Mechanism of Enzyme Action

The ability of enzymes to lower the activation energy of a reaction is a result of their structure. Enzymes are large proteins with complex, highly ordered, three-dimensional shapes produced by physical and chemical interactions between their amino acid subunits. Each type of enzyme has a characteristic three-dimensional shape, or conformation, with ridges, grooves, and pockets lined with specific amino acids. The particular pockets that are active in catalyzing a reaction are called the active sites of the enzyme.

The reactant molecules, which are called the substrates of the enzyme, have specific shapes that allow them to fit into the active sites. The enzyme can thus be thought of as a lock into which only a specifically shaped key—the substrate—can fit. This lock-and-key model of enzyme activity is illustrated in figure 4.2.

In some cases, the fit between an enzyme and its substrate may not be perfect at first. A perfect fit may be induced, however, as the substrate gradually slips into the active site. This induced fit, together with temporary bonds that form between the substrate and the amino acids lining the active sites of the enzyme, weaken the existing bonds within the substrate molecules and allows them to be more easily broken. New bonds are more easily formed as substrates are brought close together in the proper orientation. This model of enzyme activity, in which the enzyme undergoes a slight structural change to better fit the substrate, is called the induced-fit model. The enzyme-substrate complex, formed temporarily in the course of the reaction, then dissociates to yield products and the free unaltered enzyme.

Since enzymes are very specific as to their substrates and activity, the concentration of a specific enzyme in a sample of fluid can be measured relatively easily. This is usually done by measuring the rate of conversion of the enzyme's substrates into products under specified conditions. The presence of an enzyme in a sample can thus be detected by the job it does, and its concentration can be measured by how rapidly it performs its job.

Energy of reactants Activation energy

Enzymes Lower The Enrgy Barrier

Activation energy

Energy of reactants

Activation energy

Energy of reactants

Reactant

Reactant

Energy of reactants Activation energy

Reactant

Reactant

Noncatalyzed reaction Catalyzed reaction

■ Figure 4.1 A comparison of noncatalyzed and catalyzed reactions. The upper figures compare the proportion of reactant molecules that have sufficient activation energy to participate in the reaction (blue = insufficient energy; green = sufficient energy). This proportion is increased in the enzyme-catalyzed reaction because enzymes lower the activation energy required for the reaction (shown as a barrier on top of an energy "hill" in the lower figures). Reactants that can overcome this barrier are able to participate in the reaction, as shown by arrows pointing to the bottom of the energy hill.

Enzyme

Substrate A

Substrate A

Enzyme

Substrate B

(a) Enzyme and substrates

Substrate B

(a) Enzyme and substrates

Enzyme

Enzyme Substrate Complex

(b) Enzyme-substrate complex

Product C

Product D

Product C

Product D

Enzyme Substrate Complex

(c) Reaction products and enzyme (unchanged)

(b) Enzyme-substrate complex

(c) Reaction products and enzyme (unchanged)

Figure 4.2 The lock-and-key model of enzyme action. (a) Substrates A and B fit into active sites in the enzyme, forming an enzyme-substrate complex. (b) This complex then dissociates (c), releasing the products of the reaction and the free enzyme.

When tissues become damaged as a result of diseases, some of the dead cells disintegrate and release their enzymes into the blood. Most of these enzymes are not normally active in the blood for lack of their specific substrates, but their enzymatic activity can be measured in a test tube by the addition of the appropriate substrates to samples of plasma. Such measurements are clinically useful because abnormally high plasma concentrations of particular enzymes are characteristic of certain diseases (table 4.1).

Clinical Investigation Clues

Remember that Tom had elevated blood levels of acid phosphatase and creatine phosphokinase. How might these laboratory results help to explain his difficulty in urination?

What two different conditions might cause the elevated creatine phosphokinase?

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Essentials of Human Physiology

Essentials of Human Physiology

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