The crystal structure of horse met-myoglobin - I. General features: the arrangement of the polypeptide chains

Abstract

The preliminary results of an X-ray study of crystalline horse met-myoglobin are described. This protein crystallizes in the monoclinic space-group P2 1 with two molecules in general positions in the unit cell. The optical properties of the crystals show that the haem groups are parallel and lie approximately in the plane containing b and the internal bisector of β. a and c Patterson projections contain ridges of high vector density perpendicular to b and 15*4A apart; it is deduced that in real space the contents of the cell are arranged in pronounced layers of alternately high and low electron density, also perpendicular to b and 15*4 A apart. The b projections also exhibit rods of high vector density, 9*5 A apart and inclined at 20° to a . These rods are identified as the vector equivalents of the polypeptide chains, which thus run parallel to [20T] and 9*5 A apart. A Patterson projection along [20T] shows the rods well-resolved and in end-on view; by making certain plausible assumptions a Fourier projection has been made along the same direction. During shrinkage the cell dimensions change in such a way that the inter-chain distance alters very little; it is deduced that the molecules have flat sides parallel to (102) and that the liquid of crystallization is situated mainly on their [20T] faces. During shrinkage liquid is withdrawn and the molecules slide past one another along their flat sides. It is not possible to determine the whole shape of the molecule unequivocally; the most plausible model is a platelet built of four parallel co-planar lengths of chain each 54 A long (see figure 13). There are striking analogies between the structures of myoglobin and of horse haemoglobin. The type of chain folding seems to be the same in the two proteins, and probably the same as in a-keratin; and it may be that myoglobin has a structure analogous to that of one of the four layers of which haemoglobin is believed to be built up, the molecular weights of the two proteins being in the ratio 1:4.