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Show 79 college of science It is widely believed that motions in both the enzyme and the substrate are required for catalysis to take place. The nature and extent of these motions are not fully understood, however, and may differ among enzymes. We are studying these motions using a serine protease and protease inhibitor as a model for the enzyme-substrate complexes. The serine proteases are a large family of enzymes that degrade other proteins. Their functions in na-ture include the digestion of food and the precise regulation of processes such as blood clotting. One way organisms regulate serine proteases is through producing protein inhibitors that specifically bind tightly to active sites. The bound inhibitors mimic substrates but are remarkably resistant to digestion, and rigidity of the inhibitor is usually assumed to play a role in blocking the catalytic reaction. Study-ing motions within these enzyme-inhibitor complexes may provide insight into the motions required for successful catalysis in normal substrates. Trypsin and bovine pancreatic trypsin inhibitor (BPTI) are among the best-studied proteases and protease inhibitors, respectively. The flexibility of BPTI-protease complexes was studied by comparing crystal structures of closely related complexes in different crystal environments, in which the proteins might be captured in different conformations. 48 different structures were examined in 1,128 pair wise comparisons. From these comparisons, substantial differences were found to suggest BPTI does indeed have an intrinsic flexibility when bound to trypsin. The portion of the BPTI directly in contact with trypsin at the binding site is relatively rigid. Away from the binding site, BPTI exhibits a hinge-bending motion. The BPTI swings back and forth while remaining stiff at the binding site to trypsin, much like the way a door will swing on its hinges. We hypothesize that this motion described is likely related to those necessary for the enzyme to eventually digest and inactivate the inhibitor. Differences in these motions may modulate the activi-ties of different natural inhibitors and could provide a means of designing novel inhibitors for specific biochemical or therapeutic purposes. INTRINSIC FLEXIBILITY OF BPTI IN COMPLEX WITH SERINE PROTEASE Chae Kyung Yoo (David P. Goldenberg) Department of Biology University of Utah UNDERGRADUATE RESEARCH ABSTRACTS Chae Kyung Yoo David P. Goldenberg |