Horse hemoglobin crystal

FIGURE 7.7. Methods of mounting crystals, (a) A crystal mounted in a glass fiber, as used for a small-molecule crystal that does not decompose on exposure to air. (b) Diagram of the mounting of a crystal in a capillary tube, (c) and (d). A crystal of a chemically modified horse hemoglobin enclosed with mother liquor in a thin walled glass capillary. (Courtesy J. J. Stezowski). 

The classic example is monoclinic C2 horse hemoglobin studied by Max Perutz. In this crystal form it was known from density measurements that the C2 unit cell contained two entire molecules of hemoglobin. There are four asymmetric units in a C2 unit cell, however, as the International Tables show. Thus one-half of a hemoglobin, to satisfy space group symmetry, had to be related to the other half by a twofold axis. By this means horse hemoglobin was shown to possess a perfect twofold axis of symmetry well before it was demonstrated by any other means. 

The hemoglobins of erythrocytes (molecular weight 68,000) are tetramers with one heme per polypeptide the association of the four subunits results in a spheroid approximately 71 X 54 X 52 A. The position of the hemes, at least for horse hemoglobin has been determined by electron spin resonance (2H) the hemes are not parallel to each other. One pair lies in the a, b plane of the crystal while the other pair is tilted 13° above and below the plane. The three dimensional structure of hemoglobin to a resolution of 6 A has been achieved (215). 

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