What do you get when you give a hydrocarbon alcohol? A carb! I hope I don’t carboload you with nerdy jokes, but it’d be sweet if we could talk about carbohydrates! An alcohol just refers to a hydroxyl (-OH) group where O stands for oxygen & H for hydrogen. Carbohydrates are like really alcoholic lipids in that they both have a carbon backbone, but instead of just being joined to H, the carbon in a carb’s backbone is linked to hydroxyl (-OH) groups which makes it want to hang out with different m
A common theme you see in biochemistry is the idea of “building blocks” – all molecules are made up of atoms, but if you had to make complex molecules quickly on demand “from scratch” it would take forever. So, like a fast food chain that has premade components it can mix & match, your cells stock up on premade parts that can be stuck together to make the intricate macromolecules that your cells need. Different types of biomolecules have different types of parts that can interconnect. For proteins, the parts are amino acids, for nucleic acids (DNA & RNA) they’re nucleotides, for lipids they’re fatty acids & for carbohydrates, they’re sugars.
Lipids (e.g. fats, oils, waxes) which we looked at last week are mostly just carbon (C) and hydrogen, often in long hydrocarbon chains. Carbohydrates are also built off of a hydrocarbon skeleton, but about half of the Hs are replaced by OHs. more here: http://bit.ly/2Yx1Lb0
Just like people might like to hang out with different people when they’re drunk, carbon likes to hang out with different molecules when it’s attached to an alcohol group vs just an H. So, while lipids are hydrophobic (water-avoiding) and thus good for making barriers in watery solutions (e.g. forming cellular membranes that keep inside cell stuff separate from outside cell stuff) carbohydrates are hydrophilic and happy to dissolve in water (when something dissolves in water it just means it gets a full watery coat).
Why the difference? Molecules are made up of atoms which are made up of subatomic particles – positive protons hang out with neutral neutrons in a dense, central nucleus. And negatively-charged electrons whizz around them. The outermost electrons (valence electrons) are furthest from the protons’ positive pull, so they can “roam” a bit & interact with the electrons of other atoms.
You can think of it kinda like a dog walker (nucleus) walking a bunch of dogs (electrons). Valence electrons are like the most energetic dogs and when they go sniff other dogs, dog walkers can end up “sharing custody” of that pair of dogs, forming a new bond. The number of bonds an atom can form depends on how many electrons it can hold & how many it already has. Each single bond represents 2 shared electrons and double bonds 4. “Extra” unshared electrons can exist as longe pairs.
C can form 4 single bonds & O can form 3 (it likes to keep at least 1 spare lone pair). but H can only form 1. So sticking an H on something is kinda like a “dead end” – or at least it would be if it weren’t so easily removed. Because H can come off relatively easily, it’s more like a cap than a true dead end. It just hides the more reactive part and, in an alcohol, the more reactive part is the oxygen.
Oxygen hogs the electrons it shares with hydrogen, so going back to the dog-walking analogy, it’s like oxygen has dog whisperer powers it uses to convince the dog hydrogen’s walking to switch loyalty. Sometimes, the H can “give up” and leave as a proton (H+) (dogless walker) leaving the electron behind but taking the positive charge of the proton. So the oxygen’s “bitten off more than it can chew” and is now negative, so it seeks out something positive or partly positive to help neutralize it. And, when it does so, it can form new bonds and thus build new things.
We call such positive charge seekers nucleophiles because nuclei are where positive charge lives in the form of protons. Nucleophiles seek out & befriend electrophiles.
If an oxygen finds a new friend in a separate molecule, you can combine molecules. Sugars can join together hydroxyl to hydroxyl through glycosidic bonds. So you can go from single (mono) sugar pieces -MONOSACCHARIDES to 2 monosaccharides joining together to form DISACCHARIDES. Add a few more and you get OLIGOSACCHARIDES. Add a lot more and you get POLYSACCHARIDES. And you can even stick sugars onto other molecules like lipids (to get glycolipids) & proteins (to get glycoproteins).
Oxygen can also find new friends “in itself,” allowing sugars to go back and forth (interconvert) between linear, open-chain, forms & circularized forms. To do this it needs something partly positive at its “other end” to attack. And this is where carbonyls come in. A carbonyl is a C double-bonded to an O. And, just like O hogs electrons from H, it also hogs them from C, so this C is partly positive and thus electrophilic. We call this reactive C the anomeric carbon.
In aldoses, the carbonyl is part of an aldehyde – the carbonyl’s at the “very end” with just an H attached afterwards (-(C=O)-H). In ketoses, the carbonyl is part of a ketone – instead of being at the very end, there are other carbons both before & after it.
Monosaccharides can have different numbers of carbons and we have different names to describe how many carbons they have.
3 carbons – triose (e.g. glyceraldehyde, dihydroxyacetone)
4 carbons – tetrose (e.g. erythrose)
5 carbons – pentose (e.g. ribose, ribulose, xylulose) (yep – We’ve actually already seen carbs show up – in nucleic acids, which have sugars – ribose in RNA & deoxyribose in DNA)
6 carbons – hexose (e.g. glucose, galactose, mannose, fructose)
Depending on how many carbons there are and where the carbonyl’s located, you get rings with different numbers of sides. 5-membered rings are called furanoses because they’re sugars (-oses) that look like a molecule called furan. And 6-membered rings are called pyranoses because they’re sugars that look like pyran.
For example glucose & fructose are both monosaccharides with 6 carbons (hexoses) but glucose is an aldohexose – it’s a sugar (-ose) that has an aldehyde (aldo-) & 6 carbons (-hex-). Whereas fructose is a ketohexose (it has a ketone). Because fructose is a ketose & not an aldose, it can’t ring up to the very end, just to the almost-end, so you can only get a 5-sided ring (furanose) even though you have 6 C’s – the last one’s gonna have to stick off. So glucose forms pyranose rings & fructose forms furanose rings.
Depending on where additional alcohol groups are located and from which side the O attacks you’ll get different numbers and locations of -OH “legs” sticking up or down from the ring. If you’ve seen sugars labeled “L” or “D” these are just ways of telling you the “stereochemistry” – what way the legs stick out in 3D. You might also see the terminology α & β – this refers to whether the leg off the anomeric carbon sticks up or down compare to the direction the “front end” is sticking.
Monosaccharides like to hang out as disaccharides. And a few common examples are:
- maltose (aka malt sugar) is 2 glucoses linked together
- lactose (aka milk sugar) is galactose + glucose
- sucrose (aka table sugar) is glucose + fructose
Oligosaccharides (3-10 monosaccharides) are good for sticking onto other molecules like lipids and proteins to serve as kinda “flags” for signaling & offering up new bonding opportunities.
Polysaccharides can have hundreds or even thousands of monosaccharides in straight or branched chains. Some of these are used for structural function, like the cellulose that gives plant walls strength while others, like glycogen & starch are used more for energy storage (animals use glycogen, plants starch). The process of getting energy from the food you eat involves breaking down the bigger pieces & adding oxygen onto the molecules. You get more energy per gram than carbs because carbs already have some oxygen added.
Starch has 2 forms – amylose, which is unbranched, & amylopectin which has branches. Glycogen (the “animal version”) is similar to amylopectin, but it’s even more branched. We have a lot of it in our liver & muscles for a quick energy source.
Cellulose is an unbranched chain of glucose – like amylose – but the glucoses are linked up differently so instead of being coily, it’s rigid and good for keeping plant cell walls stiff.
This post is part of my weekly “broadcasts from the bench” for The International Union of Biochemistry and Molecular Biology (@theIUBMB). Be sure to follow the IUBMB if you’re interested in biochemistry! They’re a really great international organization for biochemistry.