The application of the molecular replacement method in studies on the quaternary structure of haemoglobin

It has been found that the crystallization of oxyhaemoglobin from polyethylene glycol solutions [Grabowski et al. (1978). Biochem. J. 171, 277-279] yields at least three different forms of isomorphous crystals (P2₁2₁2, a = 96.66, b = 97.88, c = 65.40 Å). 3.5 Å resolution data have been collected for two of the three identified forms. The orientations and positions of the αβ haemoglobin dimers within the asymmetric units of these crystals have been established by the molecular replacement method with computing techniques different from those used by Ward, Wishner, Lattman & Love [J. Mol. Biol. (1975), 98, 161-171] in their studies of human deoxyhaemoglobin crystals obtained from polyethylene glycol solutions. The calculations have been performed also with diffraction data from deoxyhaemoglobin and fluoromethaemoglobin + inositol hexaphosphate crystallized from polyethylene glycol by Fermi & Perutz [J. Mol. Biol. (1977), 144, 421-431]. In all cases, the individual dimers have been positioned independently and it is shown that, in the methods using a fast rotation function, three- and multidimensional residual-type translation functions may be directly applied to the structure determination of those complex structures in which the structure of only one of the two subunits present in the asymmetric unit is known. It is also shown that in all crystals studied the haemoglobin dimers are arranged in the conformation, which seems to exclude the possibility of the full oxygenation of haemoglobin crystallized from polyethylene glycol solutions.