Lecture 19 - Friday October 6 2006 Lecture 19 Announcements...

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Friday, October 6, 2006 Lecture 19 Announcements : 1. Quiz 5 results: A 20.0 B 20.8 C 20.3 2. RasMol Reviews: Sun 10/15 1 – 3PM in Carpenter Hall Red Computer Lab; and Tu 10/17 4:30 - 5:30PM in Racker Room, Biotech ground floor 3. Midterm exam: Kennedy Auditorium or Warren B45, Thursday 10/19, 7:30 - 10PM Make-up exam: Sat 10/21, 3M – 5:30PM ( e-mail Prof with conflict ). Room TBA 4. How to study for the midterm exam: a. Last year’s exam is on our web site (Blackboard). Don’t look yet at the key! Take the exam after you are 3/4 done with studying, and only then look at the key. b. Be able to answer every question from the first 5 quizzes. c. Understand every word from every lecture. d. Use the text as a supplement. 5. Internships abroad in health, education, social work: Th 10/12 4:45PM in Goldwin Smith Kaugmann Auditorium. Wednesday's lecture: Chymotrypsin step-by-step mechanism Today's lecture: Note about the small curved arrows that indicate electron flow: think of these electron flows as leading to the next intermediate . p. 132 For the family of enzymes known as the Serine proteases, here is the overall reaction equation for the catalyzed reaction: E + S ES ES' ES'' ES''' EP 2 E + P 2 P 1 The rate of enhancement over the uncatalyzed hydrolysis of peptide is about 10 10 . What if we mutate either Ser195 or His57? You might imagine that catalysis would be completely ruined. Indeed, the rate enhancement drops to about 10 5 . This is a big drop of about 100,000-fold, but there is still impressive catalytic power left! Catalytic
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power still comes from the binding, hence stabilization, of the tetrahedral transition state. A summary of some key catalytic features of chymotrypsin: Ser195-O: binds to the peptide carbonyl C (nucleophilic attack) Ð Peptide NH of Ser195 and Gly193 H-bond to the O - of the tetrahedral transition state (and tetrahedral intermediates) Enzyme shape fits the tetrahedral intermediate, thereby especially lowering TS free energy. In other words, the enzyme is specialized to fit the TS especially well, NOT to fit the substrate well.
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