NameID #Chem 135 Midterm 3 Fall 20121. (15 points) Papain (from papayas) catalyzes the hydrolysis of peptide bonds and belongs tothe family of proteins known as the cysteine proteases. The essential residues at the activesite of the enzyme are a protonated His and an unprotonated Cys. The enzyme has a bellshaped pH-rate profile with the midpoint of the rising leg at a pH of 4.2 and the midpoint of
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Name
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Chem 135 Midterm 3 Fall 2012
1. (15 points) Papain (from papayas) catalyzes the hydrolysis of peptide bonds and belongs to
the family of proteins known as the cysteine proteases. The essential residues at the active
site of the enzyme are a protonated His and an unprotonated Cys. The enzyme has a bellshaped pH-rate profile with the midpoint of the rising leg at a pH of 4.2 and the midpoint of
the falling leg at a pH of 8.2. Based on the fact that a thioester intermediate forms along the
reaction pathway, draw the reaction mechanism of papain catalyzed peptide bond hydrolysis,
depict a reaction coordinate diagram consistent with this mechanism, and assign pKa values
to the essential residues.
Solution:
N NH
E
H
R N
O
R'
H
E S
R NH
O S
R'
E N NH
E
H
R S
O
E
H
O
H
R'NH2
N NH
E
R S
O O
H
E N
NH
E
H
RCO2H
E S + N NH
E
H
The cysteine residue has pKa a value of 4.2 (since it is unprotonated at the beginning of
catalysis) while the histidine has a pKa value of 8.2.
ΔG
Reaction Coordinate
2. a) (9 points) Draw the structure of the glycerophosphplipid phosphatidyl choline, in
which a 14:0 fatty acid is esterified to the sn1 position and a 12-hydroxylated version
of an 18:1 ω9 fatty acid is esterified to the sn2 position.
Solution:
O
O
O
OH O
O P
O
O
O
N(CH3)3
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