Factors Affecting Reaction Velocity

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Chapter: Biochemistry : Enzymes

Enzymes can be isolated from cells and their properties studied in a test tube (that is, in vitro). Different enzymes show different responses to changes in substrate concentration, temperature, and pH.


FACTORS AFFECTING REACTION VELOCITY

Enzymes can be isolated from cells and their properties studied in a test tube (that is, in vitro). Different enzymes show different responses to changes in substrate concentration, temperature, and pH. This section describes factors that influence the reaction velocity of enzymes. Enzymic responses to these factors give us valuable clues as to how enzymes function in living cells (that is, in vivo).

 

A. Substrate concentration

 

1. Maximal velocity: The rate or velocity of a reaction (v) is the number of substrate molecules converted to product per unit time. Velocity is usually expressed as µmol of product formed per minute. The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (Vmax) is reached (Figure 5.6).


Figure 5.6 Effect of substrate concentration on reaction velocity.

The leveling off of the reaction rate at high substrate concentrations reflects the saturation with substrate of all available binding sites on the enzyme molecules present.

 

2. Hyperbolic shape of the enzyme kinetics curve: Most enzymes show Michaelis-Menten kinetics, in which the plot of initial reaction velocity (vo) against substrate concentration ([S]), is hyperbolic (similar in shape to that of the oxygen-dissociation curve of myoglobin). In contrast, allosteric enzymes do not follow Michaelis-Menton kinetics and show a sigmoidal curve that is similar in shape to the oxygen-dissociation curve of hemoglobin.


 

B. Temperature

 

1. Increase of velocity with temperature: The reaction velocity increases with temperature until a peak velocity is reached (Figure 5.7). This increase is the result of the increased number of molecules having sufficient energy to pass over the energy barrier and form the products of the reaction.

 

2. Decrease of velocity with higher temperature: Further elevation of the temperature causes a decrease in reaction velocity as a result of temperature-induced denaturation of the enzyme (see Figure 5.7).

 

The optimum temperature for most human enzymes is between 35°C and 40°C. Human enzymes start to denature at temperatures above 40°C, but thermophilic bacteria found in the hot springs have optimum temperatures of 70°C.


Figure 5.7 Effect of temperature on an enzymecatalyzed reaction.

 

C. pH

 

1. Effect of pH on the ionization of the active site: The concentration of protons (H+) affects reaction velocity in several ways. First, the catalytic process usually requires that the enzyme and substrate have specific chemical groups in either an ionized or un-ionized state in order to interact. For example, catalytic activity may require that an amino group of the enzyme be in the protonated form (–NH3+). At alkaline pH, this group is deprotonated, and the rate of the reaction, therefore, declines.

 

2. Effect of pH on enzyme denaturation: Extremes of pH can also lead to denaturation of the enzyme, because the structure of the catalytically active protein molecule depends on the ionic character of the amino acid side chains.

 

3. Variable pH optimum: The pH at which maximal enzyme activity is achieved is different for different enzymes and often reflects the [H+] at which the enzyme functions in the body. For example, pepsin, a digestive enzyme in the stomach, is maximally active at pH 2, whereas other enzymes, designed to work at neutral pH, are denatured by such an acidic environment (Figure 5.8).


Figure 5.8 Effect of pH on enzyme-catalyzed reactions.

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