Organic Chemistry

Problems 1031

34. There are two kinds of aldolases. Class I aldolases are found in animals and plants; Class II aldolases are found in fungi, algae, and
    some bacteria. Only Class I aldolases form an imine. Class II aldolases have a metal ion at the active site. The mechanism
    for catalysis by Class I aldolases was given in Section 24.9. Propose a mechanism for catalysis by Class II aldolases.


35. Propose a mechanism for the following reaction. (Hint: The rate of the reaction is much slower if the nitrogen atom is replaced
    by CH.)


36. The hydrolysis of the ester shown here is catalyzed by morpholine, a secondary amine. Propose a mechanism for this reaction.
    (Hint:The of the conjugate acid of morpholine is 9.3, so morpholine is too weak a base to function as a general-base catalyst
    in this reaction.)


37. The enzyme carbonic anhydrase catalyzes the conversion of carbon dioxide into bicarbonate ion (Section 1.20). It is a
    metalloenzyme, with coordinated at the active site by three histidine residues. Propose a mechanism for this reaction.


38. At the rate of hydrolysis of ester A is greater than the rate of hydrolysis of ester B. At the relative rates reverse.
    (Ester B hydrolyzes faster than ester A.) Explain these observations.


39. 2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific; the
    stereoisomers obtained as products depend on the stereoisomer used as a reactant. Explain the following observations:
    a. Both cis reactants form an optically active trans product, but each cis reactant forms a different trans product.
    b. Both trans reactants form the same racemic mixture.
    c. A trans reactant is more reactive than a cis reactant.

40.Staphylococcusnuclease is an enzyme that catalyzes the hydrolysis of DNA. The overall hydrolysis reaction is as follows:

Recall that the nucleotides in DNA have phosphodiester linkages. The reaction is catalyzed by Glu 43, and Arg 87. Propose a

mechanism for this reaction.

Ca^2 +,

H 2 O RO P OR

O−

O

RO P OH ROH

O−

O

+ +

CH 3 CO−

2-acetoxycyclohexyl tosylate

O

O OTs

CO

CH 3

1,2-cyclohexanediol diacetate

OO

CO

CH 3

CO

CH 3

+

CH 3

CH 3

AB

O

COCH 2 CH 2 NCH 3

CH 3

CH 3

O

COCH 2 CH 2 N

pH=12, pH=8,

CO 2 + H 2 O HCO 3 − + H+

carbonic anhydrase

Zn^2 +

H 2 O

O

N

H

+ CH 3 OH

CH

COCH 3

O

O

CH

CO−

O

O

morpholine

pKa

CH 3 N

HO−

H 2 O CH^3 N

Cl OH

Cl OH

1 Zn^2 + 2