Understanding proteins is basic to understanding the processes of living things. While we know the chemical formulae of proteins, learning the chemical structure of these macromolecules is more di cult. Mapping the three- dimensional structure of proteins, DNA, ribonucleic acid (RNA), carbohydrates, and viruses provides information concerning their functions and behavior. This knowledge is fundamental to the emerging eld of rational drug design, replacing the trial-and-error method of drug development. Microgravity provides a unique environment for growing crystals, an environment that is free of the gravitational properties that can crush the delicate structures of crystals. Currently, several test facilities are used to grow crystals.
The Advanced Protein Crystallization Facility (APCF) can support three crystal- growth methods: liquid-liquid di usion, vapor di usion, and dialysis. Liquid-liquid di usion was not used during Expedition 3. In the vapor di usion method, a crystal forms in a protein solution as a precipitant draws moisture in a surrounding reservoir. In the dialysis method, salt draws moisture away from the protein solution via a membrane separating the two, forming crystals. ESA has announced that due to potential di culties with the vapor di usion method that could cause experiment failure, it will no longer propose the use of this method with the APCF.
APCF-Lysozyme was one of eight protein crystal investigations that was conducted in the Advanced Protein Crystallization Facility onboard the ISS during Expedition 3. Lysozymes are produced by plants and higher organisms as a rst line of defense against harmful bacteria. Lysozymes are also used by bacterial viruses to facilitate infection or to release virions (a complete virus particle that consists of an RNA or DNA core with a protein coat sometimes with external envelopes and that is the extracellular infective form of a virus) at the end of an infection cycle. The structure of a protein and its function are directly linked. The study of high- quality lysozyme crystals will help scientists learn more about its structure and thus more about its role in bacterial infection.