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17 - Monday October 4 2010 Lecture 17 Announcements 1...

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Monday, October 4, 2010, Lecture 17 Announcements: 1. Reading and problem assignments for this week, as in LG plus reading in text 220- 2. PyMOL assignment # 6, Chymotrypsin. This is the one and only PyMOL assignment that will appear on the midterm exam. Note: no office hrs or reviews for this PyMOL until Sun 10/17! 3. Conflict for the midterm exam Th 10/21 7:30PM ? e-mail prof before Fall Break to describe your conflict. Meet briefly after class on Wed 10/6 to set the day and time. Day will probably be Sat 10/23 or Sun 10/24. Check your calendar now so that you can be ready on Wed to give your input re the make-up date. Friday's lecture: turnover number, and the concept of enzyme "efficiency" in lowering G o of TS use of pH to "inhibit" enzyme activity and find ionizable residues that might be involved in catalysis. competitive and non-competitive inhibition irreversible enzyme inhibition Today's lecture: In this course, we will consider the detailed, step-by-step mechanism of only one enzyme: chymotrypsin. This enzyme belongs to a large family of enzymes, the serine proteases . All of these enzymes have serine at the active site. Not only do many peptide bond-cleaving enzymes have a Ser at the active site, but moreover many use the same mechanism : trypsin , chymotrypsin and elastase are digestive enzymes; thrombin , part of the blood clotting mechanism, and plasmin , part of the clot dissolving process are two more serine proteases. There are others as well. X-ray diffraction shows that this family of enzymes all have a grouping of Ser-His- Asp at the active site. These residues are called the catalytic triad . The Ser, His, and Asp are not consecutive in the AA sequence, but instead are placed next to each other by the protein’s fold. The catalytic triad lowers the G o of every step of catalysis that involves the transfer of protons . It also helps to stabilize the transition state. The Ser and the His are directly involved in the catalytic steps. In contrast, the Asp has a different role: mainly to hold the His and the Ser in a precise alignment for rxn catalysis. Remember: the TS is an unlikely, improbable juxtaposition of reactants. With the enzyme holding some reactants in precise orientations, some of the unlikeliness is gone. p. 126
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17 - Monday October 4 2010 Lecture 17 Announcements 1...

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