The induced fit model of enzyme action is a modification of the lock-and-key model originally proposed by Emil Fischer in 1894 (Figure 11.7a). The lock-and-key model proposes that an enzyme/substrate pair is like a lock and key. Though it explains the specificity of enzyme/substrate pairs, it does little to explain catalysis, because a lock does not change a key the way an enzyme changes a substrate.
In 1958, Daniel Koshland proposed the induced fit model to explain enzymatic catalysis (Figure 11.7b). The model proposes that distortion of the enzyme and the substrate is an important event in catalysis. Figure 11.8 shows x-ray diffraction studies of the enzyme hexokinase both without (11.8a) and with (11.8b) glucose bound. Note that binding of glucose causes two domains of the enzyme to fold toward each other.
Enzymes do more than simply bind and position substrates, however. Enzymes
1. Bind substrate(s);
2. Lower the energy of the transition state; and
3. Directly promote the catalytic event.
The latter may occur as a result of specific amino acid side chains that physically interact with the substrate and end up promoting the reaction.
When the catalytic process is complete, the enzyme must be able to release the product or products and return to its original state for another round of catalysis.
For an enzyme (E) that catalyzes the conversion of a single substrate (S) into a single product (P), the simplest way to write the overall reaction is in two steps:
S + E <=> ES
ES -> E + P
Here, ES represents the enzyme-substrate complex. For simplicity, the first reaction is shown as reversible, while the second reaction is irreversible. For specific examples, see the triose phosphate isomerase and serine proteases below.
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