An X-ray study of horse methaemoglobin. II

A complete three-dimensional Patterson synthesis of haemoglobin has been calculated, giving the distribution of vector density in thirty-one sections through the unit cell. The sections show certain concentrations of vector density which can be interpreted in terms of polypeptide chain structure. The following are the conclusions tentatively arrived at on the evidence described in this paper. The haemoglobin molecule resembles a cylinder of 57 Å diameter and 34Å height, which consists of an assembly of polypeptide chains running parallel to the base of the cylinder. The chains show a short-range fold, with a prominent vector of 5 Å parallel to the chain direction. In addition to this the chains also contain a longer fold which may extend through the whole width of the molecule. This long fold may be due either to open chains folded backwards and forwards through the molecule or to closed loops of polypeptide chains. The average distance between neighbouring chains, or neighbouring portions of the same chain folded back on itself, is 10.5 Å. The chains are arranged in four layers which are about 9 Å apart and correspond to the four layers of scattering matter described in a previous paper. The haem groups lie with their flat sides approximately normal to the chain direction.