Abstract
The erythrocruorins (extracellular haemoglobins from annelids1) have molecular weights of 3–4 ×l06, contain 60–192 O2-binding haem moieties per molecule and are much more complex than the tetrameric vertebrate haemoglobins1–5. However, they perform the same function, carrying O2 from the respiratory surfaces to the tissues, and exhibit similar coopera-tivity in O2 binding and inhibitory heterotropic interactions between O2- and proton-binding sites (Bohr effects), although these functions show greater adaptive variation than in the vertebrate pigments2,3. Whereas erythrocruorin–O2 affinity is insensitive to the anionic organic phosphate cofactors like glycerate-2,3-bisphosphate and ATP1–3, which depress the O2 affinity of vertebrate haemoglobin inside the red blood cells, it is increased by inorganic salts6–8. This effect is important physiologically because annelids lack significant capacity for osmotic regulation and experience large fluctuations in blood electrolyte levels8,9.I show here that, in contrast to the situation in man, where anionic cofactors and protons decrease haemoglobin–O2 affinity by specifically depressing the O2 association equilibrium constant of the pigment in the deoxygenated state10–13, inorganic cations govern the O2 affinity of erythrocruorin from the burrowing, intertidal lugworm, Arenicola marina, by preferentially modifying the association constant of the (almost fully) oxygenated form. In contrast to the vertebrate mechanism, riftich optimizes O2 unloading in the tissues, this alternative control mechanism in erythrocruorin seems to be adaptive to O2 loading at the low O2 tensions generally characteristic of the microenvironments of erythrocruorin-bearing annelids1–3.
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Weber, R. Cationic control of O2 affinity in lugworm erythrocruorin. Nature 292, 386–387 (1981). https://doi.org/10.1038/292386a0
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DOI: https://doi.org/10.1038/292386a0
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