The crystal structure of myoglobin II. Finback whale myoglobin

Abstract

Myoglobin of Balaenoptera physalus (finback whale) normally forms orthorhombic crystals with space group P2 1 2 1 2 and cell dimensions a = 97·4, b = 39·8, c = 42·5 Å; the unit cell contains four molecules. The intensities of the reflexions in the three principal zones have been used to compute Patterson projections, which exhibit rod-like features parallel to z . These rods are 10 Å apart in approximately hexagonal packing and have nodes at 5 Å intervals; they are taken to be the vector equivalents of a set of quasi-parallel polypeptide chains with the same orientation in real space, the mean chain direction being about the same in all four molecules in the cell. Further evidence for this conclusion is derived from the radial distribution of intensities, which is anisotropic in the sense that there is a marked preponderance of 10 Å reflexions in the [001] zone; and from the absolute magnitudes of the reflexions, which are compatible with the hypothesis that 25 to 50% of the molecule is made up of parallel polypeptide chains. If the chains were only approximately parallel they might account for a larger proportion of the molecule. The changes in the low-order reflexions produced by variation in the electron density of the suspension medium have been used to find the position of the molecules in the unit cell; confirmation of the results is found in the Patterson projections. The orientation of the haem group relative to the crystal axes is derived from measurements of paramagnetic resonance (Bennett & Ingram 1956 a ) and of optical dichroism. It is concluded that the plane of the haem group is inclined at 41° to the mean chain direction. Unambiguous information about the structure of the molecule will only be obtained by the isomorphous replacement method; meanwhile the above results, taken in conjunction with others derived from different crystal forms of myoglobin, have been used to discuss some plausible models. The most favoured is 42 Å long (in the chain direction) and has a cross-section of 25 x 35 Å. A molecule having these dimensions might consist of two layers of; polypeptide chains, each layer containing three chains. This arrangement is compatible with one of two favoured Fourier projections derived by applying the inequality relations of Cochran (1952) to the [001] zone.