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The Green Party

NASA/Marshall Space Flight Center--Space Sciences Laboratory

Biologists conducting Space Shuttle experiments may be one step closer to shedding light on the biggest power booster on the planet: a protein in green plants called Photosystem I.

A German research team recently presented the results of their Space Shuttle experiment designed to crystallize Photosystem I molecules. According to the researchers, "This experiment has yielded the best data set thus far obtained from Photosystem I crystals."

During photosynthesis, the cells in green plants undergo two simultaneous reactions, both of which rely on a separate kind of protein. Photosystem I protein molecules use the trapped energy in sunlight to convert carbon dioxide into carbon and oxygen. This provides the plant food in the form of carbohydrates, lipids, proteins and nucleic acids - the building blocks of life. Photosystem II protein molecules use light energy to split water into hydrogen and oxygen for plant respiration.

Scientists crystallize protein molecules in order to study their complex internal structures. Because the molecules are too small to study directly under a microscope, scientists use X-ray diffraction to get a picture of the molecule.

Shining X-rays through a crystal produces a scattering pattern, which is a type of blueprint. Think of a shadow cast through a picket fence - the shape of the shadow would tell you that the fundamental building block of the fence is a rectangular board. Shining X-rays through a protein crystal indicates the protein's shape, where it's located, and ultimately how it may work.

High quality crystals - composed of ordered and repeating units of a particular protein - are required for X-ray diffraction. Some of the crystals grown in the microgravity conditions of space are more perfectly ordered than crystals grown on Earth. Microgravity can also affect the rate at which the proteins initiate new growth. Space crystals have shown a 10 to 20-fold larger volume compared to the Earth-grown counterparts.

The Photosystem I protein molecule, sometimes called "the Earth's power station," was analyzed by a scientific team representing the Max Volmer Institute for Biophysical Chemistry and Biochemistry in Berlin, Germany. The team reported their results in their Final Report published from the Life and Microgravity Spacelab (LMS) mission. The team hopes these results will give scientists a more detailed knowledge of the Photosystem I molecule's shape, exact atomic positions, and biological functions. And by using the results of the experiments on the space shuttle, scientists can improve the crystallization conditions here on Earth.

The Earth's environments - from forests to grasslands to the oceans - are direct products of the Photosystem protein molecules. From the beginning of life, Photosystem II processes in algae completely altered the atmosphere, transforming the carbon dioxide environment into an oxygen-rich one.

The two Photosystem proteins underlie the Earth's balance between water and heat and between oxygen and carbon dioxide. They ultimately supply the nutrients for almost every living thing on the planet, as well. Most of the organisms on Earth receive their sustenance directly or indirectly from photosynthetic vegetation. Without the Photosystem molecules, life as we know it would cease to exist.

The space experiments were performed on ancient organisms called cyanobacteria, formerly known as blue-green algae or blue-green bacteria. As a family, these organisms form the fundamental basis of the entire marine food web and are often called "the grass of the sea." These early ancestors of modern plant cells (chloroplasts) were the first oxygenic organisms to convert light to energy on Earth. The cyanobacterium protein used in the space investigation, from the species Synechococcus elongatus, is found abundantly today. It represents more than half of the total biomass productivity in all open ocean environments and may process up to 50 percent of the excess carbon dioxide greenhouse gasses implicated in the current global warming debate.

Burning carbon fuel such as oil and coal produces most of this excess carbon dioxide. This process currently supplies much of the world's power needs, but the fuel reserves are rapidly running out. Nonpolluting alternative fuel sources are being developed to take the place of oil and coal. In the 1970s, solar power - a clean and unlimited power source - seemed to be the most promising alternative. Harnessing the power of the Sun to power the Earth, however, has been plagued with difficulties. To generate a lot of power, you need extremely large solar panels. And what do you do for power when the sun sets?

The Space Shuttle investigation is trying to discover what features of photosynthetic proteins allow for solar energy conversion. While humans have only been developing solar power technology for a few decades, plants have been evolving for billions of years to perfect their photosynthetic technique. By studying how plants accomplish this remarkable feat, scientists hope to someday also develop systems that use light as a power source. Identifying and studying characteristics of the protein's metabolism may someday also be used for applications in pollution prevention and environmental clean-ups.

Knowing The Code

Many essential biology questions depend on knowing the structure of proteins and enzymes. By charting their shape, scientists can determine how the molecules work. But these molecules may also change shape when performing important functions, like carrying oxygen in blood hemoglobin. In photosynthesis, there are many energy producing conversion steps from sunlight to plant development and growth.

Some estimates suggest that human biology depends on the action of nearly half a million different enzymes and proteins. But we only have a three-dimensional picture of shape and function for fewer than 1 in 100 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 the reverse transcriptase enzyme that, when blocked, inhibits HIV infection.

Just as in human cells, the Photosystem proteins inside a plant cell are translated from amino acids. Amino acids have a 20 letter alphabet for each of the 20 naturally occurring amino acids (shown below as AAs). These amino acids are in turn translated from the complex array of nucleic acids in DNA (coded as the letters A,G,T and C). A description of the molecular code reads like an encrypted message:

AAs =FFLLSSSSYY**CC*WLLLLPPPPHHQQRRRRIIIMTTTTNNKKSSRRVVVVAAAADDEEGGGG Starts =
---M---------------M------------MMMM---------------M------------

Base1 = TTTTTTTTTTTTTTTTCCCCCCCCCCCCCCCCAAAAAAAAAAAAAAAAGGGGGGGGGGG
GGGGG Base2 = TTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCCCAAAAGGGGTTTTCCC
CAAAAGGGG Base3 = TCAGTCAGTCAGTGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGTCAGT
CAGTCAGTCAG...

Much work remains to be done in uncovering the shape and detailed way the Photosystem power-converting molecules achieve their efficiency. By using the results from space shuttle experiments, someday we may understand how that transformation happens in detail. Such experiments make possible the study of proteins that had once proved too difficult to dissect at the molecular or atomic size.

Life On The Edge Project

A good illustration of how photosynthesis leads to environmental balance is the terrarium. A sealed jar of carefully balanced photosynthesizing organisms can sustain themselves for long periods without exposure to outside material nutrients or gases. Such a microbial terrarium can keep its ecological balance nearly indefinitely without any care or maintenance.

The secret to this self-sufficiency is that green or purple photosynthesizing organisms generate their own source of life from the energy in light. This ability allows them to divide and multiply in a stable manner. NASA's "Life on the Edge" project tests some of the limits to this remarkable behavior. By closing several green biomass mixtures into sealed jars, these organisms are frozen within a deep freezer to a frigid -80 deg C (-112 deg F), temperatures exceeding the coldest winter weather in Antarctica (-44.5 deg C, or -48 deg F). Afterwards, the jars are thawed and opened, and scientists then grow the organisms in a culture to assess their viability. Healthy growing ecosystems have been revived from this ultimate deep freeze.

Life In A Bottle

Left shows a closed terrarium that stabilizes a natural equilibrium between photosynthetic microbial populations. The nutrient supply comes from the breakdown of residual biological matter (twigs and swamp grasses shown) beginning with microbrial sunlight energy conversion. After 5 months of sunlight exposure, a small sample is taken out and put in a sealed container, which is then frozen for two months. When it thaws, it is placed back in the sun. A sample that turns greenish-brown indicates photosynthetic activity - life rejuvenated from solar energy and Photosystem I proteins in the microbes. Right shows the rejuvenation of dilute photosynthetic populations following freezing.

One task for NASA's Life on the Edge Project is to demonstrate how a system closed to nutrients but open to sunlight energy can be self-sustaining. By looking at both extreme-loving and adaptable microbes in harsh environmental locations, the researchers hope to gain further insight into how far the environmental envelope can be pushed to still sustain life. This may help us determine whether life can remain viable after long exposure to the harsh galactic conditions of outer space.

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Information

Principal investigator

Prof. Horst Tobias Witt , Technische Universität Berlin, Max-Volmer-Institut für Biophysikalische Chemie und Biochemie, Berlin, Germany

Co-investigators

Biochemistry: Petra Fromme, Max-Vollmer-Institut at the Technische Universität Berlin, Germany; X-ray crystallography is done in the Institut für Kristallographie at the Freie Universität Berlin in the group of Prof. Wolfram Saenger; Olaf Klukas, Patrick Jordan, Norbert Krauß, Wolf-Dieter Schubert

References

Life and Microgravity Sciences (LMS) Space: Final Report, February 1998, NASA Marshall Space Flight Center, Huntsville, AL. compiled, J. P. Downey. NASA CP-1998-206960

Schubert, W.-D., Klukas, O., Krauß, N., Saenger, W., Fromme, P., Witt, H.T., Photosystem I of Synechococcus elongatus at 4 Å Resolution: Comprehensive structure analysis., J. Mol. Biol. 272, 741-769, 1997

Further Readings

de Lucas, Larry, et al. 1989. Protein crystal growth in microgravity, Science, 246: 651 (1989)
Krauß, N., Schubert, W.-D., Klukas, O., Fromme, P., Witt, H.T., Saenger, W., Photosystem I at 4 Å resolution represents the first structural model of a joint photosyn thetic reaction center and core antenna system., Nature Struct. Biol. 3, 965-973, 1996.


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