Essentials of Anatomy and Physiology

tween them, creating a new compound. The product

of the reaction, the new compound, is then released,

leaving the enzyme itself unchanged and able to cat-

alyze another reaction of the same type.

The reaction shown in Fig. 2–9B is a decomposition

reaction. As the substrate molecule bonds to the active

site of the enzyme, strain is put on its internal bonds,

which break, forming two product molecules and

again leaving the enzyme unchanged. Each enzyme is

specific in that it will catalyze only one type of reac-

tion. An enzyme that digests the protein in food, for

example, has the proper shape for that reaction but

cannot digest starches. For starch digestion, another

enzyme with a differently shaped active site is needed.

Thousands of chemical reactions take place within the

body, and therefore we have thousands of enzymes,

each with its own shape and active site.

The ability of enzymes to function may be limited

or destroyed by changes in the intracellular or extra-

cellular fluids in which they are found. Changes in pH

and temperature are especially crucial. Recall that the

pH of intracellular fluid is approximately 6.8, and that

a decrease in pH means that more Hions are pres-

ent. If pH decreases significantly, the excess Hions

will react with the active sites of cellular enzymes,

change their shapes, and prevent them from catalyzing

reactions. This is why a state of acidosis may cause the

death of cells—the cells’ enzymes are unable to func-

tion properly.

Some Basic Chemistry 39

Table 2–5 FUNCTIONS OF PROTEINS

Type of Protein Function

Structural

proteins

Hormones

Hemoglobin

Myoglobin

Antibodies

Myosin and actin

Enzymes

• Form pores and receptor sites in
  cell membranes


• Keratin—part of skin and hair


• Collagen—part of tendons and
  ligaments


• Insulin—enables cells to take in
  glucose; lowers blood glucose level


• Growth hormone—increases
  protein synthesis and cell division


• Enables red blood cells to carry
  oxygen


• Stores oxygen in muscle cells


• Produced by lymphocytes (white
  blood cells); label pathogens for
  destruction


• Muscle structure and contraction


• Catalyze reactions

BOX2–4 A PROTEIN MYSTERY: PRIONS

ing misfolding, which brings about deterioration of

brain tissue. We do not know how to destroy prions.

Prions are not living; they do not contain genetic

material or carry out processes that might be dis-

rupted by antibiotics or antiviral medications.

Standard sterilization practices that kill bacteria and

viruses do not seem to inactivate prions.

Prevention of prion disease depends upon

keeping animal brain tissue from contaminating

meat destined for human or animal consumption.

In Great Britain, where the human form of mad-

cow disease emerged and killed nearly 100 people,

butchering practices are now stringently regulated.

The first cases of mad-cow disease in Canada

and the United States were found in 2003, in cattle.

As of this writing, people have not yet been

affected.

Prions are proteinaceous infectious particles, the

cause of lethal diseases of the nervous system in

people and animals. Mad-cow disease is perhaps

the best known; its formal name is bovine spongi-

form encephalopathy (BSE). The name tells us about

the disease: Encephalopathy means that the brain is

affected, and spongiform indicates that brain tissue

becomes spongy, full of holes. People may acquire

BSE by eating beef contaminated with infected cow

brain tissue. They develop what is called variant

Creutzfeldt-Jakob disease (CJD). CJD is characterized

by loss of coordination, loss of memory and person-

ality, and death within a few months. There is no

treatment.

How do prions cause this disease? We do not yet

have the entire answer. We do know that prions

change the structure of other brain proteins, caus-

The active site of the enzyme is the part that matches
the shapes of the substrates. The substrates must “fit”
into the active site of the enzyme, and temporary
bonds may form between the enzyme and the sub-
strate. This is called the enzyme–substrate complex. In
this case, two substrate molecules are thus brought
close together so that chemical bonds are formed be-