Sticking Sugars Together

In my last blog I talked about the structure of some simple sugars like glucose and fructose. These types of sugar are the most simple ones that we will find. They are used as the building blocks for much more complicated sugars. Normally we would call the building block monosaccharides (meaning one sugar) and the more complicated sugars are called polysaccharides (meaning lots of sugars). Sucrose, which we met in my first post, could be considered a polysaccharide because it is made of two building blocks glucose and fructose. However, we would normally call this a disaccharide (meaning two sugars), since two isn’t exactly a lot.

In nature the monosaccharide building blocks that we have talked about so far are mostly used for energy and building polysaccharides. It is these polysaccharides that are really exciting, since it is these that are mostly used by our bodies for more important roles. For a brief summary of some of these roles, please see this post.

So today I want to talk first about what these polysaccharides look like and then how the cells in our body make them.

Building a polysaccharide

We have seen that one of the features of our monosaccharide building blocks are the OH groups that are attached to every carbon. These can basically be used to ‘clip’ monosaccharides together. This means that with just two hexoses (that’s a sugar with 6 carbons – I explained what sugars look like here) we can make 6 unique disaccharides. It’s actually possible to make 36 unique combinations but the first sugar will almost always clip with the OH on the first carbon. I hope you can see that when we are dealing with polysaccharides containing 10 or more building blocks, the total number of possibilities will be crazily huge!

Below are some structures of so-called N-glycans (credit Essentials of Glycobiology. 2nd edition. Chapter 8.) These are polysaccharides that are attached to nitrogen atoms in proteins (the machines that make and process everything in our body). Here we have switched to a cartoon to make them easier to see. The blue squares are glucose, green circles are mannose (similar to glucose but with the second OH group sticking up), yellow circles are galactose, then the diamond and triangle are more exotic sugars called sialic acid and fucose. The numbers are which OH group they are attached to.

glycan sugar polysaccharide complex mannose hybrid

Some common polysaccharides

These types of sugar are normally used to tune the protein, making it run as efficiently as possible. They can also be used in cell communication, like I have mentioned before. Also, people who aren’t able to make these sugars properly suffer from very serious diseases, since their bodies are able to run like they are supposed to.

The polysaccharide production line

If you just put a load of monosaccharides together in some water and waited, you would have to wait a very long time before you got any polysaccharides. This is because the OH ‘clips’ that we use to join the sugars together aren’t actually very reactive. Also, the sugars themselves don’t know which OH to attach to (remember there are 6 per monosaccharide in hexoses), so you would end up with a random mixture of polysaccharides.

For life to happen, reactions like this need to be quick and they need to produce exactly what the body needs. Our bodies have evolved proteins that do these reactions very efficiently. They must also only join the sugars together in the way that they are supposed to. These proteins are called glycosyltransferases (meaning they transfer sugars).

Most glycosyltransferases live inside two compartments inside the cell called the ER (endoplasmic reticulum) and the Golgi apparatus. This kind of acts as a production line, with each glycosyltransferase adding a particular sugar to the polysaccharide and then passing it to the next glycosyltransferase. Our cells can choose which glycosyltransferase to have on the production line, and this changes what type of polysaccharide it can make.

protein sugar synthesis glycosyltransferase glycan pathway er golgi

The polysaccharide production line. Credit: 

How glycosyltransferases work

So how are glycosyltransferases able to put sugars together so quickly and in the right order?

Firstly they use a bit a cheat to get the sugars to react. They normally label them with a tag, called an activating group, which make them more reactive. Secondly they hold the sugars together in exactly the right place to make them react. This is much more efficient than just having the sugars in water, since in water they will only react if they accidentally bump into each other in exactly the right way.

Then we have the issue of how they only do the correct reaction. Basically the glycosyltransferase is designed so that only the correct sugar can fit. In science this is often called the lock and key mechanism and I briefly introduced it last time with the galectin example. Only the right sugar reacts because only the right sugar fits.

Then the reaction is quite straight forward. The two OH groups come together to form a new sugar-sugar bond, and one water molecule is made in the process.

Sugar1-OH + Sugar2-OH => Sugar1-O-Sugar2 + H2O

So that’s it. A quick crash course in making polysaccharides. I hope that you enjoyed this blog post. If you did, please be sure to share it on social media so that more people can find out about this fascinating science. Please feel free to leave comments below and if there are any topics that you like covered, please don’t hesitate to contact me.

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