Beautiful models of macromolecules such as protems and nucleic acids are generated from files of atomic coordinates obtained usually from x-ray diffraction of crystallized samples or NMR analysis of the molecules in solution. The Protein Data Bank (PDB) is a publicly accessible repository of macromolecular atomic coordinate files that can be accessed online at http://ww.rcsb.org. Access the PDB and familiarize yourself with its homepage. How many molecular structures does it contain today? What is the "Molecule of the Month"? Download a coordinate file for the serine protease chymotrypsin by typing the accession code "lACB" into the search window. This will take you to a page describing the x-ray crystal structure of a complex between bovine alpha-chymotrypsin and the small pseudo-substrate inhibitor protein eglin-c. When and in what journal was the study reporting this structural model published? Click on the "Download File" link, select "PDB File (Text)", and download the file "lACB.pdb". This is an atomic coordinate (.pdb) file that specifies the relative positions for each atom in this protein complex as determined experimentally by x-ray crystallography. Open the file in a text viewer or word processor and look at its format. The first several hundred lines contain background information including the names of the molecules, their natural sources, how they were prepared for the experiment, statistical analysis of the model quality, and bibliographic information. Eventually, you will arrive at a long list of lines that each begin with "ATOM". These are the coordinates, listed by atom number, atom type, amino acid type, and chain number. Each "ATOM" line ends with five numbers representing the atomic position on an x, y, z axis, its "occupancy," and its "thermal factor." Close the file and download software for viewing the molecular model. There are many, such as RasMol, iMol, Swiss-PDB Viewer, and PyMol, that are available for download in a free format for educational purposes. Open the lACB.pdb file and twirl it around in the viewer. Can you identify the protease? The inhibitor protein? Can you find the enzyme's active site? What other observations can you make about serine proteases from the model of this inactivated complex?