Proteins are polymers of amino acids that are synthesized in the cell. Proteins perform a variety of functions in nature. Proteins that catalyze chemical reactions are called enzymes. Examples of enzymes are trypsin, which digests proteins in the food you eat and thrombin, which is involved in blood clot formation. Proteins are also components of receptors on cell surfaces that bind hormone molecules from the blood stream, resulting in changes in cell behavior. Proteins are important in our defense mechanism as antibodies that recognize and bind to foreign materials that enter our systems. The structural proteins actin and myosin form muscles and are involved in contractile processes of motion, heartbeat, and breathing. Proteins are also found in our cell membranes and in the coat of viruses.
Many protein structures have been determined and are available on the World Wide Web (WWW) at the Protein Data Bank http://www.pdb.bnl.gov. The three dimensional structures of these proteins can be studied in depth using a free visualization tool called Rasmol which is also available at no charge on the WWW http://www.umass.edu/microbio/rasmol/getras.htm.