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4 Problems

3 . Describe the role of each member of the "triad" in the mechanism of chymostrypsin.

4. Some enzymes show a bell shaped curve for enzyme activity versus pH. What conclusions can you reach about the nature and the function of groups in the enzyme? Name an enzyme which exhibits this characteristic.

7. In 1935, two famous scientists, J.B. S. Haldane and in J. G. Priestley, posed the following puzzle. Carbon monoxide binds to the same heme sites on hemoglobin as oxygen with an affinity 200 times as great as that for oxygen. Exposure for 1 hour to a carbon monoxide concentration of 0.1% of inspired air leads to an occupancy by CO of about 50% of the heme sites in hemoglobin, a proportion that is frequently fatal. Yet, a person whose hemoglobin concentration is reduced to 50% due to anemia may go about work as usual. What is the key to this seeming paradox using our present day knowledge of hemoglobin?

9. Draw the Fisher and Haworth projections and the chair conformation of

α-D-glucose, α-D-galactose and the 2 epimer of β-D-glucose.

Solution Preview

3. Describe the role of each member of the "triad" in the mechanism of chymostrypsin.

Response:
The three invariant residues, Ser-195, His-57, and Asp-102 form a "charge relay system." In this class of serine proteases which includes chymotrypsin, the reactive hydroxyl group of Ser-195 acts as a nucleophile attacking the carbonyl carbon of the substrate. In so doing, a tetrahedral enzyme-acyl complex is formed as an intermediate in the enzymatic mechanism. This attack is facilitated by the other two residues, His-57 and Asp-102.

Essentially, here's what happens. Asp-102 exists in the ionized form as COO-. (If the pH gets too low, i.e. below the pKa of this residue, activity drops off.) This group acts as a general-base catalyst by picking off the NH hydrogen on the imidazole ring of His-57. As a result, the other nitrogen on the ring becomes a much stronger nucleophile, which in turn acts as a general-base catalyst by pulling off the hydroxyl hydrogen of Ser-195. You see, it's a "charge relay system."

All of this happens at once. The end result? Ser-195 becomes deprotonated, i.e. ionized, and as a result becomes a very strong nucleophile to attack the carbonyl carbon of the substrate.

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4. Some enzymes show a bell shaped curve for enzyme activity versus pH. What conclusions can you reach about the nature and the function of groups in the enzyme? Name an enzyme which exhibits this characteristic.

Response:
What is the underlying reason why enzyme activity profiles exist the classic "bell curve"? It has to do with pKas and amino acid residue ionization. As we've just explained in the chymotrypsin example above, amino acid residues on enzymes, especially the ...

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