Nature's sugar high
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A team comprising French and American scientists reports they have crystallized one of the most interesting families of intensely sweet proteins, a natural molecule called thaumatin, isolated from the African Serendipity Berry (Thaumatococcus daniellii).
At right: Space-grown tetragonal thaumatin crystals from US Microgravity Payload 2.
Using otherwise similar crystallizing conditions, the space crystal showed a nearly 25% larger volume compared to its earth-grown counterparts and yielded nearly twice the crystalline order. Scientists hope to use the space-grown crystals to improve the biological understanding of how these molecules work based on detailed knowledge of their shape and exact atomic positions. According to the study, the visual quality of the space crystals "appeared virtually flawless, with no observable imperfections, striations or anomalies."
This is one of several stories summarizing results from the 16-day Life and Microgravity Spacelab (LMS), which flew June 20-July 7, 1996, aboard Space Shuttle Columbia (STS-78, at launch, left). It featured 40 scientific investigations from 10 countries. Its record development and cost - each experiment cost about half of most Spacelab experiments - make LMS an example of how future space station missions can control experiments remotely from locations around the globe. LMS results were recently published by NASA (see below). The investigation in this story used the European Space Agency's Advanced Protein Crystallization Facility.
Other LMS stories:
The complex and costly management of human diabetes, obesity, and oral health has spawned a widespread search for natural sugar substitutes that are both non-caloric and safe. The calorie-free thaumatin protein, sometimes called nature's "artificial sweetener" was analyzed by scientists from the University of California, Irvine and the Institute for Molecular Biology in Strasbourg, France.
In a control study, the team compared space-grown thaumatin crystals with some previously obtained from on earth in a conventional laboratory. They found that the space crystals provided 30% more real information about the molecule's shape. This moves the investigation closer to revealing the biological function of these complex molecules
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According to their report, the space crystals
reinforce the conclusion of other reports based on different macromolecules that a microgravity environment provides distinct advantages. In the best of only a few thaumatin crystals grown in microgravity, compared with many more trials conducted on earth, the microgravity grown crystals were consistently and significantly larger, and substantially more defect free. This is the first experiment to produce space crystals by multiple methods, both suggesting the same conclusion: crystals grown in microgravity can be significantly improved in their x-ray diffraction properties when compared with those grown on earth.
The natural proteins as a group are the sweetest compounds ever discovered. The sweet taste - which depends on nearly 100 different sensory receptors on the tongue - can be detected in the presence of thaumatin at concentrations well below one part protein molecule per 100 million parts of water. On a scale in which 0 refers to no sweetness, 1 refers to table sugar or sucrose, then thaumatin is nearly off the scale at 3,000, more than 10 times sweeter than other sugar substitutes like saccharin or aspartame.
Because these kinds of complex sensory-stimulating proteins typically require binding to specific taste receptors, much of their biology remains to be worked out in the kind of studies done on the space shuttle and using modern tools of biological crystallography. Already within the bulk commercialization by biotechnology companies, Tate & Lyle's product, Talin, is marketed from thaumatin. Also, at the Unilever Research Laboratory in The Netherlands, the gene for this sweetener has been cloned into biological production using the microorganisms E. coli and yeast to substitute for the original African shrub.
As a non-caloric sweetener, thaumatin has attracted attention as a candidate for control of obesity, oral health and diabetic management. Thaumatin already is being marketed as a nutritional supplement in blood sugar stabilizers for childhood behavioral problems and the more than 3.5 million sufferers from attention deficit disorder. Among soft drink consumers alone, nearly 20.6 million tons of chemicals are used around the world - nearly 4 kilograms per capita, with a growth of about 20% towards the end of the decade.
Control of diabetes, the most common metabolic disease in the world, largely hinges on managing sugar levels in the bloodstream. According to a recent study published in the Journal of Clinical Endocrinology and Metabolism, one out of every 7 health care dollars, or $105 billion, goes to the treatment of diabetes-related complications. Individual diabetics spent an average of $9,493 on health care in 1992, the latest data available, compared with $2,604 for people without diabetes, the study said. Nearly 600,000 people per year are diagnosed as diabetic in the US. The National Institutes of Health proved that diabetic patients who can maintain blood-sugar levels as close as possible to normal can significantly slow the disease.
Some estimates suggest that human biology depends on the action of nearly half a million different enzymes and proteins. In fewer than 1 case in 100, we have a three-dimensional picture of shape and function of these complex chemicals. Since 1984, the Space Shuttle has carried experiments to determine the structures of large, biologically important molecules. This research has compiled results for a host of human diseases ranging from insulin (for the control of diabetes) to one enzyme called reverse transcriptase that can be blocked to inhibit HIV infection.
In comparing more than 33 such different biological molecules crystallized on the Shuttle and also in similar conditions on earth, space produced larger space crystals in 45% of the cases and new structures in nearly 20% of the cases. As many as half the space crystals had a 10% or better improvement in the x-ray brightness or the crystallographic resolution. Both are important to determining these large molecules' shape and exact atomic positions.
www.microgravity.com General information about science in low-gravity including protein crystal growth. Protein Crystal Growth Tutorial Describes how nearly perfect crystals are grown and analysed.
Microgravity News, Summer 1996 periodic newsletter; this on on LMS
Microgravity Research Program Office at Marshall has a wealth of information and background on various microgravity projects
Life and Microgravity programmatic information from NASA headquarters.
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Department of Molecular Biology and Biochemistry University of California, Irvine
CNRS - Centre National de la Recherche Scientifique, Strasbourg, France