Myoglobin Crystals Research Articles

The binding of cupric and zinc ions to crystalline sperm whale myoglobin

The binding of cupric and zinc ions to crystalline sperm whale myoglobin has been studied by X-ray diffraction. Difference Fourier projections of crystals containing Cu 2+ or Zn 2+ have revealed in each case a single site of attachment. Study of these sites in relation to the known structure of the sperm whale myoglobin crystal makes it possible to characterize their environment. Imidazole side chains are very probably involved in binding both Cu 2+ (histidyl A10) and Zn 2+ (histidyl GH1). Two other side chains, asparaginyl GH4 and lysyl A14, are both potential binding groups for both sites (which are only 7 A apart). The existence of the latter two ligands common to the two sites would be in accordance with chemical evidence indicating that, in solution, Cu 2+ and Zn 2+ compete for the sites of highest affinity. Chemical evidence also supports the conclusion that imidazole side chains act as ligands.

Magnetic susceptibility measurements on type A myoglobin crystals

Measurements have been made of the difference between the magnetic susceptibilities in the directions of the a and b crystal axes in type A acid-met myoglobin crystals, at temperatures between room temperature and 90°K. Below about 150°K a measurable anisotropy has been found. This has been interpreted in terms of a splitting between the ground doublet and the first excited doublet of less than 15 cm -1 , compared with the earlier estimate from electron resonance work of the order of 10 cm -1 or greater.

The crystal structure of myoglobin, V. A low-resolution three-dimensional Fourier synthesis of sperm-whale myoglobin crystals

The study of type A crystals of sperm-whale has now been extended to three dimensions by using the method of isomorphous replacement to determine the phases of all the general X-ray reflexions having d > 6 Å, and a three-dimensional Fourier synthesis of the electron density in the unit cell has been computed. Data were obtained from the same derivatives which had been used in the previous two-dimensional study (Bluhm, Bodo, Dintzis & Kendrew 1958), in the course of which the x and z co-ordinates of the heavy atoms had been determined. Several methods were used to determine the y co-ordinates from the three-dimensional data; with a knowledge of all three co-ordinates of each heavy atom it was possible to establish the phases of nearly all the reflexions by a graphical method. The three-dimensional Fourier synthesis was evaluated on a high-speed computer from these phases and from the observed amplitudes of the reflexions. A resolution of 6 Å was chosen because it should clearly reveal polypeptide chains having a compact configuration such as a helix. The electron-density map was in fact found to contain a large number of dense rod-like features which are considered to be polypeptide chains, probably helically coiled. In addition, a very dense flattened disk is believed to be the haem group with its central iron atom. Finally it was possible to identify the boundaries of the protein molecules by locating the intermolecular regions containing salt solution. An isolated myoglobin molecule has dimensions about 45 x 35 x 25 Å and within it the polypeptide chain is folded in a complex and irregular manner. For the most part the course of the chain can be followed, but there are some doubtful stretches, presumably where the helical configuration breaks down; a crude measurement of the total visible length of chain suggests that about 70% of it may be in a helical or some similarly compact configuration. The haem group is near the surface of the molecule.

258. Electron microscope study of finback whale myoglobin crystals: D. E. Bradley, Nature183, No. 4666, 941, 4 April 1959

258. Electron microscope study of finback whale myoglobin crystals: D. E. Bradley, Nature183, No. 4666, 941, 4 April 1959

A pseudo-orthorhombic crystal form of horse myoglobin

The crystal symmetry and Patterson projections of a monoclinic, pseudo-orthorhombic form of horse myoglobin crystal are described (true symmetry P21, pseudo-symmetry P22121). This unit cell is very closely related to the monoclinie form of the same protein previously described: their a and b dimensions are virtually the same, while c in the new form has double the value of c sin fl in the monoclinie form. The Patterson projections are also closely related to those of the monoclinic form. These results make it possible to obtain a general idea of the relationships between the two forms and to make plausible suggestions about the mode of packing of the molecules.

Preliminary X-ray data for horse and whale myoglobins

Keilin & Schmid (1948) have recently described methods of preparing large crystals of whale myoglobin. Myoglobin is a red pigment found in muscle cells and, like haemoglobin, combines reversibly with oxygen.