One small step for sugar

Unlike its genomic, proteomic, and transcriptomic predecessors, a comprehensive glycome project (see "Sugars get an 'ome of their own," 18[15]:45–7, Aug. 2, 2004) may have to include telescopes and spacesuits. Last month, astronomers announced that they've detected sugar molecules in the coldest gas and dust clouds of the central Milky Way. The intensity of the signal was surprisingly strong, hinting that these clouds, the birthplace of stars and planets, may also be churning out the building blocks of Earth's life.Previous searches have turned up biological molecules, including amino acids, in interstellar space and in meteorites that have crashed into Earth. But until recently, there was no evidence for the extraterrestrial existence of the molecules that form part of our plasma membranes and string together our bases. "I think sugars were a missing link, literally and figuratively," says Richard Cummings, director of the Oklahoma Center for Medical Glycobiology.That changed in 2000 when a group of astronomers led by Jan M. Hollis of NASA's Goddard Space Flight Center in Greenbelt, Md., picked up the spectral signature of glycolaldehyde molecules in an interstellar cloud called Sagittarius B2, 26,000 light years away. Glycolaldehyde is the simplest of all sugars, with just two carbon atoms, two oxygens, and four hydrogens. Telescopes can detect its presence by measuring the characteristic frequencies of radio waves that the molecule absorbs and emits when it changes energy levels.After their discovery, Hollis and his colleagues began searching Sagittarius B2 for signs of the 3-carbon glyceraldehydes, one synthetic step up from glycolaldehyde. They didn't find any, but they did find something unexpected: Glycolaldehyde molecules were detected thriving at just eight degrees above absolute zero. "These low temperatures imply that there could be, at the galactic level, a massive amount of this stuff," says Cummings.Some scientists think they might know how all of it formed. So-called molecular clouds, like Sagittarius B2, contain dust coated in icy mixtures of small molecules, including water, ammonia, carbon monoxide, and methanol. When shock waves triggered by nearby star formations ripple through space, they hit the dust grains, possibly initiating the chemical reactions that create glycolaldehyde and amino acids, and maybe even more complex molecules waiting to be discovered.The new molecules then disperse into cold, dusty clouds at the edge of a planetary system. Because glycolaldehyde, at least, can survive such frigid temperatures, Cummings says passing comets "could load themselves up with these things and they would be stable" for a long haul toward the inner solar system and the planets that reside there."Look at the moon," Cummings points out. "Look at the impacts. There have been a tremendous number of bodies impacting us over billions of years. So it could be that a significant amount of material was brought" to Earth from outer space.Whether that material included glycolaldehyde, though, "is a big, big 'if,"' according to Jeffrey L. Bada, a biogeo-chemist at the Scripps Institution of Oceanography in La Jolla, Calif. Even if roving comets or asteroids picked it up, the sugar might not have survived an entry through Earth's atmosphere. Sugar molecules also are notoriously unstable, which has led some biologists to reject an "RNA first" model of the origin of life and instead suggest that the first self-replicating life on Earth did not contain sugars.Even if molecules delivered by comets and asteroids were not the seeds of life, the power of the chemistry that creates these freezing sugar reservoirs has far-reaching implications. "You can make these compounds so easily, even in the harsh environment of cold interstellar space," Bada says. "If you take them and you have a nice little warm planet, then the chemistry is going to be even more robust." So is the basic chemistry that led to the origin of life on Earth happening somewhere else? Or has it happened in the past? "I would say that the chances are probably pretty darn good," says Bada.In any case, it's a pretty spectacular find for a sugar scientist of this world. Says Cummings: "For those of us who are stuck here on Earth, building our oligosaccharides one sugar at a time, it's kind of interesting to think about where they all really came from."- Melissa Phillips

One small step for sugar

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