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February 2010

First NMR Signal of a Copper Site in Azurin Obtained

High-field, low-temperature NMR harvests unprecedented characterization of metalloproteins

Results: Metalloproteins, such as the copper-containing azurin, play a major role in catalyzing electron transfer in cellular reactions. Understanding how their structure relates to function can give rise to advances in biotechnology and medicine. To accurately characterize the nature of the bonding—covalent vs. ionic vs. hybrid—around azurin's metal center, a team of researchers from Pacific Northwest National Laboratory and the University of Nebraska at Lincoln obtained the first high-field nuclear magnetic resonance (NMR) signal of a copper site in a copper protein.

Using NMR to study copper-containing proteins is a technique previously ignored because copper's inherent NMR properties give rise to broad NMR lines that are difficult to detect. In this study, the researchers integrated 800-MHz NMR spectroscopy and the Chinook supercomputer, both housed at the Department of Energy's EMSL. This combination of tools turned the properties of the copper ion into an advantage; the researchers gained an "exquisitely sensitive measure of the electron density around the nuclear site," revealing previously unknown details about the electronic structure and environment of azurin's copper center. Their results were published in the Journal of the American Chemical Society.

Why it Matters: Developing the novel ability to use high-field NMR to characterize the electronic structure of metalloproteins containing metals such as the copper isotope 65Cu provides a powerful new tool that advances the field of bioinorganic chemistry—the study of how living systems use nonliving elements to perform fundamental biological functions. The team's strategy broadens the arena of NMR samples and is applicable to any diamagnetic metalloprotein—those proteins that create a magnetic field in opposition to an externally applied magnetic field.

solid-state NMR
On the right is the reconstruction of the solid-state NMR lineshape of copper (I) in azurin, shown in the structure at left, with its corresponding simulation.

The advent of high magnetic fields and theoretical supercomputing capabilities allows opportunities for scientists to perform previously impossible experiments and validate the results with powerful calculations and models. The team used these advances to fill gaps in azurin's data pool. Data were then reproduced using theoretical calculations, confirming details of the electronic structure. In addition to yielding new information on azurin, this study helps demonstrate that high-field, low-temperature NMR, when combined with electronic structure theory, provides a detailed and precise probe of metal nuclei in metalloproteins.

What's next: Understanding metalloproteins at the molecular level provides researchers with the knowledge necessary to alter, support, inhibit, or direct these proteins' functions in support of human health, energy production, and environmental remediation.

Acknowledgments: This work was supported by the National Institutes of Health and DOE's Office of Science through PNNL's Science Undergraduate Laboratory Internship program. EMSL is a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at PNNL. The research team includes Andrew Lipton, Robert Heck, Bert de Jong, and Paul Ellis, PNNL; and Amy Gao, Xiongjian Wu, Adrienne Roehrich, and Gerard Harbison, University of Nebraska.

Reference: Lipton AS, RW Heck, WA de Jong, AR Gao, X Wu, A Roehrich, GS Harbison, and PD Ellis. 2009. "Low Temperature 65Cu NMR Spectroscopy of the Cu+ Site in Azurin." Journal of the American Chemical Society 131(39):13992 - 13999. doi: 10.1021/ja901308v

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