Organic Chemistry

1024 CHAPTER 24 Catalysis

RO

NH

H

NAM

CH 2 OH

CH 3 CO−O

O−

O

O

O

CO

CO

HO NH

NAG

CH 2 OH

CH 3 C

CH 2

Asp 52

O

O

CO

O

CH 2

Asp 52

CH 2

CH 2

Glu 35

CO

O

H H

CH 2

CH 2

Glu 35

D E

RO

NH

CH 2 OH

CH 3 C

O

D

HO

HO NH

CH 2 OH

CH 3 CO

O O

E

RO

NH

CH 2 OH

CH 3 CO−O

O

HO

OH

CO

HO

NH

CH 2 OH

CH 3 C

CH 2

Asp 52

O

O

D E

O

CO

CH 2

H

CH 2

Glu 35

Figure 24.9

Proposed mechanism for the lysozyme-catalyzed hydrolysis of a cell wall.

PROBLEM 19

If were used to hydrolyze lysozyme, which ring would contain the label, NAM or

NAG?

A plot of the activity of an enzyme as a function of the pH of the reaction mixture

is called a pH–activity profileor a pH–rate profile(Section 18.6). The pH–activity

profile for lysozyme is shown in Figure 24.10. It is a bell-shaped curve with the maxi-

mum rate occurring at about pH 5.3. The pH at which the enzyme is 50% active is 3.8

on the ascending leg of the curve and 6.7 on the descending leg. These pH values cor-

respond to the values of the enzyme’s catalytic groups. (This is true for all bell-

shaped pH–rate profiles, provided that the values are at least two units apart.

If the difference between them is less than two units, the precise values of the

catalytic groups must be determined in other ways.)

pKa pKa

pKa pKa

pKa

H 2 O^18