ENZYMES Proteins  Elmhurst College
Lock and Key Theory Coenzyme - NAD+ Enzyme Regulation  Chemistry Department
Carboxypeptidase Enzyme Inhibitors  Virtual ChemBook

Mechanism of Enzyme Action

Introduction - Enzyme Characteristics:

The basic mechanism by which enzymes catalyze chemical reactions begins with the binding of the substrate (or substrates) to the active site on the enzyme. The active site is the specific region of the enzyme which combines with the substrate. The binding of the substrate to the enzyme causes changes in the distribution of electrons in the chemical bonds of the substrate and ultimately causes the reactions that lead to the formation of products. The products are released from the enzyme surface to regenerate the enzyme for another reaction cycle.

The active site has a unique geometric shape that is complementary to the geometric shape of a substrate molecule, similar to the fit of puzzle pieces. This means that enzymes specifically react with only one or a very few similar compounds.

Lock and Key Theory:

The specific action of an enzyme with a single substrate can be explained using a Lock and Key analogy first postulated in 1894 by Emil Fischer. In this analogy, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme).

Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock. This is illustrated in graphic on the left.

QUES: Using a diagram and in your own words, describe the various lock and key theory of enzyme action in relation to a correct and incorrect substrate.

Induced Fit Theory:

Not all experimental evidence can be adequately explained by using the so-called rigid enzyme model assumed by the lock and key theory. For this reason, a modification called the induced-fit theory has been proposed.

The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much. Other molecules may be too small to induce the proper alignment and therefore cannot react. Only the proper substrate is capable of inducing the proper alignment of the active site.

In the graphic on the left, the substrate is represented by the magenta molecule, the enzyme protein is represented by the green and cyan colors. The cyan colored protein is used to more sharply define the active site. The protein chains are flexible and fit around the substrate.

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Nature of Active Site and Substrate Interaction:

Enzymes have varying degrees of specificity. Some enzymes have absolute specificity for one substrate and no others, while other enzymes react with substrates with similar functional groups, side chains, or positions on a chain. The least specific enzymes catalyze a reaction at a particular chemical bond regardless of other structural features.

Much experimental work is devoted to gaining an understanding of the nature of the active site in an enzyme. Since enzymes are proteins, the nature of amino acid side chains in the vicinity of the active site is important. The specific amino acid side chains have been determined for many enzymes. The active site for carboxypeptidase A will be used to illustrate the principles involved as shown in the graphic on the left.

The substrate (space filling gray,blue red) can interact with the active site through opposite charges, hydrogen bonding (shown in yellow), hydrophobic non-polar interaction, and coordinate covalent bonding to the metal ion activator as shown in magenta. The numbers behind the amino acids indicate the sequence position of the amino acid in the protein. The white lines represent the wire frames of the other amino acids in the enzyme.

The carbonyl bond is activated by interaction with the Zn ions. This leads to the addition of -OH from water to the carbonyl to produce an acid and the ultimate rupture of the C-N bond.

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