Section 24.9 Enzyme-Catalyzed Reactions 1025345
pHEnzyme activity678pK pKa^ = 6.7
a^ = 3.8>Figure 24.10
Dependence of lysozyme activity on
the pH of the reaction mixture.The given by the ascending leg is the of a group that is catalytically active
in its basic form. When that group is fully protonated, the enzyme is not active. As the
pH of the reaction mixture increases, a larger fraction of the group is present in its basic
form, and as a result, the enzyme shows increasing activity. Similarly, the given by
the descending leg is the of a group that is catalytically active in its acidic form.
Maximum catalytic activity occurs when the group is fully protonated; activity decreas-
es with increasing pH because a greater fraction of the group lacks a proton.
From the lysozyme mechanism shown in Figure 24.9, we can conclude that Asp 52
is the group with a of 3.8 and Glu 35 is the group with a of 6.7. The pH–
activity profile indicates that lysozyme is maximally active when Asp 52 is in its basic
form and Glu 35 is in its acidic form.
Table 23.2 shows that the of aspartic acid is 3.86 and the of glutamic acid is
4.25. The of Asp 52 agrees with the of aspartic acid, but the of Glu 35 is
much greater than the of glutamic acid. Why is the of the glutamic acid residue
at the active site of the enzyme so much greater than the given for glutamic acid in
the table? The values in the table were determined in water. In the enzyme, Asp 52
is surrounded by polar groups, which means that its should be close to the de-
termined in water, a polar solvent. Glu 35, however, is in a predominantly nonpolar mi-
croenvironment, so its should be greater than the determined in water. We have
seen that the of a carboxylic acid is greater in a nonpolar solvent because there is
less tendency to form charged species in nonpolar solvents (Section 10.10).
Part of the catalytic efficiency of lysozyme results from its ability to provide differ-
ent solvent environments at the active site. This allows one catalytic group to exist in
its acidic form at the same surrounding pH at which a second catalytic group exists in
its basic form. This property is unique to enzymes; chemists cannot provide different
solvent environments for different parts of nonenzymatic systems.
PROBLEM 20When apples that have been cut are exposed to oxygen, an enzyme-catalyzed reaction
causes them to turn brown. They can be prevented from turning brown by coating them
with lemon juice Explain why this is so.Mechanism for Glucose-6-phosphate Isomerase
Glycolysisis the name given to the series of enzyme-catalyzed reactions responsible
for converting D-glucose into two molecules of pyruvate (Sections 19.21 and 25.1).
The second reaction in glycolysis is an isomerization reaction that converts D-glucose-
6-phosphate to D-fructose-6-phosphate. Recall that the open-chain form of glucose is
an aldohexose, whereas the open-chain form of fructose is a ketohexose. Therefore,
the enzyme that catalyzes this reaction—glucose-6-phosphate isomerase—converts an
aldose to a ketose (Section 22.1). Because, in solution, the sugars exist predominantly
1 pH='3.5 2.pKapKa pKapKa pKapKapKapKa pKapKa pKa pKapKa pKapKa pKapKapKapKa pKa