An enzyme tunnel in 3D
HZI scientists discover possible new approach to tuberculosis therapy
At high resolution and in 3D - Scientists from the Helmholtz Centre for Infection Research (HZI) and the University of Basel solved the structure of the enzyme, EgtB. Some bacteria utilise EgtB to produce the vitamin ergothioneine, for example Mycobacterium tuberculosis which causes Tuberculosis. Experiments with closely related organisms indicate that the tuberculosis pathogen can be defeated by the immune system more easily if it lacks ergothioneine. The scientists published the structure of EgtB and its mechanism of action in the professional magazine, "Angewandte Chemie". Their results may contribute to a new tuberculosis therapy.
Ergothioneine is a substance whose function is not yet known, but which has attracted the interest of researchers so much that they deciphered its exact synthetic pathway. The substance is a vitamin that is produced by fungi and some bacteria and is ingested by humans with their food. The tuberculosis pathogen, Mycobacterium tuberculosis, also produces ergothioneine. When scientists studied closely related microorganisms, they noted that these can defend themselves less well against reactions of the immune system when they become unable to produce the vitamin. "If we succeed to inhibit ergothioneine or its synthesis in Mycobacterium tuberculosis, we might have a new approach to a tuberculosis therapy," says Prof Wulf Blankenfeldt from the HZI. According to estimates of the World Health Organization, nine million people were newly diagnosed with Tuberculosis in 2013.
But how could this type of tuberculosis medication attack? To answer this question, the researchers deciphered the three-dimensional structure of an enzyme that produces ergothioneine. This enzyme is called EgtB and uses amino acids, the building blocks of our proteins, for its starting materials. It connects a carbon atom to a sulfur atom and thereby attaches two amino acids to each other in a central step. It does not do this same way as previously known enzymes: "Each enzyme has a so-called active centre, in which the starting substances bind and in which the reaction proceeds," says Allegra Vit, who investigates EgtB in the course of her doctoral thesis at the HZI in collaboration with the research group of Prof Florian Seebeck at the University of Basel. "Our experiments show that the sulfur atom present in one of the amino acids interacts directly with the active centre of EgtB."
The scientists used a method called x-ray crystallography to resolve this process up to the smallest detail and to determine the arrangement of individual atoms. These form some kind of tunnel that contains the active centre. There is an iron atom at the end of the tunnel that is responsible for the enzymatic activity of EgtB.
"We used the detailed analysis of the enzyme structure and of interactions between the atoms of the enzyme and of the starting substances to decipher the mechanism of action of EgtB," says Blankenfeldt. The work of the scientists also lays the foundation for the search for a substance that might inhibit EgtB in the tuberculosis pathogen. (Dr Birgit Manno)
Goncharenko, K. V., Vit, A., Blankenfeldt, W. and Seebeck, F. P. (2015), Structure of the Sulfoxide Synthase EgtB from the Ergothioneine Biosynthetic Pathway. Angew. Chem. Int. Ed., 54: 2821–2824. DOI:10.1002/anie.201410045.