A team led by structural biologists at The Scripps Research Institute (TSRI) has taken a big step toward understanding the intricate molecular mechanism of a metabolic enzyme produced in most forms of life on Earth.
The finding, published in the January 9 issue of Science, concerns nicotinamide nucleotide transhydrogenase (TH), an ancient evolutionary enzyme found throughout the animal kingdom as well as in plants and many simpler species. The enzyme is part of a process key to maintaining healthy cells and has also recently been linked to diseases such as diabetes and cancer.
“Despite its importance, TH has been one of the least-studied of mitochondrial enzymes,” said TSRI Associate Professor C. David Stout. “Our new study helps clear up some mysteries—suggesting how the enzyme structure might harness protons and indicating that its two sides are able to alternate functions, always staying in balance.”
Powering the Cell
In humans and other higher organisms, TH enzymes work within mitochondria, the tiny, double-hulled oxygen reactors that help power most cellular processes.
As a mitochondrion burns oxygen, it pumps protons (hydrogen atoms denuded of their electrons) out of its inner compartment (“matrix”), creating an excess of these charged particles just outside its inner membrane. TH enzymes, which are fixed at one end within this membrane, allow a one-by-one flow of protons back through the membrane within the matrix. This process—which is similar to that which makes ATP, the cell’s universal source of energy—has also been linked to the production of a compound called NADPH, which is crucial for defusing oxygen free radicals to maintain cell health.
Stout’s laboratory and others have previously described portions of the TH enzyme that protrude from the membrane into the mitochondrial matrix. But a precise understanding of TH’s mechanism has been elusive. In its entirety, the enzyme has an exceptionally loose structure that makes it hard to evaluate using X-ray crystallography, the standard tool for determining the structures of large proteins at atomic-level resolution.
“Key details we’ve been lacking include the structure of TH’s transmembrane portion, and the way in which the parts assemble into the whole enzyme,” said Josephine H. Leung, a graduate student in the Stout laboratory who was lead author of the study.
New Clues to a Dynamic Structure
In the new study, thanks to technology developed by Professor Vadim Cherezov, now of University of Southern California, Leung and her colleagues were able for the first time to form crystals (neatly lined-up groupings) of the TH transmembrane portion and use X-ray crystallography to determine its structure—to an atomic-level resolution of 2.8 angstroms (280 trillionths of a meter).
The team also was able to grow crystals of the whole TH enzyme. These yielded a much lower-resolution structural image, but the researchers were able to enhance the resolution to 6.9 angstroms by plugging in data from crystallography of individual TH portions. In a further study, Professor Bridget Carragher and colleagues at the TSRI-based National Resource for Automated Molecular Microscopy (NRAMM) imaged individual copies of the enzyme to 18 angstroms using electron microscopy. Stout emphasized that such seamless collaborations at TSRI made this work possible: “Only an environment as at Scripps would enable the study of transhydrogenase.”
The electron microscopy data confirmed that TH naturally exists as a “dimer”—two identical copies bound together—and provided major clues to how TH manages to work in this conformation.
Directly above TH’s transmembrane structure, just inside the mitochondrial matrix, is the “domain III” structure that binds NADPH’s precursor molecule, NADP+, during conversion to NADPH. Structural biologists haven’t understood how two such structures could work side by side in the TH dimer and not interfere with each other’s activity. The new structural data suggest that these side-by-side structures are highly flexible and always have different orientations.
“Our most striking finding was that the two domain III structures are not symmetric—one of them faces up while the other faces down,” said Leung.
In particular, one of structures is oriented apparently to catalyze the production of NADPH, while the other is turned towards the membrane, perhaps to facilitate transit of a proton. The new structural model suggests that with each proton transit, the two domain III structures flip and switch their functions. “We suspect that the passage of the proton is what somehow causes this flipping of the domain III structures,” said Leung.
But much work remains to be done to determine TH’s precise structure and mechanism. For example, the new structural data provide evidence of a likely proton channel in the TH transmembrane region, but show only a closed conformation of that structure. “We suspect that this channel can have another, open conformation that lets the proton pass through, so that’s one of the details we want to study further,” said Leung.
“There are many experiments to follow,” Stout said.
Other co-authors of the study, “Division of labor in transhydrogenase by alternating proton translocation and hydride transfer,” were Robert B. Gennis, professor of biochemistry and biophysics at the University of Illinois at Urbana-Champaign, and a research associate in his laboratory, Lici A. Schurig-Briccio, who produced whole TH proteins for analysis and characterized the activity of TH when mutated at key structural sites; Jeffrey A. Speir of NRAAM; former NRAAM member Arne Moeller, now at Aarhus University; and Mutsuo Yamaguchi, staff scientist in the Stout laboratory at TSRI.
Support for the study was provided by the National Institutes of Health (5R01GM061545) and by the National Institute of General Medical Sciences (1R01GM103838, GM095600, GM073197 and P41GM103310).
About the Scripps Research Institute
The Scripps Research Institute (TSRI) is one of the world's largest independent, not-for-profit organizations focusing on research in the biomedical sciences. TSRI is internationally recognized for its contributions to science and health, including its role in laying the foundation for new treatments for cancer, rheumatoid arthritis, hemophilia, and other diseases. An institution that evolved from the Scripps Metabolic Clinic founded by philanthropist Ellen Browning Scripps in 1924, the institute now employs about 3,000 people on its campuses in La Jolla, CA, and Jupiter, FL, where its renowned scientists—including three Nobel laureates—work toward their next discoveries. The institute's graduate program, which awards PhD degrees in biology and chemistry, ranks among the top ten of its kind in the nation. For more information, see www.scripps.edu
Madeline McCurry Schmidt | newswise
Transport of molecular motors into cilia
28.03.2017 | Aarhus University
Asian dust providing key nutrients for California's giant sequoias
28.03.2017 | University of California - Riverside
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
29.03.2017 | Materials Sciences
29.03.2017 | Physics and Astronomy
29.03.2017 | Earth Sciences