Effects of Temperature and pH

An increase in temperature will increase the rate of non-enzyme-catalyzed reactions. A similar relationship between temperature and reaction rate occurs in enzyme-catalyzed reactions. At a temperature of 0° C the reaction rate is immeasurably slow. As the temperature is raised above 0° C the reaction rate increases, but only up to a point. At a few degrees above body temperature (which is 37° C) the reaction rate reaches a plateau; further increases in temperature actually decrease the rate of the reaction (fig. 4.3). This decrease is due to the fact that the tertiary structure of enzymes becomes altered at higher temperatures.

A similar relationship is observed when the rate of an enzymatic reaction is measured at different pH values. Each enzyme characteristically exhibits peak activity in a very narrow pH range, which is the pH optimum for the enzyme. If the pH is changed so that it is no longer within the enzyme's optimum range, the reaction rate will decrease (fig. 4.4). This decreased enzyme activity is due to changes in the conformation of the enzyme and in the charges of the R groups of the amino acids lining the active sites.

The pH optimum of an enzyme usually reflects the pH of the body fluid in which the enzyme is found. The acidic pH optimum of the protein-digesting enzyme pepsin, for example, allows it to be active in the strong hydrochloric acid of gastric juice. Similarly, the neutral pH optimum of salivary amylase and the alkaline pH optimum of trypsin in pancreatic juice allow these enzymes to digest starch and protein, respectively, in other parts of the digestive tract.

Although the pH of other body fluids shows less variation than that of the fluids of the digestive tract, the pH optima of different enzymes found throughout the body do show significant differences (table 4.3). Some of these differences can be exploited for diagnostic purposes. Disease of the prostate, for example, may be associated with elevated blood levels of a prostatic phosphatase with an acidic pH optimum (descriptively called acid phosphatase). Bone disease, on the other hand, may be associated with elevated blood levels of alkaline phosphatase, which has a higher pH optimum than the similar enzyme released from the diseased prostate.

10 20 30 37 40 100 Temperature (°C)

■ Figure 4.3 The effect of temperature on enzyme activity. This effect is measured by the rate of the enzyme-catalyzed reaction under standardized conditions as the temperature of the reaction is varied.

■ Figure 4.4 The effect of pH on the activity of three digestive enzymes. Salivary amylase is found in saliva, which has a pH close to neutral; pepsin is found in acidic gastric juice, and trypsin is found in alkaline pancreatic juice.

© The McGraw-Hill Companies, 2003

Table 4 .3 pH Optima of Selected Enzymes

Enzyme

Reaction Catalyzed

Optimum

Pepsin (stomach)

Digestion of protein

2.0

Acid phosphatase (prostate)

Removal of phosphate

5.5

group

Salivary amylase (saliva)

Digestion of starch

6.8

Lipase (pancreatic juice)

Digestion of fat

7.0

Alkaline phosphatase (bone)

Removal of phosphate

9.0

group

Trypsin (pancreatic juice)

Digestion of protein

9.5

Monoamine oxidase

Removal of amine group

9.8

(nerve endings)

from norepinephrine

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