Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymes are Wonderful Catalysts 49
Problems
(1) Rationalise the rate accelerations observed for the phenyl esters (8) and (9)
in Figure 3.5 which contain tertiary amine groups. What type of catalysis is
operating in this case
(2) Explain why the hydrolysis of the substituted benzoic acid (A) at pH 4 is
1000-fold faster than the hydrolysis of the corresponding methyl ester (B)
under the same conditions.
O
O
Ph
pH 4
buffer
OH
OH
+ PhCHO
CO 2 R
(A) R = H
(B) R = Me
CO 2 R
(3) (a) Incubation of compound (C) under alkaline conditions leads to a
unimolecular reaction of rate constant 7:3 10 2 s 1 to give a bicyclic
product – suggest a structure. Incubation of compound (D) with phenoxide
(PhO ) leads to a bimolecular reaction of rate constant 10 6 m 1 s 1 . Work
out the eVective concentration of phenoxide in (C) and comment on the rate
acceleration observed.
R
O
(C) R = OH
(D) R = H
(3) (b) Epoxide hydrolase catalyses the hydrolysis of a wide range of
epoxide substrates. Its active site contains an aspartate residue which is
essential for catalytic activity. Given that the enzyme is most active at pH
8–9, propose two possible mechanisms for the enzymatic reaction, and
suggest how you might distinguish them experimentally. Comparing the
k cat of 1.1 s 1 for epoxide hydrolase with the rate constant for reaction of
(C) above, suggest how the enzyme might achieve its additional rate acceleration.
(4) Incubation of haloalkane dehalogenase (see Figure 3.19) with substrate
under multiple turnover conditions in H 18
2
O leads to incorporation of one
atom of 18 O into the alcohol product. However, incubation of a large
quantity of enzyme with less than one equivalent of substrate in H 18
2 O
revealed that the product contained no 18 O. Explain this observation.