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2009-09-16_-_Lecture_6 - Biochemistry 461 FS 2009 Michigan...

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Biochemistry 461 FS 2009 Michigan State University Proteins: Three-Dimensional Structure September 16, 2009 Lecture 6 In a β sheet , the polypeptide chain takes on an extended conformation ( β strand ), and hydrogen bonds are formed between adjacent strands. The peptide planes give the β sheet a pleated shape Sheets can be antiparallel or parallel in terms of the N-to-C backbone : In either case, the side chains are directed above and below the plane of the sheet. Side chain of residue i points up, Side chain of residue i+1 point down Secondary Structure: The β Sheet (from previous lecture) 2
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Secondary Structure: The β Sheet (from previous lecture) The figure on the left nicely demonstrates the “pleating” of the peptide planes The figure on the right demonstrates that β sheets can stack on top of one another 3 Parallel β -strands Secondary Structure: The β Sheet (from previous lecture) Each residue H-bonds with two different residues Antiparallel β -strands Each residue H-bonds with only one different residue 4 In both parallel and antiparallel β sheets, the NH and CO of same residue are aligned Which structure is more stable? Why?
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Connecting Secondary Structure: β Turns Characteristics of β turns: Connect the ends of two adjacent strands in a β sheet Involves a 180° turn C=O of first residue forms H-bond with N-H of 4th residue Gly and Pro are common in β turns -- Why? 5 Best Method: NMR Chemical Shifts correlate with φ,ψ H-bonds slow down H-D exchange 2 nd Best Method: Circular Dichroism (CD) Differential absorption of left- and right- handed circularly polarized light.
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