The haemoglobin enzyme (original) (raw)

Physiology

Nature volume 401, pages 437–439 (1999)Cite this article

Haemoglobin is well known for its function in the vascular system of animals, transporting oxygen from the lungs or gills to peripheral tissues. It also aids, both directly and indirectly, the transport of carbon dioxide and regulates the pH of blood. Three years ago, a fourth function for mammalian haemoglobin was proposed1 by J. S. Stamler and his collaborators — they suggested that the allosteric mechanism of haemoglobin is involved in the nitric oxide (NO)-mediated regulation of blood flow. Haemoglobin is a model allosteric protein2, yet it has no catalytic activity, leading the late Jeffries Wyman (a prominent protein scientist) to dub it an “honorary enzyme”2,3. Now, a report by Minning et al.4 on page 497 of this issue effectively removes that honorific by showing that haemoglobin from a parasitic nematode, Ascaris suum, is a true enzyme, serving as an NO-activated deoxygenase.

By its original definition, the term haemoglobin is used for the proteins contained in blood, haemolymph or coelomic fluid. However, proteins in other, non-circulating fluids or in single-celled organisms may also have this name. Haemoglobin is widely distributed among prokaryotes, unicellular eukaryotes, plants and animals. It comprises a polypeptide chain called globin, associated with protohaem IX — a planar complex of iron and protoporphyrin IX. These structural units form tetramers in vertebrate red blood cells, but in invertebrates the combinations are more diverse, ranging from the monomeric haemoglobin of protozoa to the gigantic annelid extracellular haemoglobin which has 144 subunits. The physiological function of haemoglobin is attributed to its ability to bind dioxygen reversibly, depending on the partial pressure of oxygen (_P_O2). To function as an oxygen carrier, the oxygen affinity of haemoglobin (which is measured by the reciprocal of _P_O2 at 50% saturation) must be comparable to _P_O2 in the environment.

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  1. Division of Physiology and Biosignalling, Osaka University Graduate School of Medicine, Osaka, 565-0871, Japan
    Kiyohiro Imai

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  1. Kiyohiro Imai
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Imai, K. The haemoglobin enzyme.Nature 401, 437–439 (1999). https://doi.org/10.1038/46707

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