Today I will talk about the structure of sugars. What are they made of? What do they look like? And most importantly, how does this affect how they work?
Sugars are made up of three chemical elements: carbon, oxygen and hydrogen. These are put together so that there are twice as many hydrogen atoms as carbon and oxygen.
For example, lots of common sugars are hexoses, which means that they have 6 carbons, 6 oxygens and 12 hydrogens. And that there is one of the really interesting things about sugars. You can make lots of completely different molecules with exactly the same atoms. In fact, it is possible to make 24 different sugars all with the formula C6H12O6!
Below are three common sugars: glucose, galactose and fructose. All three of these sugars have exactly the same chemical formula, C6H12O6. The only difference is some of the atoms are shuffled around a bit. In fact, between glucose and galactose, the only difference is that the H and OH on the fourth carbon are swapped! As we will see in a bit, this tiny change can have a massive impact on how they behave in our body.
In the pictures above, I have drawn the sugars as a long straight chain. However, most of the time sugars exist as rings. They do this by forming a chemical bond between the C=O group and one of the OH groups. I think this YouTube video demonstrates it nicely (thanks to Manojit Majumder for the video!):
So how do sugars decide which OH to react with? Well in chemistry, rings with either 5 or 6 atoms are normally the most stable. This is because these size of rings have the least amount of strain between the atoms. Glucose and galactose like to have rings with 6 atoms. In fructose, we have moved where the C=O group is, so it prefers rings with 5 atoms.
Now let’s redraw these sugars in their ring form. They look quite different now don’t they? In case you haven’t seen this type of diagram before, it’s called the skeletal structure. The carbons aren’t explicitly written, but we know that there is one at each point where no letter is written. Also, the thicker lines mean that part of the ring is pointing towards us.
With these drawings I think it is easier to understand why such small changes in the chemical structure mean that these sugars behave very differently. For example, the only difference between glucose and fructose is that we’ve moved where the C=O group is, but that has forced it to have a completely different 3D shape (and it’s the shape of the molecule that often lets our bodies read them differently). The difference between glucose and galactose also becomes more obvious. Now we can see that the OH on the fourth carbon is pointing in a completely different direction: out to the side in glucose and straight upwards in galactose.
To illustrate how significant this difference is in nature, I will use galectins as an example. Galectins are a type of protein (the machines of our cells) that use galactose as a signal to trigger lots of different cellular processes. It would be really bad if the galectin was accidentally triggered by glucose because then these processes would happen in an uncontrolled way. To do this, shape of galectins has evolved to exactly complement galactose; in particular that OH group that is sticking upwards. In the picture below, you can see the upward OH group forming a special type of bond, called a hydrogen bond (yellow line), with the galectin. As the OH group is pointing the wrong way in glucose, the galectin is unable to recognise it.
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