Let’s have a talk *with* sugars! Because, as glycobiologists show us, these molecules have a lot to say – about much more than just “here’s some energy”! Sugars (aka carbohydrates or “carbs”) play lots of roles but you usually only hear about a couple: energy storage is an important role some sugars play (like glycogen in animals and starch in plants), & sugars like cellulose are touted for their structural support roles. In those roles, sugars “act alone” but sugars can also be used to “dress up” other molecules. Basically, cells can cover their surface with (as well as secrete) carefully-made “forests” of “sugar trees” anchored to proteins, lipids, etc. to allow for cellular communication, immune system surveillance, etc. It’s really sweet stuff!
Nucleic acids (DNA & RNA) with their nucleotide “alphabet” and proteins with their amino acid “alphabet” tend to get most of the attention. But did you know there’s also a sort of sugar alphabet? Its letters are monosaccharides like glucose & galactose and they can get attached (conjugated) in a bunch of different ways to a bunch of different things (proteins, lipids, other sugars). These different combos have different names (proteoglycans, glycoproteins, glycolipids, etc.) and today I want to help you make sense of what these sometimes sense-ational molecules are & what they do.
Let’s start with “What the heck is a sugar anyway?” In one of the cases where the jargon-y terms can actually make things less confusing, the technical term for a sugar is a carbohydrate. As the name implies, these are “hydrated carbons” – but it’s not like you can just pour some Gatorade on a diamond and, voila! Instead, there are certain defining characteristics…
The basic formula for an (unmodified) carb is Cx(H₂O)x. so you have an average of 1 water molecule per carbon atom (and you need to have a chain of at least 3 carbons). In order to cram that many (equivalents of) water molecules in, carbs take the form of olyhydroxy aldehydes or polyhydroxy ketones. This mumbojumbo just means that there are lots of (poly) (-OH) (aka hydroxyl) groups as well as a carbonyl (C=O) group. If that (C=O) is at the end of the carbon chain, we call it an aldehyde and if its somewhere in the middle we call it a ketone.
If you’re on the lookout for sugars, your clues are lots of -OH groups (and names that in “-ose”).
The simplest sugars are called monosaccharides. They have 3-9 carbons, but usually 5-7 (we call the 5-C ones pentoses & the 6-C ones hexoses). Since they can have aldehyde or ketone groups, they can be further classified as aldoses & ketoses. So, for example, glucose is an aldohexose (6-carbon aldehyde sugar), fructose is a ketohexose (6-carbon ketone sugar), and ribose is an aldopentose (5-carbon aldehyde sugar). note: A lot of times you can’t actually “see” the carbonyl (C=O) because monosaccharides use it to “ring-ify” (I think this is easiest to explain visually, so check out the pics). During the ringification, this carbonyl turns into a hydroxyl (-OH) & depending on how they ring-ify, it can be sticking up or down from the ring (positions termed α (if it sticks down) & β if it sticks up).
note: The rings can be drawn a few different ways because the atoms in the rings can move a bit to try to get more comfy. They stay connected so the whole structure has to adjust – you can think of it kinda like if you were to squish one of those water bottle cap rings in different ways. A lot of the common monosaccharides hang out in a squashed version called the “chair conformation” which is how I’m drawing them in most of the pics.
Based just on what I’ve told you so far, monosaccharides might seem pretty boring – especially if you compare them to amino acids (protein letters). As a refresher, there are 20 (common) amino acids and they have a wide variety of “side chains” (aka R groups) with different sizes, charges, etc. Compared to those, monosaccharides might seem like pretty lame letters. I mean, they just have a lot of -OHs. But a lot of protein side chains are actually pretty “boring” – in fact, a few of the most exciting are serine, threonine, & tyrosine – which are exciting *because* they have -OHs. an -OH can take you many a place! And instead of just having one, these sugars have many. -OH the many places you can go!
-OHs are so valuable because that H can “easily” be swapped out for something bigger & better. I was careful to add the modifier “unmodified” to the carb formula I showed you above (Cx(H₂O)x) because monosaccharides are commonly modified. For example, they can have oxygens removed (be reduced) to give you “deoxy sugars” (like the deoxyribose in DNA); they can be oxidized to carboxylic acids to give you sugar acids like glucuronic acid (glucoronate) which the liver uses to flag potentially harmful compounds; they can be attached to nitrogen-y groups to give you amino sugars like glucosamine & N-acetylglucosamine; they can be sulfated (have an -SO₃⁻ stuck on) to add some negative charge.
And those modifications can occur on “any” of the multiple OH’s, so there are countless theoretical monosaccharides, but in humans there are 9 main ones that serve as the “glycan alphabet” (glycan is a term used to refer to sugars attached to other things). The mother of them all is D-glucose (Glc). All of the other letters can be made from it through a bit of metabolic magic (metabolism refers to the making (anabolism) & breaking (catabolism) of molecules). Those others are:
- D-galactose (Gal)
- D-mannose (Man)
- N-acetyl-D-glucosamine (GlcNAc)
- N-acetyl-D-galactosamine (GalNAc)
- D-glucuronic acid (GlcU)
- N-acetylneuraminic acid (Neu5Ac) aka NANA aka N-acetylsialic acid (Sia)
- note: derivatives of Neu5Ac are also referred to as sialic acids which can be confusing…
- D-xylose (Xxl)
- L-fucose (Fuc)
note: The “D” & “L” refer to how the sugars’ “last” OH groups are oriented in space relative to the simplest monosaccharide, D-glyceraldehyde. Don’t confuse these D & L with the designations for “dextrorotatory” and “levorotatory,” which are official terms used to describe the way in which certain molecules rotate light. Some “D-sugars” really are dextrorotatory (including D-glucose which is why doctors tend to call it dextrose seemingly to confuse us…). But other D-sugars are levorotatory.
Monosaccharides can link together (through glycosidic bonds) to give you:
- disaccharides: 2 monosaccharides linked together. A couple examples are sucrose, which is a disaccharide made up of glucose and fructose; and lactose, which is a disaccharide made up of glucose and galactose
- oligosaccharides: shortish chains of monosacharides like raffinose, the carb in broccoli that we can’t digest but bacteria in our guts can… and they let of gas as a byproduct…
- polysaccharides: (sometimes massively) long chains of monosaccharides. A couple of examples are storage carbs like glycogen & starch and structural carbs like cellulose & chitin.
Amino acids can “only” link to one another in one way – using their generic backbone part. So you can only get straight chains of amino acids (we call these chains “peptides” – the long ones are polypeptides & they fold up to give you functional proteins). But sugars can link to one another in multiple ways (e.g. 1,4 or 1,2 where the numbers refer to the “address” of the carbon the linkage comes from). This gives you a lot a lot of variety and the opportunity for lots of branching. So you can end up with massive branched sugar trees.
Branching is great for energy storage because there are lots of ends to start chewing from when you need energy. Both plants & animals use highly-branched carbs to store glucose – starch amylopectin stores energy in plants & glycogen stores energy in animals. Glycogen is a highly branched polymer of tens of thousands! of α-1,4 and α-1,6 linked glucose monomers. When cells need energy, they can break down the glycogen in a process called glycogenolysis. Since there are lots of ends, lots of glucoses can be broken off at the same time by glycogen phosphorylase. Similarly, the plant starch amylopectin is also made up of thousands of α-1,4 and α-1,6 linked glucose monomers, but it’s not as massive as glycogen and it only branches every 24-30 glucose residues, instead of 6-12 like glycogen does. So glycogen is bigger & more branched.
But you don’t *always* want a lot of branching because it doesn’t make for very sturdy structures. Imagine you have a pile of glue-covered sticks you want to bundle together to hold up your tent. If the sticks are branched it’s hard to get them close together and you have less points of contact for the glue to go to work. But if your sticks aren’t branched, you can easily stick them together. The biochemical equivalent of this is at play in structural carbohydrates. Instead of glue, you have lots of -OH groups which can form weak partial-charge-based interactions.
The difference can be dramatic – compare cellulose and glycogen. Both are made up of just glucose, but cellulose is unbranched and β-1,4 linked. This linkage allows to to form long straight chains that can stick to gather for sturdiness.
For even better stickiness, many structural carbohydrates have modifications like amidation and oxidation which provide additional binding opportunities. For example, chitin, a structural carb found in fungal cell walls & the exoskeletons of insects & crustaceans (crabs, etc.) has the same linkage as cellulose, but instead of plain glucoses it has N-acetylglucosamines. This makes chitin super strong & stiff.
But “structure” doesn’t have to mean “stiff.” Some sugars play structural roles by forming gels. Those -OH groups love water, and water loves them, so sugars can “soak up” water to form gels. Depending on how much sugar vs. water and what type of modifications the sugar has, these gels can have different properties. In the lab, we use the sugar agarose to make gels we use to separate DNA pieces by size through “electrophoresis” (using charge to get the molecules to travel through the gel). That’s a really easy way to see sugar gels at work, but they’re also at work in our bodies, forming things like the cushioning around the bones in our joints & the bouncy-ball-ness of our eyeballs!
A lot of the time, sugars work together with other molecules in “glycoconjugates” where sugar chains (glycans) get attached to non-sugar things. And this teamwork is accompanied by team names that can be confusing…
The order of the names of these “hybrids” indicates which is dominant – the first one is the minor component and the last one is the main component. So, glycoproteins are proteins with sugar chains attached; proteoglycans are sugars with a sprinkling of protein; peptidoglycans are sugars with some peptides helping link them together; and glycolipids are sugars linked to the lipid molecules making up membranes. They have a lot of diversity and, correspondingly, a lot of diverse functions. Here are a few examples.
This is where you have the protein-ness dominating, with sugars attached. I know I said “minor” but some glycoproteins can actually be “mainly” sugar – ~1/2 of all proteins have at least one glycan & some glycoproteins are up to 60% carb by mass. Glycoproteins are often found in the outer leaflet of the plasma membrane, facing the cellular exterior, or secreted into the extracellular matrix.
The sugar chains can be O-linked or N-linked. O-glycosylation is where a sugar chain gets added through the -OH group of a Serine (Ser) or Threonine (Thr) protein letter (or a hydroxylated lysine or proline). N-glycosylation is where a sugar chain gets added through the amine group of an Asparagine (Asp) letter.
In addition to needing to use different enzymes, O- & N-glycosylation differ in several important ways. In O-glycosylation, individual sugars are added one at a time, but for N-glycosylation, the same “starter tree” gets added in the beginning of the protein processing process at spots on the protein containing the consensus recognition sequence Asn-X-Ser/Thr, where X is any amino acid other than proline (proline’s that awkward-shaped amino acid whose side chain swings back around and attaches to the backbone making things tricky). Throughout the protein processing steps, this tree gets whittled down to a common core, which can then be added onto.
This whittling might sound kinda pointless and energy-wasting – I mean, why add something just to remove it, right? But it serves as an important quality control measure and makes sure that the protein gets directed to were it needs to go in the right order, etc. Unlike proteins, sugars are *not* genetically encoded – only the enzymes responsible for making, placing, and processing the sugars are. So our cells rely on the selective expression of these, their compartmentalization to different membrane-bound protein-processing centers inside of cells (e.g. Golgi bodies), etc. to make sure that the “right” sugars get added in the right places.
For example, proteins bound for secretion have a signal sequence that directs them to a membrane-bound room within the cell called the Endoplasmic Reticulum (ER) while it’s being made. From there it gets passed through a variety of compartmentalized pouches of the “Golgi body” and those compartments have different modifying enzymes that can trim & modify it. Since all of this is happening inside of membrane-bound rooms, the protein never has to really see the inside of the cell, so it can be optimized for the environment it’s gonna face when it gets released.
If a protein isn’t folded properly, it won’t get whittled correctly so it won’t get permission to go to the next compartment and, if it can’t get its act together, it will be directed to the protein shredder (proteasome) instead. But if the whittling goes well, it can get modified and then those modifications can direct it different places to get modified in more ways and/or get released outside the cell or onto the cell surface.
Depending on which “versions” of the modifying enzymes a person has, they can make slightly different sugars. This is the case with the ABO blood groups, which refer to the sugar groups displayed on the surface of blood cells. There’s this glycosyltransferase enzyme (sugar adder/mover) made by the ABO gene. People with type “A” blood have (2 copies of) one version of it that adds on an N-Acetylglucosamine. People with type “B” blood have (2 copies of) a different version of it that adds on a galactose. People with type “AB” blood have 1 copy of each of those so they make and display both. And people with type “O” blood have a total dud version of the enzyme that can’t add on any sugar, so they only display the core pentasaccharide (5 sugar) part (H antigen).
The reason this matters (and how it was discovered) is because these sugars can serve as antigens (things that antibodies recognize). People who have type O blood won’t be used to seeing A & B antibodies, so if you give them blood with A and/or B antigens, they’ll see that blood as foreign and attack it. Similarly if you give type A people B and/or AB or if you give type B people A and/or AB. This is why we say AB is “universal recipient” and O is “universal donor.” note: Rh factor is the (+/-) thing and it involves a different protein. also note that the ABO sugars can be attached to lipids to anchor them to the cell surface as we’ll discuss, or they can be attached to proteins and secreted, but not all people have the secreted version which is why CSI shows sometimes talk about secreters vs. non-secreters.
Here, we flip around the starring roles – with proteoglycans, the sugars get the starring role and the protein just serves to kinda hold them all together. These sugars are GlycosAminoGlycans – large polysaccharides with a lot of modifications. These modifications include lots of amino sugars (hence the A in GAG) but they also often include things like oxidation to carboxylate or the addition of sulfate groups. GAGs can have different patterns of modifications that serve as a sort of code that is “read” by proteins in the extracellular matrix (ECM) (the molecularly-rich environment in between cells). How it works is that the different modifications make for different binding sites that complement different proteins’ structures. So then those proteins can bind, which can be useful in several ways. Sometimes, binding may get the proteins to shape-shift (undergo a conformational change) that activates or inactivates it. Other times, binding might serve to just keep the protein there. And, since GAGs can be really long, they can bind multiple proteins and serve as “hubs” or “scaffolds” at which that multi-protein reactions can take place.
In addition to those more specialized functions, proteoglycans have “generic” ones. Modifications like carboxylation & sulfation add negative charge. And when there’s negative charge, you know metal cations (positively-charged molecules) are gonna wanna come play! Sodium ions (Na⁺) drop in to serve as counter ions, and they bring water with them. So these proteoglycans soak up a bunch of water, gel-like. Different ones (which include heparan sulfate, chondroitin sulfate & keratin sulfate) have different gel-ness-es & thus are best suited for different things – from tense tendons & strong nails, to the shock absorbers between your joints, & the mucus in your nose! Yup, “mucus” is a proteoglycan consisting of a mucin protein covered with sugars! There are actually dozens of mucin proteins and the proteins involved in mucin & in all the other proteoglycans vary a lot – which they can do since it’s the sugars starring here!
Here, sugars get an even bigger role. Peptides play supporting roles (literally) – short chains of amino acids serve as bridges between carbohydrate chains for extra sturdiness. These are seen in bacterial cell walls, and many antibiotics target them. More on that here: http://bit.ly/penamp
We’ve talked about how a lot of the sugars coating our cells are anchored onto glycoproteins embedded in the plasma membrane (the phospholipid lipid bilayer surrounding each cell). Another way our sugar trees can get anchored to the cell surface is by attaching directly to the lipids themselves, and these are what we call glycolipids. There are several different subclasses of them with different names (of course…) These include lipopolysaccharides, glycosylphosphatidylinositol (GPI), cerebrosides, & gangliosides.
Lipopolysaccharide (LPS) is a glycolipid present in bacteria’s outer membrane, with different bacterial species having different sugar combos in their LPS. These sugars provide Gram-negative bacteria (ones w/o a strong cell wall) with a barrier that makes it hard for would-be attackers to get to the membrane itself (and potentially into the bacteria). So it serves to help protect the bacterial membrane from things like antimicrobial molecules and viruses. But not humans – our immune system recognizes LPS and attacks it. May sound great, but too much LPS can cause septic shock (a sort of immune system overreaction when bacteria are in your blood). You might have heard of LPS by another name – endotoxin – just like that nickname suggests, LPS is toxic to our bodies and is responsible for some of those symptoms of bacterial infections people experience – this includes fever (LPS is a fever-inducer aka pyrogen). To prevent over-reaction we have enzymes that can break it down
Glycosylphosphatidylinositol (GPI) is a short sugar chain that serves to anchor non-membrane-spanning proteins to the outside of the cell. One end of the sugar links to a phospholipid head and the other end hooks up (through a phosphoethanolamine linker) to a protein. So, with a GPI-anchored protein you have lipids, carbs, & proteins all working together! Molecular teamwork is so cool! Our bodies use a bunch of these proteins to do a bunch of different things including helping cells recognize each other and acting in signal pathways.
Cerebrosides & gangliosides are glycolipids that I’m not sure what do but there’s a bunch of them in nervous tissues & if there are problems making or breaking them you get issues.
Wrapping up our sweet science…
One thing I learned (or relearned since I probably did learn it in undergrad but then forgot) is that glycogen (that’s the animal storage carb remember) is attached to a central protein called glycogenin, which actually is a glucosyltransferase that does the pioneering work of adding on the first glucoses (before passing the job over to glycogen synthase & a branching enzyme).
Having glycogenin as a central hub allows glycogen to form granules that can chill in the cytoplasm (general cellular interior) until called upon. The main glycogen storage locations are the liver and the muscles. When blood sugar levels drop, glycogenolysis starts in the liver and the glucoses newly freed travel through the bloodstream to tissues in need! (your brain & red blood cells are a couple of the main reliers on this process).
I had always thought of glycogen as just tangles of sugar, and apparently most scientists had too. Because when a scientist named Bill Whelan discovered glycogenin people didn’t believe him but he used his biochemistry brilliance to prove ‘em wrong!
Whelan served for a time as president of the International Union of Biochemistry and Molecular Biology (IUBMB) which is a great note to end on because this has been one of my weekly “broadcasts from the bench” for them Be sure to follow the IUBMB if you’re interested in biochemistry (now on Instagram @the_iubmb)! They’re a really great international organization for biochemistry.⠀
This week I’d also like to thank Professor John Tansey, a chemistry, the Director of Biochemistry and Molecular Biology at Otterbein University in Ohio. I first met him at an ASBMB conference where he caught my attention because he was there with undergrads and he was so excited for and supportive of them. And this warmed my heart so much because a lot of profs at big institutions sometimes overlook undergrads and undergrad education is hugely under-attention-paid-to at many places. This is one reason I really want to become an undergrad professor at a small school and why I love being able to help teach undergrads (and everyone else following along) through this social media/blog stuff. Anyways, Dr. Tansey kept in touch on Twitter & when he wrote a biochemistry textbook he sent me an advanced copy (no strings attached). And it’s really good – and its where I learned a lot of what I told you today (glycobiology wasn’t covered that much in my undergrad classes). The book is published by Wiley & called “Biochemistry: an Integrative Approach” – it isn’t out yet but keep an eye out for it! (and keep an eye out for some short & sweet sugar science stories I learned about in it).
more on all sorts of things: #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0