Cooperative dimeric and tetrameric clam haemoglobins are novel assemblages of myoglobin folds

Cooperative functioning of many protein systems depends on communication between different subunits of those systems. Perhaps the best understood cooperative protein system is the vertebrate haemoglobin tetramer, in which the subunits share a similar tertiary structure (the myoglobin fold) with each other and with myoglobins and haemoglobins from at least four different animal phyla and leguminous plants. Blood clams have cooperative tetrameric haemoglobin. In view of previous reports concerning the role of dimers in the vertebrate tetramer, the clam haemoglobins represent a very interesting model system. We report here the low-resolution three-dimensional crystal structures of the dimeric and tetrameric cooperative haemoglobins from the blood clam Scapharca inaequivalvis. We find that clam haemoglobins are made of myoglobin-like subunits but their assembly to form dimers and tetramers is quite different from that of vertebrate haemoglobin. The arrangement of the subunits provides a simple structural explanation for haem-haem interaction in the dimer and tetramer.