X-ray laser to slay sleeping sickness parasite

The world’s most powerful X-ray laser has revealed the three-dimensional structure of an important enzyme vital to the functioning of the parasite that causes sleeping sickness.

An international team of scientists led by the German Electron Synchrotron (DESY) used the X-ray laser at SLAC National Accelerator Laboratory to elucidate the 3D structure of cathepsin B.  This enzyme is key to the sleeping sickness parasite’s (Trypanosoma brucei) survival, and knowing its structure will enable the design of new drugs to inhibit the single-celled organism.Their findings are published in Science.

“This is the first new biological structure solved with a free-electron laser,” said Henry Chapman from the Centre of Free-Electron Laser Science (CFEL).

The sleeping sickness parasite threatens more than 60 million people in sub-Saharan Africa but current drug treatments are not well tolerated, cause serious side effects and the parasite is becoming increasingly drug resistant.

Cathepsin B allows the parasite to break down the proteins of its victims. The team hope to prevent the disease by mimicking a natural inhibitor that deactivates the enzyme until the parasite invades its host’s bloodstream.

Unfortunately, the enzyme is so similar to a human enzyme that blocking it could harm the patient. This is why the researchers needed more detailed information about its structure in order to design drugs in the future that only target the parasite’s enzyme.

“These images of an enzyme, which is a drug target for sleeping sickness, are the first results from our new ‘diffract –then-destroy’ snapshot X-ray laser method to show new biological structures which have not been seen before,” explained John Spence, Arizona State University Regents’ Professor of Physics.

The researchers discovered a novel way to grow crystals of the enzyme for analysis by cultivating them inside live insect cells (a technique called in vivo crystallization), freezing the enzyme in its natural inactive state. They then streamed the crystals into the laser’s path, producing patterns in a detector that were used to reconstruct the enzyme and its inhibitor in 3D. The X-ray laser’s pulses are so intense and fast that they can capture structural information of enzymes without damaging the samples.

The researchers are now working to crystallise proteins relevant to other parasites and viruses.