“Trick or treat?”- Time for Halloween! This month we’re looking at all things creepy crawlies and gorey with cobwebs and blood. Let’s look at haemoglobin and the protein in spider silk!
It’s the end of October and millions of children (and adults) are celebrating Halloween. The rules are simple. You can either provide a treat: chocolate, sweets, candy or even fruit (!?), or you can receive a trick. Speaking of tricks… What if I told you that one of the strongest materials is the width of a human hair? If someone also said that material was produced by a small creature, you would definitely think it was a trick. Surprise – this is no trick! It is very much real. What is this amazing material and what is producing it? The answer is one of Halloween’s favourite creepy crawlies… Spiders, and the protein they make to create a web as strong as steel with their silk.
Now for a treat
How about a treat to make up for our earlier trick? Well, in this Halloween edition, we are going to be looking at not one, but two molecules. Apart from spiders, another common feature of Halloween and horror is blood. For most animals, including us, blood is a thick, red fluid that is constantly being pumped around our bodies. Blood is wonderfully fascinating, an incredible transport system, delivering and receiving all types of vital packages around the body. In every red blood cell currently zooming around our bodies is a protein vital for transporting oxygen. The iron containing protein, haemoglobin.
Prepare yourself for both a trick and a treat as we take a look at spider silk and haemoglobin.
Proteins are like Lego (other building blocks are available)
The two molecules we’ll look at this month are both proteins. They’re more like entire factories than a single molecule – often with lots of different units all coming together.
Proteins are macromolecules made up from a set of building blocks known as amino acids. Proteins are made by combining amino acids in different orders to create unique chains. Each protein performs its own functions. The instructions for making proteins from amino acids are written in your DNA. A bit like a biological instruction booklet for Lego. If you follow one set of instructions you might make a car, but follow another and get a castle. The components for each of these are the same (e.g. Lego blocks), but the order in which they are put together is what makes the thing what it is.
At Halloween we see lots of decorations. I bet you’re seeing a lot of cobwebs at the moment (or someone needs to tidy your house). What molecules make this spooky spidery silky substance?
Inside every spider is an incredible organ called a spinneret. Some spiders have up to 8 of these things! The spinneret is used to combine tiny molecules together, stringing out a chain of individual components to make a long thread with delicately balanced properties. This is where the spider uses protein to create silk. You can see a spinneret in action here.
If you were going fishing, you wouldn’t want to have a heavy iron chain hanging off the end of your rod, would you? You’d need a delicate, thin wire – such as a fishing line. When a spider is searching for food (and the protein in that food), they make this delicate, thin, light but incredibly strong silk to build their webs. These threads are 1,000 times thinner than a single human hair.
Spiders don’t just use their proteinaceous silk for building webs! Some make tiny ‘handheld’ nets that they can throw over prey to ensnare them. Some make tiny booby traps that as soon as prey steps on it, catapults them up into the air! Others have made silk that is nearly invisible to insects… All the different properties spiders need to make these different silks come from changing around the building blocks – the amino acids – and the order they are placed into the protein chain that becomes spider silk.
Choose your weapon
What about if you wanted to fly? Well humans are pretty bad at that and we need to build gigantic metal structures to be able to do it! Spiders are so much better than us at engineering, and once again they turn to their spinnerets to make another delicate, super long chain of silk that they cast off into the sky. This protein-packed spider silk thread is so light that it can be pulled around in the air, but is strong enough to pull the spider with it! It’s like they have made their own kite out of this silk. That’s why it’s often called Spider Kiting! However, tying silk around a spider and expecting it to work as a kite is unlikely (and we highly advise against trying).
Aren’t you glad spiders can’t fly? Oh wait…
Speaking of planes… this spider silk is actually so strong that a strand as thick as a pencil could actually stop a plane! It’s five times stronger than steel (if your piece of steel was also a fraction the width of a human hair…) and almost as strong as kevlar. In fact… one type of spider has a web that was measured as actually being stronger than kevlar! That’s right, stronger than a material made to be bulletproof!  It’s even been tested on Mythbusters Jr!
It’s all about the order
The strength and properties of the silk are tuned by changing the ratios of the chemicals the silk is built from. Remember we said that proteins are made by chaining together lots of smaller units? Well, this really is like building something out of building blocks! If you have lots of shapes that go together nicely and build a strong foundation – you can make some strong structures out of it!
In the case of spider silk and the webs they make from that silk, the three main types of building blocks are proline, alanine and glycine – and these combine to make up the giant protein molecules. So what are the recipes available for spider silk?
Tinker, Tailor, Spider, Spy..
Adding more glycine (the blue block above) to the spider protein makes the silk stretchier and more elastic. Adding alanine (the yellow block) makes the silk stronger. Keeping the water content in the web is also important for strength. There are some amazing videos here.
So if changing around these three building blocks – alanine, proline and glycine – can give us all these different properties of spider silk, imagine how complicated and intricate the things you could make with 20 different blocks would be! 
Now let’s have a look at one of the most incredible proteins there is. It’s so great, you’re literally only alive because of it. Every time you take a deep breath, this molecule picks oxygen up and then flies around your body, delivering that precious oxygen to every single cell in your body. Cool, huh?
That molecule is haemoglobin. To see how important it is, let’s have a little look at the red goop that you try so hard to keep inside your body – blood.
Blood glorious blood
Believe it or not, blood makes up around 7% of your total body weight – that means that it is super important! Blood carries out a lot of different functions, such as carrying nutrients around the body, and it is one of these functions that gives it its characteristic bright red colour: transporting oxygen.
Blood is made up of lots of different types of cells but we are going to focus on one type – red blood cells. These specialised cells are designed for one specific function which is to pick up oxygen from the air that you breathe and deliver it all around your body to make energy.
Red blood cells need some extra special help to carry oxygen so they use a molecule called haemoglobin, which also happens to be the reason that your blood is bright red. You wouldn’t survive without haemoglobin and just to prove how important it is, let’s talk numbers…
The average adult has around
5 litres of blood
in their body. Each millilitre of that blood contains around
5,000,000,000 red blood cells.
Each of those red blood cells contains around
280,000,000 molecules of haemoglobin..
This means that an average adult’s body contains around
molecules of haemoglobin.
*(That’s 7 sextillion. That’s a lot.)
Haemoglobin – your tiny best friend
Haemoglobin’s design makes it really good at its job. It’s basically the same shape as a teeny tiny doughnut but is made up of four protein molecules instead of dough and icing and is a brilliant example of how the shape of a molecule helps it to perform its job to the best of its ability.
Grand designs and gassy times
Each of these four proteins is a special shape known as an alpha helix, which is a bit like a corkscrew, and each of these proteins contains a single atom of iron which can really easily pick up oxygen and let it go again when it needs to be delivered to somewhere in your body. It is this ability of the iron atom to flip flop between holding and letting go of oxygen that makes haemoglobin so good at its job.
To make matters even more impressive, each molecule of haemoglobin can carry four oxygen molecules at once! Remember that crazy big number we calculated earlier? Now multiply that by four. That’s a lot! In fact, haemoglobin is so good at binding to oxygen that it allows your blood to transport 27 times more of it around your body than if it was left to float around on its own.
Too much of a good thing
Unfortunately, haemoglobin’s ability to bind to gases doesn’t always work in our favour as sometimes things come along that bind more tightly than oxygen, such as carbon monoxide. The iron atoms struggle to let go of carbon monoxide because the bonds are so strong, and this is the cause of carbon monoxide poisoning. Fortunately for us, our body creates a whole new set of red blood cells roughly every 21 days so it’s not all doom and gloom! On a side note, this is exactly why it is so important to check your carbon monoxide alarm in your home – do it now!
Red vs blue: Ironing out the truth
So back to the “blood red” business – it is the iron in the haemoglobin molecules that gives blood this characteristic colour. Blood that isn’t currently carrying oxygen is a really dark red colour but turns much brighter cherry colour when oxygen binds to the iron atoms. You might have heard that deoxygenated blood is actually blue, but this is a bit of an optical illusion I’m afraid – if you really want to see some blue blood you need to find a squid! Instead of haemoglobin, their oxygen-carrying molecules are called haemocyanin as they contain copper rather than iron and this makes their blood bright blue !
How was that for a treat?
Seeing is believing, and whether you believe in ghosts and the supernatural you cannot deny the existence of spiders, spider webs or the fact that blood is red. However, what you may not know is just how amazing spider silk is, and how this seemingly frail material that you dust out of the corner of rooms is an incredible feat of engineering. For many of us, blood is simply something you lose when we cut or graze our skin. However, the building blocks of blood is equally awesome, and deep within those red blood cells is a tiny mode of transport, picking up oxygen and dropping if off all around the body. Both of these amazing things owe their existence to proteins, which themselves are build from 20 individual blocks.
We hope that you enjoy celebrating halloween (safely, of course) and remember to think about protein next time you see spider webs or blood. Trick or treat!?
 What is the strongest material? It actually isn’t as simple as you may think to work that out! There are different types of ‘strong’. Some materials can stretch really far before they break. Some materials resist being stretched and stay exactly as they are until they shatter. Other materials can’t be squashed. The problem with a lot of this is that to find out how strong a material is… you have to keep stretching or squeezing it until it breaks! You can only test these materials once. If you want to find out more about these different kinds of strength you can read more here. BACK TO POST
 The 20 different building blocks we mention here are the ‘essential amino acids’ and they make up every protein in every living thing! Some of them can be made inside our bodies while others are only found in our food. You can read more about amino acids here. BACK TO POST
Alex is a molecular biologist/animal scientist.
Marcus has a Ph.D. in Theoretical Chemistry from the University of Birmingham studying the photo-chemistry of aromatic molecules and the green fluorescent protein.
Phil has a Ph.D. in chemistry but now works in an engineering department as a full-time tinkerer – taking STEM out of the lab or workshop and into classrooms.