Copper-binding mystery solved

Methanotrophs, bacteria that use methane as a food and energy source, have to acquire copper ions from their environment because their primary metabolic enzyme uses it to metabolize methane. Some methanotrophs produce natural products called methanobactins to acquire copper they need from the surrounding media.

Many methanobactins have two oxazolone-thioamide groups that act like pincers to grab copper tightly. Enzymes form these groups in MbnA, a methanobactin precursor peptide made by bacterial ribosomes, but researchers have not been able to show how the groups are created.

Amy C. Rosenzweig of Northwestern University and coworkers have now shown that two proteins, called MbnB and MbnC, combine to form a redox-active iron enzyme complex that oxidizes an MbnA cysteine to an oxazolone-thioamide group in the bacterium Methylosinus trichosporium OB3b. They reported the findings in Science (2018, DOI: 10.1126/science.aap9437) and in a Sunday presentation in the Division of Inorganic Chemistry at the ACS national meeting in New Orleans.

“This work is cool,” said professor David W. Graham of Newcastle University, who led a group that first structurally characterized the methanobactins in 2004. “Ever since we discovered and named the methanobactins, we speculated on how they were made.”

Many nonmethanotrophic bacteria, including human pathogens, possess MbnB and MbnC, so the findings could make it easier to figure out how those microbes create methanobactin and methanobactin-like natural products. The Northwestern group believes engineered methanobactins could lead to drugs for copper-accumulation disorders such as Wilson disease, a genetic condition characterized by toxic copper buildup. Methanobactins may also have antibiotic activity, Rosenzweig said.

Researchers have found MbnA, MbnB, and MbnC genes together in every methanobactin gene cluster they studied. Rosenzweig and coworkers proposed in 2013 that MbnB and MbnC catalyzed the four-electron cysteine oxidation that produces each oxazolone-thioamide group.

What enabled them to prove it, Rosenzweig said, “were the incredible hard work of first author Grace E. Kenney, who pioneered the project, and a multidisciplinary approach that included bioinformatics, biochemistry, mass spectrometry, spectroscopy, and genetics.” The oxidation reaction’s detailed molecular mechanism still isn’t known, and finding it may be one of the group’s next steps.