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From oxygen transport to melanin formation: Activation mechanism of key enzymes explained

04 June 2009 Universität Mainz

Pandinus imperator, the emperor scorpion, is not only popular as a pet, but is also of interest for research purposes. The reason for this is its blue blood, which transports oxygen and distributes it throughout the body. Like tyrosinase, the key enzyme in melanin synthesis, the blue blood pigment hemocyanin found in the emperor scorpion and other arthropods belongs to a group of special molecules that occur in all organisms and that have many different functions: coloring the skin, hair and eyes, immune response, wound healing or the brown discoloration of fruit. "When these enzymes mutate, this may result in albinism, or in birth marks when production of the pigment melanin increases, as often seen in melanoma," explains Professor Heinz Decker of Johannes Gutenberg University Mainz. The biophysicist has been studying hemocyanins and the associated tyrosinases for the past 20 years. In cooperation with researchers, Dr. Cong and Dr. Chiu, from the Baylor College of Medicine in Houston he has now been able to show for the first time exactly how the enzymes become active, thereby fulfilling their various functions. This work was published in the journal Structure on 13 May.

The researchers investigated the hemocyanin molecules of the emperor scorpion with the aid of cryo-electron microscopy. This is done by dissolving the molecules in an extremely thin film of water and then freezing it. The use of this technology means that the water does not form crystals, but an amorphous film of ice, which can then be examined by means of electron microscopy. "The benefit of this method lies in the fact that we can use it to penetrate the inside of the molecules and therefore see exactly what takes place there," says Decker. The molecules house the "active center", the part of the enzyme that carries out its function. Access to the active center is at first blocked. Once the researchers have triggered an appropriate stimulus  the structure changes. "We have seen that a specific domain of the molecules must move before the door to the active center is opened, thus triggering enzyme activity. This allows bulky phenols to reach the active center as a substrate and be converted into active quinones by bonding with oxygen; these quinones can then independently synthesize to melanin". For many years, Decker had been proposing this activation mechanism as a hypothesis in his work, but now it has been directly observed for the first time.

The observations made regarding the oxygen transport molecule hemocyanin can also be applied to tyrosinases. Hemocynanin is so closely related to tyrosinases that it can even be converted into tyrosinases by means of the activation mechanism described. This, too, has been demonstrated in several experiments. New opportunities have thus been created for an improved understanding of disorders or diseases such as albinism and malignant melanoma. The cosmetics industry is interested in this interrelationship, as the color of the skin and hair is determined by the formation of melanin. The food industry could make use of the information to prevent the discoloration of fruit, such as banana peels for example, by inhibiting this mechanism.

This study was funded by the National Center for Research Resources, the Roadmap Initiative for Medical Research (in Houston) and the German Research Foundation (DFG, SFB490) as well as the newly established Research Focus of Computational Sciences in Mainz (CMS) and Research Center for Immunology in Mainz.

http://www.cell.com/structure/abstract/S0969-2126(09)00150-6

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