Ascaris Haemoglobin: New Tricks for an Old Protein

Parasite biochemists have long been intrigued by the possible physiological role of Ascaris perienteric fluid haemoglobin, and a recent paper in Nature by Minning et al.1 has added a new perspective, combining structural data with new experimental findings. Ascaris, like several nematodes, has multiple forms of haemoglobin. The intracellular body-wall form resembles vertebrate myoglobins, with a molecular mass of 40.6 kDa, a single haem group and a P50O2 of 0.1 mmHg. The extracellular perienteric fluid haemoglobin, on the other hand, is an octomer (328 kDa) and has the highest affinity for oxygen of any known haemoglobin, the P50O2 being 0.0015 mmHg, nearly 25 000 times lower than that of mammalian haemoglobin. This high affinity reflects the extremely low dissociation constant for oxygen (koff at 20°C for Ascaris perienteric haemoglobin is 0.004 S−1, compared with 12 S−1 for human haemoglobin). The affinity of the Ascaris perienteric haemoglobin for carbon monoxide, however, is similar to that of mammalian haemoglobin2