NatureA research team of eight Danish and Norwegian scientists and their one Brazilian colleague have achieved a long-sought goal of scientists since the 1960s: to determine the atomic-level structure of the proteins that bind together to prevent hemoglobin – the protein that carries oxygen and gives blood its red color – from reaching toxic levels of concentration in the body and possibly damaging vital organs. The study was published in the September 20, 2012 issue of the journal Nature. According to Professor Cristiano Luis Pinto de Oliveira of the Physics Institute of the University of São Paulo (IF-USP) who took part in the study, the discovery may contribute to the development of medicines that relieve episodes of hemoglobin toxicity in persons suffering from congenital blood diseases and malaria.
Transported by the erythrocytes, or red blood cells, hemoglobin molecules consist of four subunits, each containing one iron ion that easily reacts with other chemical elements, especially oxygen atoms. This makes hemoglobin the main diffuser of oxygen throughout the body’s tissues.
When this protein emerges from the erythrocytes, however, it separates into two dimers (each with two subunits), and leaves the iron ions highly exposed. High concentrations of hemoglobin in this form may be toxic. The problem is more acute in the kidneys, where there is a tendency for this molecule to accumulate since it is a kidney’s job to purify the blood. Patients who also suffer from this disorder are those with certain hereditary conditions as well as carriers of the malaria parasite whose hemocytes constantly rupture, releasing excess hemoglobin into the plasma.
The human body has a natural mechanism for eliminating excess amounts of this protein. Haptoglobin, another protein that circulates in the blood, binds to the hemoglobin dimers, enveloping and neutralizing their iron ions. This hemoglobin-haptoglobin (Hb-Hp) complex is in turn removed from the blood by cells known as macrophages. They do this through proteins called CD 163 receptors. By bonding to specific locations, the receptors “hook” and remove the Hb-Hp structures from the bloodstream.
Researchers have understood the atom-by-atom structure of hemoglobin since 1959 when they first managed to grow a crystal of the macromolecule and watched how it interfered with the passage of x-rays, a phenomenon known as diffraction. But until recently, they have not been able to do this with haptoglobin and the Hb-Hp complex. “The main difficulty was the peculiar shape of the haptoglobin, which made it hard to crystallize,” explains Oliveira.
Under the coordination of biomedical scientist Soeren Moestrup of Aarhus University in Denmark, the researchers began working in 2006 to find the ideal conditions for purifying and crystallizing the desired substances. Their efforts combined several biochemical and biophysical techniques. Oliveira collaborated on the analysis of how X-rays are scattered by the molecules in an aqueous solution. The technique enabled him to confirm that the shape of crystallized molecules is the same as those dissolved in water. The high-resolution structures obtained show how the haptoglobin is formed out of the two subunits (in light blue and dark blue in the figure), connected in a way never before seen in proteins. They also show how the Hb-Hp complex is formed and how it binds to the CD 163 receptors.
ANDERSEN, C.B.F. et al. Structure of the haptoglobin–haemoglobin complex. Nature. v. 489. 20 Sept. 2012.