Are you familiar with the term blue bloods? It refers to aristocrats coming from privileged, noble families that are wealthy and powerful. But society elites aren’t the only blue bloods. There are squiggly and creepy crawly creatures living among us that literally have blue blood. Does it make them royals of the animal world?
Blood is essential to life. This precious fluid fights infections, delivers nutrients and gases to cells, and ferries away waste products. All animals have developed a method to transport oxygen and nutrients around their bodies, but the method varies among species. In all vertebrates and some invertebrates (e.g. octopuses, squids), the heart pumps blood into arteries and veins causing it to circulate through the body inside the closed circulatory system. Invertebrates like insects, spiders, aquatic arthropods and molluscs have circulatory systems that do not contain blood vessels. In these open circulatory systems, a fluid called hemolymph circulates in the interior of the body in direct contact with the tissues. Compared with closed circulatory systems, open circulatory systems generally work at low pressure and are not well-adapted for rapid response.
Blood vs. Hemolymph
Hemolymph can be considered analogous to blood, but unlike blood, hemolymph does not contain red blood cells and hemoglobin. Hemolymph is composed of fluid plasma mostly made up of water, but it also contains ions, carbohydrates, lipids, glycerol, amino acids, hormones, pigments and suspended cells. Hemolymph directly absorbs nutrients from food and oxygen from the breathing pores or lungs. Instead of using hemoglobin to carry oxygen, organisms with open circulatory systems use other respiratory pigments to carry oxygen through their body.
Is blood always red?
The red colouring of blood comes from the iron-containing protein hemoglobin. Oxygen binds to the iron atom in the bound heme molecule and is transported to cells where it is released. The heme molecule in oxyhemoglobin absorbs all colours of light but reflects red, thereby accounting for the red colour of the blood. But red is not the only colour of blood - animal blood comes in a rainbow of colours. Invertebrates that have blood analogue hemolymph, use metal-binding proteins other than hemoglobin. These contain copper, iron or vanadium resulting in blue, green, violet or yellow colouring, respectively.
The chemistry of different colours of blood. There are four major classes of respiratory pigments: hemoglobin, hemocyanin, erythrocruorin-chlorocruorin, and hemerythrin. Hemoglobin and chlorocruorin are globins containing a heme group where an iron atom is coordinated by a porphyrin. Interestingly, vanadium-binding metalloproteins (vanabins) turn yellow when exposed to oxygen, but despite this they are not involved in oxygen transport and therefore are not classed as respiratory pigments. From left to right, PDB: 1BUW, 1LL1, 4V93, 1I4Y, 1VFI.
Hemocyanin
Hemocyanin is a respiratory pigment found only in some invertebrates - molluscs (e.g. snails, octopuses, squids) and arthropods (e.g. scorpions, spiders, horseshoe crabs, lobsters). It is an extracellular protein, i.e. it is not carried in blood cells, but it floats freely dissolved in the hemolymph. Hemocyanin is a copper-containing protein which gives hemolymph a blue colour when oxygenated, yet is colourless when deoxygenated. The oxygen-binding site is composed of a pair of copper cations (oxidation state +I when deoxygenated, +II upon oxygenation) which are directly coordinated by imidazole rings of 6 histidines.
Deoxy and oxy forms of hemocyanin. When an O2 molecule coordinates to the active site, copper (I) ions are oxidised to copper (II) and the protein changes from colourless to blue. Three histidine residues coordinate each copper ion (His 173, 177, 204 and His 324, 328, 364). PDB: 1LLA (deoxy), 1OXY (oxy).
Three-dimensional structures of molluscan and arthropod hemocyanins are different. The hemocyanin molecules found in the blood of arthropods are built as hexamers or multiples of hexamers, i.e. 6, 12, 24, 36, or 48 subunits depending on the species in which they are found. Each subunit is organized into three domains. Domain I contains five to six α-helices, domain II contains a four α-helix bundle and encompasses the di-copper centre and domain III is a seven stranded antiparallel β-barrel. In contrast, molluscan hemocyanins form hollow cylindrical decamers or multi decamers. Their polypeptide chains are large and each consists of 7 or 8 functional units, each with a di-copper centre. The structure of the functional unit is commonly composed of two domains: the α- or core domain consisting of a four α-helix bundle that houses the di-copper centre and the smaller β-domain incorporating a seven stranded antiparallel β-barrel.
Different structural organisation of molluscan and arthropod hemocyanins. While molluscan hemocyanins appear as a decamer or multiples of decamers in which each subunit consists of two domains, arthropods have much smaller hemocyanin aggregates (hexamers or multiples of hexamers) in which subunits are formed by three domains. Despite these structural differences they have the same function and same active site (centre image: copper atoms in orange surrounded by six conserved histidines). Structures EMDB: EMD-5100, PDB: 1LL1, 1OXY for arthropods, EMDB: EMD-1648, PDB: 1JS8, 3QJO for molluscs. (Image credit: Dev. Comp. Immunol., Volume 45, Issue 1, July 2014, Pages 43-55, https://doi.org/10.1016/j.dci.2014.01.021)
The animal kingdom is hugely diverse. Its members vary in their shape and size, physiology, habitat and behaviour. For any creature larger than a few millimetres, transport of oxygen from the surface to tissue by a simple diffusion is too inefficient. Thus, it is no surprise that larger organism with higher metabolic needs evolved proteins specialised for oxygen transport. But it doesn’t matter whether it’s a tiny bug or a massive whale, they all rely on oxygen to stay alive.
Did you know?
1. Hemocyanin has been identified as one of major allergens of shrimp and other seafood.
2. The open circulatory system is one of several reasons why there are no giant insects. This type of circulatory system is less efficient than closed circulatory system and cannot move oxygen efficiently enough to power large bodies. Interestingly, while octopuses and squids are considered molluscs, they have evolved closed circulatory systems. This is why they are able to reach huge sizes – colossal squid are thought to reach almost 50 feet (≈ 15 m) in length, and weigh up to 1,650 pounds (≈ 750 kg)!
3. The red colour that you see upon squashing a housefly or fruit fly is actually pigment from the animal's eyes.
Romana Gáborová
About the artwork
Daisy Wallman, age 13, The Perse School in Cambridge, UK
Red blood cells contain the protein hemoglobin, which carries oxygen from the lungs to all parts of the body. They also remove carbon dioxide from the body, bringing it to the lungs to exhale. Each human red blood cell contains approximately 270 million hemoglobin molecules, each carrying four heme groups to which oxygen binds. These heme groups contain iron and give blood its red colour. Octopuses and horseshoe crabs have blue blood because the protein transporting oxygen in their blood, hemocyanin, contains copper, instead of iron, making their blood appear blue rather than red. Hemocyanin is much bigger than hemoglobin and can bind 96 oxygen atoms. Unlike hemoglobin, hemocyanin floats freely in blood. In addition, it is a much more stable molecule that can function at temperatures up to 90°C or even in creatures living in cold climates.
(Explore in 3D at PDBe.org/1lla/3d)
View the artwork in the virtual 2021 PDB Art exhibition.
Sources:
- Hemocyanins
- Molluscan hemocyanin: structure, evolution, and physiology
- Diverse immune functions of hemocyanins
- Geometric and electronic structure differences between the type 3 copper sites of the multicopper oxidases and hemocyanin/tyrosine
- Identification of a novel allergen from muscle and various organs in banana shrimp
