Protein Crystal Growth 31 Objectives

This area of space biotechnology has both a basic science component and an applied science component. The basic science component seeks to understand the fundamental physics and chemistry of macromolecular crystal growth by utilizing microgravity to study aspects of the crystal growth process that are masked by gravity on Earth. The applied science component of the program uses microgravity to produce higher quality crystals that are subsequently used in ground research to produce more detailed and more accurate atomic structures of macromolecules. Examples of this research include structural biology research, biological nanotechnology, and biomolecular self-assembling materials.

Biological macromolecules such as proteins, enzymes and viruses play a key role in the complex machinery of life. They possess active sites which make them bind or interact with other molecules in a very specific manner that determines their biological function. They intervene in the regulation, reproduction and maintenance mechanisms of living organisms, and they can be the cause of diseases and disorders. Pharmaceutical drugs are molecules that inhibit the active sites of macromolecules and, in principle, are intended to affect only the targeted macromolecule.

The vast majority of current drugs are the result of systematic testing, first at molecular level, then at a clinical level. This extensive process significantly increases the cost of the product. With a detailed knowledge of the 3D structure of a macromolecule, biochemists can restrict the range of drugs to be tested. Furthermore, with a rational drug design approach, one may attempt to synthesize a drug targeted exclusively on a specific macromolecule. That means a drug will perfectly bind to the macromolecule and inhibit its biological function while remaining inert vis-à-vis other macromolecules.

The 3D structure of the macromolecule can be discovered through the analysis of crystals by X-ray diffraction: X-rays are passed through a single crystal at various angles. The resulting diffraction patterns are analyzed using computers to estimate the size, shape, and structure of the molecule. A flawed crystal will yield a blurry and/or weak diffraction pattern, whereas a well-ordered crystal will yield a sharp and/or strong diffraction pattern and thus useful information about the structure. So, the better the quality of the crystal, the faster and the more accurate the determination of the structure and the faster the identification of a drug (Binot 1998).

During the 1990's, there was explosive growth in the number and complexity of macromolecular structures being determined by X-ray crystallography, as evidenced by the exponential increase in the number of structures published and submitted to the Protein Data Bank'. This growth has been made possible by the convergence of a large number of new technologies, including the following:

a. Improved systems for cloning and expressing wild-type and mutant proteins;

b. Improved protein and nucleic acid purification techniques;

1 The Protein Data Bank is a repository for 3D structural data of proteins and nucleic acids. This data, typically obtained by X-ray crystallography or NMR spectroscopy, is submitted by biologists and biochemists from around the world, is released into the public domain, and can be accessed for free at http://www.wwpdb.org/

c. Immortalization of crystals by cryogenic freezing;

d. Very high brilliance X-ray synchrotron sources;

e. Fast, accurate area detectors with high dynamic range;

f. Super fast, inexpensive computers;

g. Readily available software packages for data acquisition and reduction, phasing, and refinement.

For the most part, however, protein crystallization is done in much the same trial-and-error manner it was a decade ago, although easier and faster since the introduction of reagent kits and the growing use of automated systems. It is still more art than science. One of the main goals of crystallization in microgravity has been the growth of crystals in space that are of better quality than those available on the ground (National Research Council 2000).

Figure 8-05. View of a bubble formed as a result of a Zeolite Crystal Growth experiment in the Destiny laboratory on the International Space

Station. This experiment has shown that the bubbles could cause larger number of smaller deformed crystals to grow. Photo courtesy of NASA.

Figure 8-05. View of a bubble formed as a result of a Zeolite Crystal Growth experiment in the Destiny laboratory on the International Space

Station. This experiment has shown that the bubbles could cause larger number of smaller deformed crystals to grow. Photo courtesy of NASA.

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