Essentials of Anatomy and Physiology

With respect to temperature, most human enzymes
have their optimum functioning in the normal range
of body temperature: 97to 99F (36to 38C). A tem-
perature of 106F, a high fever, may break the chemi-
cal bonds that maintain the shapes of enzymes (see
Fig. 2–9C). If an enzyme loses its shape, it is said to be
denatured, and a denatured enzyme is unable to func-
tion as a catalyst. Some human enzymes, when dena-
tured by a high fever, may revert to their original
shapes if the fever is lowered quickly. Others, however,
will not. (An example of irreversible denaturation is a
hard-boiled egg; the proteins in the egg white and
yolk will never revert to what they were in the original
egg.) A high fever may cause brain damage or death

because enzymes in the brain have become perma-

nently denatured.

You already know that metals such as lead and mer-

cury are harmful to humans and that both may cause

serious damage to the nervous system and other body

tissues. These heavy metals are harmful to us because

they are very reactive and block the actions of our

enzymes. Fig. 2–9D depicts what happens. Notice that

the heavy metal ion bonds with part of the active site

of the enzyme and changes its shape. The substrate

molecule cannot fit, and the enzyme is useless. Many

other chemicals are poisonous to us for the very same

reason: They destroy the functioning of our enzymes,

and essential reactions cannot take place.

40 Some Basic Chemistry

Active site

Enzyme

Substrates Enzyme-substrate

complex

Enzyme

Product

Enzyme

Substrate Enzyme-substrate

complex

Enzyme

Products

Enzyme

Enzyme

Denatured enzyme

Heavy-metal ion

or toxin

Nonfunctional

enzyme

A

B

C

D

Figure 2–9. Active site theory. (A) Synthesis reaction. (B) Decomposition reaction.

(C) The effect of heat. (D) The effect of poisons. See text for description.

QUESTION:Which of these four pictures best represents the effect of an acidic pH on an

enzyme, and why?