12 - Wednesday, September 22, 2010 Lecture 12 Announcements...

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Wednesday, September 22, 2010 Lecture 12 Announcements 1. The PyMOL exercise for this week is #4: Hemoglobin. 2. Prof. Feigenson will meet with any 3310 students who would like to discuss “Applying to graduate school-- what are they looking for?", this Friday, 9/24 from 2:55 – 4PM in Comstock B108. 3. Volunteer medical work over Winter Break? Check out one of the these 3 info sessions for Medlife: 5 PM in Rockefeller 122 on Th 9/23, Tu 9/28, or Tu 10/5. See www.medlifeweb.org/ for more information. Monday’s lecture: Comparison of Mb and Hb structure; Introduction to the sigmoid curve; Ion pairs stabilize the deoxy form of Hb, but NOT the oxy form Today's lecture: We can also turn to a general picture of why IV structure can lead to “modified action” (allostery and cooperativity) for Hb, and also for the many other proteins that show this "structure switching" behavior: 1. Think of Mb single chain as a “device” that evolved to bind oxygen. It does a great job of binding oxygen, perfected over hundreds of millions of years of evolution. 2. When this single polypeptide chain binds oxygen, small changes occur in the 3-d structure. For Mb, these small changes have no influence - they pose no constraint. The small changes in structure occur throughout the Mb, but there is no “resistance” to these small changes. Now, what happens if you bring four of these “O 2 -binding devices” together and stick them tightly to each other? The four devices come together in a quaternary structure that optimizes the subunit contacts . This is an important principle! The IV structure is more stable the tighter the binding of the subunits to each other. In the case of Hb, we notice especially the 8 ion pairs and the ion pairs to BPG. This IV structure, with strong, optimized subunit contacts, is called the tight, or T state (also termed "tense"). Any change in this state is strongly resisted by numerous interactions. 3. When in this T-state, the numerous small 3-d changes that occur as a result of oxygen binding are resisted by the contacts between the chains. The small changes
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occur throughout the 3d structure when oxygen binds, but at the subunit interfaces any such small changes are resisted by the strong binding. 4.
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12 - Wednesday, September 22, 2010 Lecture 12 Announcements...

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