An X-ray study of horse methemoglobin.

This research examines the structure of horse hemoglobin using three-dimensional X-ray techniques. The analysis reveals that hemoglobin resembles a cylinder composed of polypeptide chains, which present both a short and longer range folds, the latter potentially extend through the entire molecule.

Methodology

• The researchers used a three-dimensional Patterson synthesis method with X-ray diffraction techniques to study the haemoglobin from a horse.
• The researchers then calculated the distribution of vector density in thirty-one sections through the unit cell. This method gives information about the relative positions of atoms within the molecule.

Results

• The representations of the distribution of vector density showed concentrations which are interpreted in terms of polypeptide chain structure.
• The structural analysis revealed that the haemoglobin molecule resembles a cylinder measuring 57 angstrom in diameter and 34 angstrom in height.
• The cylindrical structure is formed by parallel polypeptide chains. These chains exhibit a short-range fold with a notable vector of 5 angstrom parallel to the chain direction.
• Alongside this short-range fold, the chains contain a more extensive fold which potentially spans the entire width of the molecule.
• This extensive fold could be attributed to open chains folding back and forth through the molecule or to closed loops of polypeptide chains.
• The mean distance between neighboring chains, or segments of the same chain folded back onto itself, is approximately 10.5 angstrom.

Additional Findings

• The polypeptide chains are organized in four layers, which are about 9 angstrom separately. These layers correspond to the four layers of scattering matter described in a previous study by the researchers.
• The haem groups in the molecule sit in such a manner that their flat sides are almost normal to the chain direction. This orientation of haem groups is important for the molecule’s functional properties, as the arrangement provides an advantageous geometry for the interaction with oxygen.