ch369_f09_enz_2_notes -...

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Today is the second of 3 lectures on enzymes. (chapter 6 & 7 of text)
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Chymotrypsin - catalyzes peptide bond hydrolysis. Active site of chymotrypsin is in red. Asp Ser His
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Mechanism, 1st step: Histidine extracts proton from serine:
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Serine attacks peptide:
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Enzyme active sites are micro-environments where intermediates can be stabilized that would  not be stable in water. Tetrahedral intermediate
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Tetrahedral intermediate stabilized in  chymotrypsin active site. By stabilizing the tetrahedral intermediate, chymotrypsin lowers the activation energy  for the reaction.  So instead of taking years to cleave a peptide bond, chymotrypsin can  cleave 190 peptides per second. uncatalyzed reaction catalyzed  reaction
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Look one last time at the first step of chymotrypsin mechanism.
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At pH 7, you would  not  expect Histidine to remove a proton from Serine (because pKa  of His is 6, pKa of serine is 12). But pKa’s of amino acids can have unusual pKa’s in enzyme active sites !
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A nice example of how local structure can alter pKa’s pKa = 10 10 9 9 10 10 8.5 10 10 4.7
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A nice example of how local structure can alter pKa’s pKa = 10 10 9 9 10 10 8.5 10 10 4.7 Having the amino groups close together alters the pKa (3 positive  charges close together are unstable).
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Ser His In the chymotrypsin catalytic triad, the negative charge on Asp stabilizes  the positively charged Histidine. This allows the Histidine to accept a proton from Serine in the catalytic 
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This note was uploaded on 01/21/2010 for the course CH 369 taught by Professor Kbrowning during the Fall '07 term at University of Texas at Austin.

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ch369_f09_enz_2_notes -...

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