The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin.

To have blue-blood is usually taken as an aspiration, or more likely delusion, of aristocratic pedigree. In reality it is a sign of cyanosis because in our blood the haemoglobin will change from its red colour to blue when it de-oxygenates, which explains why your veins have a bluish colour. The exact reverse, however, is found in those animals that employ a copper-based respiratory protein, specifically haemocyanin (or hemocyanin) because this is colourless when de-oxygenated and turns blue when carrying oxygen.

Arthropod and mollusc haemocyanin

Haemocyanin is an excellent example of molecular convergence in respiratory proteins, although the same applies to haemoglobin, myoglobin and haemerythrin, all based on iron rather than copper. One intriguing difference between haemocyanin and the globins is that the latter appear to be genomically conservative, whereas the haemocyanins are much more labile with evidence of extensive intron addition and movement (at least in molluscs). Haemocyanin occurs in both the arthropods and molluscs, and is almost certainly of independent origin although it is possible that ultimately both derive from a very ancient copper protein. So why are they convergent? The crucial similarity is that each dioxygen is held by two atoms of copper that each have a close association with three amino acid ligands, specifically histidines (this is of more than passing interest because the remarkably convergent enzyme carbonic anhydrase uses zinc in association with histidine, and it transpires that this amino acid is repeatedly employed in the active sites of enzymes). The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin.

Haemocyanin protein structures

The reason why the haemocyanins are thought to be unrelated in arthropods and molluscs is because they have radically different protein structures, clearly suggesting independent origins. 

Arthropod haemocyanin

In brief, in the arthropods the basic unit is a six-fold structure, hence a hexamer (each subunit is c.75 kDa), self-assemble into different configurations of up to 6 x 8 units. It is also clear the haemocyanin has evolved extensively within the arthropods, and in certain cases such as the spiders the changes in the haemocyanin have been linked to changes in respiratory efficiency and activity. Haemocyanin is probably primitive to the arthropods, and the fact that very similar molecules (phenoloxidases, see below) are known in the tunicates (deuterostomes) suggests that this protein has a deeper ancestry. Thus they occur in the onychophorans, as well as many crustaceans and chelicerates (e.g. spiders), as well as the myriapods. Interestingly, haemocyanin is only found in some primitive insects, whereas as a general rule they use haemoglobin.

Mollusc haemocyanin

In molluscs, haemocyanin occurs in a radically different protein, basically as an enormous (on a molecular scale) cylinder based on a decamer arrangement. Why is the protein so enormous, with a size of up to 450 kDa? The answer seems to be that to sweep up enough oxygen the concentration of protein would have to become so high that the osmotic stress on the blood would be potentially lethal. The answer, therefore, is to aggregate the protein into large units.

As with the arthropods molluscan haemocyanin protein has clearly evolved within the phylum. For example, in the cephalopods (and chitons) it is simply decameric, whereas in the snails and bivalves it is didecameric. It is also not surprising to learn, given the many striking convergences between octopus (and other cephalopods) and the vertebrates, that the synthesis and breakdown of the haemocyanin is much more tightly controlled than in the other molluscs. In addition, the haemocyanin is evidently capable of highly effective gas exchange, even though by and large haemoglobin is probably the more effective respiratory protein. Indeed, it seems slightly puzzling why cephalopods have not shifted to haemoglobin. First, in other molluscs notably the snails and in the buccal mass associated with the radula of chitons haemoglobin is employed. Similarly, while some primitive insects employ haemocyanin, insects such as flies and bees have (presumably recruited) haemoglobin. Why not the cephalopods?

Co-option of haemocyanins: from immunity to moulting

Haemocyanins are also evolutionarily important because at least in the arthropods they have been co-opted for new functions, notably in the innate immune response and also sclerotization and melanisation of the cuticle. In addition, the cryptocyanins are evidently very important in the moulting cycle of at least some crustaceans, rising to very high levels in the pre-moult stage and having a major role in the building of the new skeleton. Closely related proteins, the hexamerins of insects and the so-called cryptocyanins, have a very similar structure but have lost the copper and can no longer bind oxygen. They are used as storage proteins, and fascinatingly are also employed to regulate the production of castes in the termites, an important group of insects that have evolved eusociality and types of agriculture independently. Such co-option recalls, of course, the example of the crystallins and is a reminder of the versatility of biology at the molecular level.

Closely related to the haemocyanins are the phenoloxidases, and these are critical in defence against fungal and microbial attack. The key role of phenoloxidases is to catalyze monophenols to diphenols, and thereby compounds that are effective in destroying pathogenic fungi and other microbes. Indeed it is possible that primitively this was their function, with co-option occurring as the respiratory needs of the early animals came to the fore.

A very interesting and apparently uninvestigated question is the extent to which the convergences between cephalopod and arthropod haemocyanins extend to physiological similarities. Cephalopods, of course, do not moult so we would not expect cryptocyanin analogues, but what about storage proteins and more especially assistance with immunity? Here a further convergence would be unsurprising. Or is there a constraint in as much as the decamer construction of cephalopod haemocyanin and/or its ancestry forbids its employment for such functions?