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28 Cards in this Set
- Front
- Back
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When the substrate closely approaches the catalytic site with proper orientation what happens to the enzyme conformation?
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the enzyme conformation probably changes to give a strained E-S complex
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A hydrophobic pocket in the enzyme has what effect on E and S interaction?
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it lowers the surrounding dielectric constant allowing electrostatic interaction between E and S.
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What is electrostatic interaction?
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noncovalent interaction between oppositely charged atoms or groups
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Describe acid-base catalysis of enzymes
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Chemical groups groups can often be made more reactive by adding or removing a proton. Enzyme side chains act as proton donors and acceptors.
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Describe covalent catalysis of enzymes
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in some enzymes a nucleophilic side chain forms a unstable covalent bond to the substrate eg. serine proteases
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chymotripsin (CT) catalyzes the hydrolysis of what?
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peptide bonds next to aromatic side chains
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the active site on CT involves what major residue?
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serine 195, CT is a serine protease
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How was the presence of serine in CT determined?
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by labeling the serine with diisopropylfluorophosphate (DFP).DFP irreversibly inhibits serine so that the enzyme is no longer functional
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Ser 195 is close to what other residue in the active site?
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His 57
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What effect does His 57 have on Ser 195?
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It acts as a general base catalyst converting the serine to its anion and making it a better nucleophile for attack at the carbonyl carbon to be hydrolyzed.
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the alignment of the active site is essential for what?
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normal function of protease
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describe the active site of CT
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it consists of Ser, His, and Asp residues that form a triad providing stabilization
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In the initial enzyme-substrate complex of CT how are Asp, His, and Ser arranged?
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they are aligned
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IN the mechanism of CT the aromatic ring of the substrate's phenylalanine residue is seated in a what?
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hydrophobic binding pocket
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IN the initial substrate-enzyme complex of CT, the substrate's peptide bond is hydrogen-bonded to what groups?
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the amide NH groups Ser 195 and Gly 193
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In the intial E-S complex of CT the nucleophilic hydroxyl oxygen of Ser 195 does what?
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it launches a nucleophilic attack on the carbonyl carbon of the substrate
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In the CT mechanism the oxyanion that forms as neg. charge moves to the carbonyl oxygen is stabilized by hydrogen bonds to what?
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the amide NH of ser 195 and gly 193
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What is the first tetrahedral intermediate formed in the mechanism of CT?
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Ser, it is the only covalent bond made between enzyme and substrate
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The tetrahedral intermediate decomposes to form what?
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the covalently bound acyl-enzyme intermediate
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What is believed to facilitate the decomposition of the tetrahedral intermediate?
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His 57 acting as a general acid
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After the first tetrahedral is formed, what happens to the C-N bond in the substrate?
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it is cleaved and the original serine proton transfers to the nitrogen
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When the acyl-enzyme intermediate is formed what happens to a portion of the substrate?
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the amino end is released
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the nucleophilic attack of water on the acyl-enzyme intermediate results in what?
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formation of the second tetrahedral (oxyanion) intermediate. His 57 again serves as a general base catalyst
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What is the final step of the CT mechanism?
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the bond between the serine oxygen and the carbonyl carbon is broken releasing a C-term Phe protein and Ser is again hydrogen bonded to His 57
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All serine proteases require what to function?
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Ca 2+
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metal proteases usually have what?
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Zn
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What makes cysteine proteases different from serine proteases?
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they have cystein instead of serine
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Instead of water cysteine proteases have what?
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S-H
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