NEWS OF THE WEEK
SELENIUM DOUBLES UP IN PROTEINS BIOCHEMISTRY: Diselenide bonds could play a role in redox regulation
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HE FIRST REPORT of a selenium-selenium bond in naturally occurring proteins characterizes the linkage's low redox potential and suggests that it might play a role in regulation of redox levels in cells (Proc. Natl. Acad. Sci USA, DOI: 10.1073/ pnas.0703448104). The bond, described by a K E Y L I N K A G E Two selenocysteine residues team led by biochemist Vad(left) within the same protein are in redox equiim Gladyshev at the Univerlibrium with their diselenide-linked form (right) sity of Nebraska, unites two nearby selenocysteine residues in members of a protein family found mainly in aquatic animals and bacteria. Mass spectrometry sequencing verified the selenium-selenium bond and revealed a characteristic isotope signature of a compound containing two selenium atoms.
EARLIEST DIAMONDS STIR UP DEBATE GEOCHEMISTRY: Gems in zircon grains may revise notions of early Earth events
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Tiny needlelike diamond embedded in zircon is billions of years old.
DISCOVERY of tiny diamonds embedded in the most ancient known minerals adds a new twist to the ongoing debate over at what point, billions of years ago, the molten, nascent Earth began to cool and crust over. Relic grains of hardy zircon previously found in the Jack Hills of Western Australia are believed to be up to 4.4 billion years old—hardly younger than the 4.5 billion-year-old Earth itself. Their various well-studied properties are taken as evidence that a crust of cooler rock was forming and plate tectonics were at work earlier than thought in previous decades. The latest discovery—diamonds within zircon grains up to 4.25 billion years old—could either give weight to, or detract from, the cool-old-Earth theory. The diamonds were discovered by Thorsten Geisler, researcher at the Institute of MinWWW.CEN-0NLINE.ORG
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The two linked selenocysteines are located in these proteins' active site and are organized in a manner reminiscent of the pair of catalytic cysteines found in the active site of thioredoxins. These proteins use a reversible disulfide bond to reduce cellular substrates. On the basis of this similarity, Gladyshev suggests that the new protein family may play a similar role in redox regulation. The diselenide bond doesn't react with small-molecule reducing agents, indicating that it has a very low redox potential. A team led by Phil Dawson, a protein chemist at Scripps Research Institute, reported last year that a synthetic protein with an engineered diselenide bond can be partially reduced by thioredoxin. This represents a potential avenue for reduction of diselenide bonds in a catalytic cycle. "Many people had suggested that these linkages were too oxidatively stable to be useful under physiological conditions," Dawson says. The identification of a naturally occurring diselenide bond, however, suggests that nature can harness its low redox potential. Other known selenocysteine-containing proteins tend to also have a naked cysteine residue, and the two amino acids form a reversible selenium-sulfur bond. It isn't yet clear why diselenide-containing proteins overwhelmingly occur in aquatic organisms and what mechanisms exist for reducing the diselenide bond in vivo, but the nature of this linkage suggests a role for these proteins in cellular redox regulation.—CARMEN DRAHL
eralogy at Westfalische Wilhelms Universitât, Munster, Germany; graduate student Martina Menneken; and colleagues in Australia. The researchers report their findings in Nature (2007,448,917). Raman spectroscopy indicates that the diamonds' characteristics are similar to those of diamonds formed under ultra-high pressures, Geisler tells G&EN. That complicates things. As Ian S. Williams, an earth sciences researcher at Australian National University, in Canberra, points out in a commentary accompanying the report, that finding would seem to indicate that the diamonds weren't produced in relatively low-temperature molten rock. Perhaps, a high-pressure event produced the diamonds, which were then recycled into younger material. Or perhaps the diamonds were created during the convection processes of Earth's early mantle. "Like the authors, I'm not sure what to make of it," notes Harry Green, geology and geophysics professor at the University of California, Riverside. Whatever the mechanism, "the data appear to be solid," he says. Geisler's group has plans for further study, including analyses of the isotopic composition of the diamonds' carbon. "Any information about the very early Earth is fantastic," Martin Van Kranendonk at Geological Survey of Western Australia says in a statement. "It's like a Christmas present for geoscientists."—ELIZABETH WILSON
AUGUST 27, 2007
