Researchers have identified a key molecule in the biological conversion of mercury to toxic methylmercury, a finding that could lead to innovative ways to mitigate mercury poisoning in the environment.
Mercury is notoriously toxic, but it reaches its most dangerous form as methylmercury — a compound so harmful that mere billionths of a gram can cause severe neurological damage to a developing fetus. This dangerous compound often enters human bodies via seafood, creating a significant public health concern.
Scientists at the Stanford Synchrotron Radiation Lightsource (SSRL) at the U.S. Department of Energy’s SLAC National Accelerator Laboratory have uncovered a critical aspect of this transformation. Utilizing high-energy X-rays, they have identified a molecule called S-adenosyl-L-methionine (SAM) as a major player in the biological methylation of mercury.
“Nobody knew how mercury is methylated biologically,” co-author Riti Sarangi, a senior scientist in SSRL’s Structural Molecular Biology program, said in a news release. “We need to understand that fundamental process before we can develop an effective methylmercury remediation strategy. This study is a step toward that.”
Published in the Proceedings of the National Academy of Sciences, the study addresses a crucial gap in understanding how methylmercury is produced.
Industrial emissions of mercury often end up in bodies of water, where microbes transform the metal into methylmercury. This compound then accumulates as it moves up the food chain, ultimately affecting human health.
One major mystery for researchers has been the exact mechanism by which microorganisms convert mercury into its methylated form. The task is complicated by the fact that the responsible protein system, known as HgcAB, exists in minuscule quantities within microbes. It is also highly sensitive to light and oxygen, making it difficult to study.
Over a decade-long effort involving multiple national laboratories and universities, a team led by Steve Ragsdale, David Ballou Collegiate Professor and professor of biological chemistry at University of Michigan, developed a new protocol to produce stable amounts of HgcAB. This breakthrough enabled the team to delve deeper into understanding the protein’s function.
“We’ve worked with a lot of very difficult proteins, but this one had everything you would not want to have in a protein if you wanted to purify it. It was very complicated,” Ragsdale said in the news release.
Once purified, the HgcAB samples, kept in liquid nitrogen and shielded from light, were analyzed at SSRL.
Macon Abernathy, an associate scientist at SSRL, conducted extended X-ray absorption fine structure spectroscopy to examine the samples.
“SSRL’s X-ray spectroscopy facilities are especially equipped to study biological samples and have powerful detector systems that can resolve the extremely weak signals of ultra-dilute protein samples like these,” Sarangi added.
Previous theories suggested that the methyl group necessary for methylmercury came from methyltetrahydrofolate.
However, this study reveals that SAM is actually the methyl donor. This discovery narrows down the main actors in the methylation process, potentially aiding in the development of environmental remediation strategies.
“No one has tried it yet, but perhaps analogs of SAM could be developed that could address methylmercury in the environment,” added Ragsdale.
This study marks a significant leap toward understanding and potentially mitigating a hazardous environmental issue.