5. You discover a new cysteine protease similar to papain. Cysteine proteases are proteolytic enzymes that utilize a cysteine residue in the active site for the nucleophilic attack of a peptide bond, while a second residue acts as a base for proton abstraction in the reaction (acid base catalysis). In this novel enzyme the pKa of the cysteine residue is significantly lowered by the active site environment to pKa-4. a. Knowing that the protease shows highest activity between pH 4-6, what could be the identity of the base residue acting as a in the reaction? Explain and write the expected mechanism for the reaction. S.

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Chapter1: Biochemistry: An Evolving Science
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5. You discover a new cysteine protease similar to papain. Cysteine proteases are
proteolytic enzymes that utilize a cysteine residue in the active site for the nucleophilic attack
of a peptide bond, while a second residue acts as a base for proton abstraction in the reaction
(acid base catalysis). In this novel enzyme the pKa of the cysteine residue is significantly
lowered by the active site environment to pKa=4.
a. Knowing that the protease shows highest activity between pH 4-6, what could be the
identity of the base residue acting as a in the reaction? Explain and write the expected
mechanism for the reaction.
SH
он
Transcribed Image Text:5. You discover a new cysteine protease similar to papain. Cysteine proteases are proteolytic enzymes that utilize a cysteine residue in the active site for the nucleophilic attack of a peptide bond, while a second residue acts as a base for proton abstraction in the reaction (acid base catalysis). In this novel enzyme the pKa of the cysteine residue is significantly lowered by the active site environment to pKa=4. a. Knowing that the protease shows highest activity between pH 4-6, what could be the identity of the base residue acting as a in the reaction? Explain and write the expected mechanism for the reaction. SH он
b. Assuming that the optimal pH for the reaction strictly depends exclusively on the
protonation state of the active site residues at which pH do you expect maximal enzymatic
activity? Explain your rationale.
His
CH,
H-N NH
Inactive
PK.
His
Cys
H-NNH
Active
His
CH,
EN NH
Inactive
c. What is the fraction of deprotonated Cys and protonated B at the calculated optimum
pH?
d. How these fractions are changing at 1 pH unit below the optimal pH? Which fraction of the
enzyme will be active at this pH?
Transcribed Image Text:b. Assuming that the optimal pH for the reaction strictly depends exclusively on the protonation state of the active site residues at which pH do you expect maximal enzymatic activity? Explain your rationale. His CH, H-N NH Inactive PK. His Cys H-NNH Active His CH, EN NH Inactive c. What is the fraction of deprotonated Cys and protonated B at the calculated optimum pH? d. How these fractions are changing at 1 pH unit below the optimal pH? Which fraction of the enzyme will be active at this pH?
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