaline by adding a methyl (-CH3) group onto the end of of it. That’s done by Phenylethanolamine N-methyltransferase (PNMT), with the extra methyl coming from the cofactor S-adenosylmethionine (SAM). Only cells with PNMT can do this conversion, and it’s not found in most neurons (though it is made from a small group of neurons in the brainstem). Instead, most of it is in the adrenal gland (specifically in the inner part called the medulla) & its activity is regulated by corticosteroids – hormones like cortisol made in the outer part of the adrenal gland (the adrenal cortex)
But noradrenaline or adrenaline making isn’t Tyr’s only fate.
The substantia nigra actually gets its name (Latin for “dark substance”) from this brain region having a dark blue-black color, and this is caused indirectly by having a lot of tyrosine in that area because tyrosine can get converted into the pigment molecule melanin. Melanin making starts with a different -OH adder – Tyrosinase – which converts tyrosine to DOPA & then instead of getting used to make dopamine, etc. it gets further oxidized – converted to dopaquinone which can get further modified & link up in different ways to give you different forms of melanin. So problems with tyrosine metabolism can also lead to albinism.
Problems with tyrosine metabolism can also lead to thyroid problems because the thyroid gland relies on tyrosine to make the thyroid hormones it sends out to help regulate energy, growth, etc. throughout your body. There are 2 major thyroid hormones which are made from adding iodine to tyrosines & sticking them together: thyroxine (also known as T4 or L-3,5,3′,5′-tetraiodothyronine) & triiodothyronine (T3 or L-3,5,3′-triiodothyronine)
What if you’re done with it? How is Tyr catabolized (broken down)?
Similarly to Phe, it is both GLUCOGENIC (can be used to make glucose (blood sugar) (entering the pathway as fumarate) and KETOGENIC (can be used to make fats & ketone bodies) (entering the pathway as acetoacetate) http://bit.ly/2OROZkQ The breakdown requires multiple steps each with their own specialized enzymes and each with the opportunity to get mutated and cause diseases called tyrosinemias which result in buildup of tyrosine and or the “partway-there” products
Breaking down catecholamines requires catecholamine-O-methyltranserase (COMT) & monoamine oxidase (MAO). If MAO sounds familiar it’s likely because MAO inhibitors can be used as antidepressants
Tyrosine gets its name from the “tyros” which is Greek for cheese. It was discovered by the German chemist Justus von Liebig in 1846 from casein (a protein in milk and cheese). A couple years later, Warren de la Rue found it in an insect, and then Emil Erlenmeyer (of flask fame) and Lipp synthesized it.
The OH also opens up the possibility for tyrosine to get phosphorylated like we saw with threonine. http://bit.ly/2DSvdQ3 So having a Tyr on your protein gives you the chance to have a bulky, negatively-charged phosphate group on your protein, which can change its shape and/or activity.
how does it measure up?
coded for by: UAC, UAU
chemical formula: C9H11NO3
molar mass: 181.191 g·mol−1