Essay on the Models for Enzyme-Substrate Complex

Based on the high specificity of binding of the substrate to the enzyme, two hypotheses for the enzyme-substrate complex have been) proposed:

Lock-and-Key hypothesis:

Fischer proposed this hypothesis in 1890 the chemical nature of the enzyme was not known. However, the experimental observations that enzymes are very much specific to their substrate led him to propose the lock-and-key hypothesis.

He assume that the substrate molecule must have complementary shape to fit closely with enzyme molecule in the same way as the key (substrate) fits into the lock (enzyme) a shown in Some authors now prefer to rename the lock-and-key hypothesis as direct-fit hypothesis for the reason you will be cleared in the following paragraph.

Induced-Fit hypothesis:

It is now well known that the three dimensional structure of many enzymes are flexible. The binding of the substrate induces a conformational change in the active site of the enzyme so that the substrate fits perfectly well with the enzyme to form the ES complex.

This was postulated by Daniel E. Koshland, Jr in 1958. Hexokinase is good example of an enzyme that exhibits this type of conformational change.

Catalysis

The details of the catalytic mechanism differ from one type of enzyme to another. But the facts discussed above are involved in all enzyme catalysis. When E and S are mixed, the S molecule comes into close contact with the E molecule.

A temporary intermediate compound called ES complex is formed. The S molecule is transformed into the P molecule by catalytic mechanisms which differ from enzyme to enzyme. The P moves away from the E molecule. The E, now freed, interacts with another S molecule to repeat the process of catalysis.