Scientists at MIT have demonstrated the use of a new technology to study the mechanism of an important chemical phenomenon called proton-coupled electron transfer (PCET). This advance, detailed in a paper published in September 2011 in the Journal of the American Chemical Society, brings us a step closer to understanding a mysterious process at the heart of all life.
The research team, which included graduate student Ellen Minnihan and Professor JoAnne Stubbe of MIT’s Chemistry Department, focused on ribonucleotide reductase (RNR), an enzyme found in all living organisms. RNR converts nucleotides, the building blocks of RNA, to deoxynucleotides, the building blocks of DNA. PCET, which is how the class of RNRs found in organisms ranging from bacteria to humans initiates this transformation, works by moving an unpaired electron, or radical, over a long distance within the enzyme. This process has always been a little mysterious to scientists, but the real the mystery is why nature would choose such a complicated mechanism to conduct what in chemical terms is a fairly simple transformation.
The team investigated the mechanism of PCET by inserting unnatural amino acids into the process. In RNR, the radical breaks down its long journey into short distances by using a chain of amino acids as stepping-stones. By engineering the protein to replace the amino acids with artificial substitutes, the scientists were able to track the radical’s movement. “Scientifically, this process has not been documented anywhere else in nature,” Minnihan said. “Because we cannot study the mechanism in the natural enzyme, the unnatural amino acid allows us to reveal details that would otherwise be undetectable.”
Their observations provide concrete evidence of the radical’s “hopping” movement as well as a clearer picture of how it takes place. Because RNR is a drug target, better understanding this mechanism may help scientists to design better therapeutics against infectious disease and cancer. For instance, because RNR creates the building blocks of DNA, a clearer picture of its workings could reveal novel strategies for shutting down DNA production in tumor cells.
The next step is to build on the data to explain the complexity of the transfer mechanism, which Minnihan speculates has to do with regulating the enzyme’s activity in the cell. Minnihan, who plans to pursue a career in research science, said that these puzzles at the molecular level inspire her as a chemist. “Every year I am increasingly surprised by the complexity of nature,” she said. “Here, we’re studying this phenomenon where the chemical reaction itself, if written down on paper, is simple, yet the process in nature is so complicated.”