In lobsters, crabs, shrimps, and other crustacea, oxygen is transported by means of large extracellular proteins termed the hemocyanins.

These respiratory pigments are usually present at high concentrations (g/l) in the plasma and are often almost the only protein present in this fluid. The circulatory system in these organisms is quite unlike that of vertebrates. Instead of pumping blood at high pressure through arteries, veins, and capillaries, as in the vertebrates, the crustaceans employ a low pressure system in which the blood is circulated in the spaces or sinuses around the internal organisms. Hemocyanins are also found in many molluscs (such as the keyhole limpet, the whelks, and conches) and in arachnids (spiders and scorpions):

When deoxygenated, the hemocyanins are almost colorless, but become pale blue upon oxygenation. By contrast with hemoglobins, the hemocyanins are copper-containing proteins and are, in fact, the only copper proteins capable of reversibly binding oxygen. One finds hemocyanin in both land and aquatic (both fresh and salt water) animals. The hemocyanins form a variety of aggregates in different organisms with molecular weights ranging to up to nearly 10 million D. Electron microscopy has provided some striking pictures of the complex architecture of these molecules.

However, in the past few years it has been the techniques of mass spectrophotometry and DNA cloning and sequencing that have provided breakthroughs in our understanding of the basic structure of these proteins. See the references below for some recent papers.

As a side note, it is of interest that hemocyanins have been found to be of great use in immunology, particularly keyhole limpet hemocyanin. These hemocyanins are very immunogenic, and advantage is taken of this in raising antibodies against small proteins and peptides, which are often conjugated to the hemocyanin prior to injection.

The hemerythrins are, despite their name, non-heme iron oxygen-binding pigments found both in cells, generally of the coelomic fluid, and tissues (myohemerythrin) of organisms from four different phyla:

and in a singular annelid worm (Magelona). These respiratory pigments are essentially colorless when deoxygenated but turn a violet-pink in the oxygenated state. The oxidized methhemerythrins are lemon-yellow.

The discovery of the hemerythrins dates back to the 1800s, and crystals were obtained as early as 1933. Oxygen binds to non-covalently coordinated Fe residues that are not contained in porphyrin structures as they are in the hemoglobins and myoglobins. The subunit of hemerythrin has a mass of approximately 13 kD and is folded in a very similar manner in all species (the hemerythrin fold).

Yet the state of aggregation can vary considerably, from monomeric in Thermiste zostericola (a), tetrameric in Phascolopsis gouldii (b), and trimeric in Siphonosoma (c,).

Thermiste

Siphonosoma

Phascalopsis

You can explore the three-dimensional structure of some of these hemerythrins in more detail in Chime by clicking on the buttons below. Be sure to utilize the various coloring and display modes to get a full appreciation for the folding modes of these proteins. In some cases, organisms possess more than one hemerythrin. This is the case, for example, in the sipunculid worm Dendrostomum zostericolum, where different hemerythrins are present in the vascular system and in the coelomic cavity. It has been proposed that the different oxygen affinities of these hemerythrins could provide an oxygen transport system, facilitating diffusion of oxygen from the vascular system into the coelom.

In some species, there are also intracellular hemerythrinshemocyaninshrins) whose function in oxygen binding and storage is surmised to be similar to the functions of myoglobins. From a genetic standpoint, the sporadic presence of hemerythrins of very similar three-dimensional folding patterns in four very distant phyla raises intriguing questions about the phylogenetic heritage of these proteins. Were the genes maintained in functional form in a silent state across hundreds of millions of years of evolutionary history? If so, how was functionality maintained even in the absence of selective pressure? One could also argue that perhaps the organisms that today express hemerythrins might have gained them through horizontal transfer. A very similar situation exists with the invertebrate hemoglobins (see the topic on non-vertebrate hemoglobins).

These green extracellular respiratory pigments are found in the plasma of four families of polychaete worms. Some of these worms have both hemoglobin and chlorocruorin coexisting in their blood. Of the four major groups of oxygen-binding proteins, the chlorocruorins most closely resemble the hemoglobins. Instead of the protoporphyrin IX prosthetic group found in hemoglobins and myoglobins, the chlorocruorins have a similar heme, but with a vinyl group substituted by a formyl group on one of the pyrole rings. It is this prosthetic group that gives the chlorocruorins a distinctive dichroic coloration, being a deep red/green in concentrated solutions and green when more dilute. The oxy and deoxy forms of these pigments have absorption spectra that closely mimic those seen in hemoglobin, but moved toward the red end of the visible spectrum. In overall structure, the chlorocruorins closely resemble the high-molecular-weight extracellular hemoglobins, with molecular weights in the 2-3 million D range. Electron micrographs indicate that the molecules are built up from hexagonal discs with a diameter of about 230 angstroms. Both oxy and deoxy forms appear to have the same structure. As with the hemeocyanins, DNA sequencing and mass spectrophotometric analyses are providing new insight into the structural characteristics of these proteins.

There have been relatively few studies on the functional properties of the chlorocruorins. Even though they have considerably lower oxygen affinities than typical hemoglobins, they appear to serve as oxygen transport pigments, being oxygenated at the gills and deoxygenated at the tissues. These complex respiratory pigments display a high degree of cooperativity in oxygen binding, which is pH-dependent.

An older paper that provides a useful overview of ligand binding to the hemerythrins:
Klotz, I. M., Durtz, D. M. 1984. Binuclear oxygen carriers: Hemerythrin. Acc. Chem. Res., 17, 16.

Smith, J. L. et al. 1983. The structure of the trimeric hemerythrin from Siphonosoma. Nature Vol 303, 86.

Lontie, R., ed. 1984. Copper Proteins and Copper Enzymes. CRC Press: Boca Raton, Fla. Vols I-III.

Preaux, G., Gielens, C. 1984. Hemocyanins. From Lontie, R., ed. 1984. Copper Proteins and Copper Enzymes. CRC Press: Boca Raton, Fla.

Green, B. N. et al. 1988. The quaternary structure of chlorocruorin Biochemistry Vol 37, 6598-6605.

The sipunculids,
the priapulids,
the brachiopods,