The incorporation of copper atoms instead of iron, leads to the blood being colourless when deoxygenated, and blue when oxygenated. They also bind to oxygen in a different manner to haemoglobin, with two copper atoms binding to each oxygen molecule.
It doesn’t stop there; green blood, too, is possible, in some species of worms and leeches. This is an interesting one, in that the individual units of chlorocruorin, the protein leading to a green blood colouration, are actually very similar in appearance to haemoglobin. In fact, they’re near identical – the only different is an aldehyde group in the place of a vinyl group in the chemical structure (although the name might suggest otherwise, chlorocruorin doesn’t contain any chlorine atoms).
Despite this minor difference, a noticeable colour change is the result – deoxygenated blood containing chlorocruorin is a light green colour, and a slightly darker green when oxygenated. Oddly, in concentrated solutions, it takes on a light red colour. A number of organisms that have chlorocruorin in their blood also have haemoglobin present as well, resulting in an overall red colouration.
Chlorocruorin isn’t always necessary for green blood, however, as the green-blooded skink lizard illustrates. This lizard is found in New Guinea, and despite its blood containing haemoglobin like other vertebrates, its blood is a distinctive green colour. The colour is due to a difference in how they recycle haemoglobin. Humans recycle haemoglobin in the liver, by breaking it down first into biliverdin, and then bilirubin. The lizards, however, aren’t capable of breaking down biliverdin any further, so it accumulates in their blood, giving a green colour intense enough to overpower the red colour of haemoglobin.
Finally, violet blood is also possible, albeit in a limited range of marine worms (including the rather unfortunately named penis worms). This colour is caused by yet another different respiratory pigment, this time one called haemorythrin. Haemorythrin contains individual units which themselves contain iron atoms; when deoxygenated, the blood is colourless, but when oxygenated it is a bright violet-pink. Like most of the other respiratory pigments, it’s a lot less efficient than haemoglobin, in some cases only having around a quarter of the oxygen carrying capacity.
What’s perhaps most interesting about the varying colours of blood is that it showcases evolution coming up with different solutions to the same problem – in this case, transporting oxygen. It’s funny to think that, if our blood incorporated copper-containing respiration pigments instead of iron, we might all be slapping on a different colour fake blood for Halloween instead of red!
Comments