Organic Chemistry

Section 24.9 Enzyme-Catalyzed Reactions 1027

PROBLEM 21

When D-glucose undergoes isomerization in the absence of the enzyme, three products re-

sult:D-glucose,D-fructose, and D-mannose (Section 22.5). Why is D-mannose not formed

in the enzyme-catalyzed reaction?

PROBLEM 22

The descending leg of the pH–rate profile for glucose-6-phosphate isomerase indicates

that one of the amino acid side chains at the active site of the enzyme has a value of

9.3. Identify the amino acid side chain.

Mechanism for Aldolase
The substrate for the first enzyme-catalyzed reaction of glycolysis is a six-carbon
compound (D-glucose). The final product of glycolysis is two molecules of a
three-carbon compound (pyruvate). Therefore, at some point in the series of en-
zyme-catalyzed reactions, a six-carbon compound must be cleaved into two three-
carbon compounds. The enzyme aldolasecatalyzes this cleavage (Figure 24.12).
Aldolase converts D-fructose-1,6-diphosphate into D-glyceraldehyde-3-phosphate
and dihydroxyacetone phosphate. The enzyme is called aldolase because the
reverse reaction is an aldol addition reaction (Section 19.13). The reaction pro-
ceeds as follows:

• In the first step of the aldolase-catalyzed reaction, D-fructose-1,6-diphos-
  phate forms an imine, with a lysine residue at the active site of the enzyme
  (Section 18.6).


• A tyrosine residue functions as a general-base catalyst in the step that cleaves the
  bond between C-3 and C-4. The molecule of D-glyceraldehyde-3-phosphate
  formed in this step dissociates from the enzyme.


• The enamine intermediate rearranges to an imine, with the tyrosine residue now
  functioning as a general-acid catalyst.


• Hydrolysis of the imine releases dihydroxyacetone phosphate, the other three-
  carbon product.

PROBLEM 23

Propose a mechanism for the aldolase-catalyzed cleavage of D-fructose-1,6-diphos-

phate if it did not form an imine with the substrate. What is the advantage gained by

imine formation?

PROBLEM 24

In glycolysis, why must D-glucose-6-phosphate isomerize to D-fructose-6-phosphate

before the cleavage reaction with aldolase occurs? (See p. 1026.)

PROBLEM 25

Aldolase shows no activity if it is incubated with iodoacetic acid before D-fructose-

1,6-diphosphate is added to the reaction mixture. Suggest what could cause the loss of

activity.

pKa

3-D Molecule:

Aldolase