Organic Chemistry

1016 CHAPTER 24 Catalysis

Figure 24.4N
The structure of hexokinase before
binding its substrate is shown in
red. The structure of hexokinase
after binding its substrate is shown
in green.

The energy released as a result of binding the substrate to the enzyme can be used

to induce a change in the conformation of the enzyme, leading to more precise binding

between the substrate and the active site. This change in conformation of the enzyme

is known as induced fit. In the induced-fit model, the shape of the active site does not

become completely complementary to the shape of the substrate until the enzyme has

bound the substrate.

An example of induced fit is shown in Figure 24.4. The three-dimensional structure of

the enzyme hexokinase is shown before and after binding glucose, its substrate. Notice

the change in conformation that occurs upon binding the substrate.

The following are some of the most important factors that contribute to the remark-

able catalytic ability of enzymes:

• Reacting groups are brought together at the active site in the proper orientation for
  reaction. This is analogous to the way that proper positioning of reacting groups
  increases the rate of intramolecular reactions (Section 24.6).


• Some of the amino acid side chains of the enzyme serve as catalytic groups, and
  many enzymes have metal ions at their active site that act as catalysts. These
  species are positioned in orientations relative to the substrate needed for cataly-
  sis. This is analogous to the rate enhancements observed for intramolecular catal-
  ysis by acids, bases, and metal ions (Section 24.7).


• Groups on the enzyme can stabilize an intermediate and, therefore, the transition state
  leading to the intermediate by van der Waals interactions, electrostatic interactions,
  and hydrogen bonding (Figure 24.1b).

3-D Molecules:

Hexokinase;

Hexokinase bound to its

substrate

lock-and-key model induced-fit model

a. b.