Blue Blood of the Emperor Scorpion X-rayed

Photo: Hemocyanin

Hemocyanin of the emperor
scorpion: model of the 24-meric
protein complex and electron
density; © University of Mainz

The emperor scorpion (Pandinus imperator) is not only one of the biggest scorpions in the world, but it also has one remarkably large protein, namely hemocyanin. Hemocyanin is a protein complex made up of 24 subunits that functions as blood pigment. It is one of the largest known proteins, comparable in size to ribosomes or even small viruses. For the first time ever, scientists have now successfully grown crystals from the emperor scorpion’s hemocyanin. With the help of x-rays, these crystals allow for a more precise analysis of the structure of the protein.

Up to now, cryo-electron microscopy has primarily been used to examine large protein structures such as hemocyanin. This method has its disadvantages, however, because its resolution is not sufficient to be able to differentiate between single atoms. With x-ray crystallography, on the other hand, protein structure can be more precisely determined. It is even possible to determine the spatial arrangement of individual atoms. Scientists rely on this knowledge about the detailed molecular structure of these protein complexes in order to be able to understand how these proteins function.

Hemocyanins are extraordinarily large respiratory proteins that transport oxygen in the blood of mollusks and arthropods. While these blue blood proteins bind oxygen between two copper atoms, human hemoglobin binds oxygen to iron atoms. Hemocyanin fascinates biologists because, depending on the animal species, up to 160 oxygen binding sites within a single protein complex must communicate with one another in order to bind, transport, and release oxygen in the blood. Referred to as cooperativity, this phenomenon occurs only in nature and could potentially be used in nanotechnology applications to build molecular switches. Structure determination at an atomic resolution is necessary in order to be able to understand this process in detail.

This is the decisive first step toward successful x-ray structure determination because protein crystals are necessary to diffract x-rays so that the structure of the protein can be determined. Crystallisation, however, is especially difficult for large protein complexes. "It is a little bit like a game of chance," Professor Elmar Jaenicke describes the crystallisation process, because the process is dependent on a number of factors such as the pH-level, the salinity of the solution, or the temperature. "The decisive step is always crystal nucleation," which, according to Jaenicke, can take months and requires a lot of patience. Sometimes, it even takes several years to optimize the conditions for crystallisation.

MEDICA.de; Source: Johannes Gutenberg-University of Mainz