( hool Drennan's collaborator and co-aut...)MIT chemist discovers secret behind nature's medicines

hool, Drennan's collaborator and co-author of the study published in the March 16 issue of Nature. Looking at the SyrB2 enzyme that the microorganism Pseudomonas syringae uses to produce the antifungal agent syringomycin, he discovered it had a single iron atom in the protein's active site, the part responsible for the chemical reaction.

Drennan and her graduate student Leah C. Blasiak, who was first author of the study, crystallized SyrB2 and then used X-ray crystallography to discover the physical structure of the protein. The X-rays scatter off the crystal, creating patterns that can be reconstructed as a three-dimensional model for study.

Normally, iron-containing enzymes have three amino acids that hold the iron in the active site. In this enzyme, however, one of the typical amino acids was substituted with a much shorter one.

That smaller substitute leaves more room in the active site -- enough space for the halide, in this case a chloride ion, to casually slip inside and bind to the iron, without the grand theatrics chemists had anticipated. After the iron and the chloride bind, the protein closes down around the active site, effectively pulling the trigger on the gun.

"We were surprised," Drennan said. "The change in activity required for an enzyme to be capable of catalyzing a halogenation reaction is so radical that people thought there must be a really elaborate difference in their structures. But it's just a smaller amino acid change in the active site. Things are usually not this simple, but there's an elegant beauty in this simplicity," and it may be what gives other enzymes the prowess required for making other medicinally valuable halogenated natural products, too.

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Source:Massachusetts Institute of Technology


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