Human Physiology, 14th edition (2016)

Enzymes and Energy 93

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.
See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.

24

pH

Enzyme activity

6810

Pepsin Salivary

amylase

Trypsin

cofactor requirement do not have a properly shaped active site

in the absence of the cofactor. In these enzymes, the attach-

ment of cofactors causes a conformational change in the pro-

tein that allows it to combine with its substrate. The cofactors

of other enzymes participate in the temporary bonds between

the enzyme and its substrate when the enzyme-substrate com-

plex is formed ( fig. 4.5 ).

allows it to be active in the strong hydrochloric acid of gastric
juice ( fig. 4.4 ). 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, respec-
tively, in other parts of the digestive tract.

Figure 4.5 The roles of cofactors in enzyme

function. In ( a ) the cofactor changes the conformation of the

active site, allowing for a better fit between the enzyme and

its substrates. In ( b ) the cofactor participates in the temporary

bonding between the active site and the substrates.

Substrates

Enzyme

(a) Cofactor

(b)

CLINICAL APPLICATION

The pH optima of digestive enzymes reflect differences in

the pH of different regions of the digestive system, such as

neutral for saliva, acidic for gastric juice, and alkaline for

pancreatic juice. These are taken into account in enzyme

replacement therapy for pancreatic insufficiency (often

due to pancreatitis or cystic fibrosis). Differences in the pH

optima of enzymes found in the cells of internal organs are

useful in laboratory blood tests. For example, a rise in blood

levels of acid phosphatase or alkaline phosphatase, which

have different pH optima, may indicate diseases of the

prostate or bone, respectively ( table 4.3 ).

Cofactors and Coenzymes

Many enzymes are completely inactive when isolated in a pure
state. Evidently some of the ions and smaller organic molecules
that are removed in the purification procedure play an essential
role in enzyme activity. These ions and smaller organic mol-
ecules needed for the activity of specific enzymes are called
cofactors and coenzymes.
Cofactors include metal ions such as Ca^2 1 ,  Mg^2 1 , 
Mn^2 1 ,  Cu^2 1 ,  Zn^2 1 , and selenium. Some enzymes with a

Table 4.3 | pH Optima of Selected Enzymes

Enzyme

Reaction

Catalyzed

pH

Optimum

Pepsin (stomach) Digestion of protein 2.0

Acid phosphatase (prostate) Removal of

phosphate group

5.5

Salivary amylase (saliva) Digestion of starch 6.8

Lipase (pancreatic juice) Digestion of fat 7.0

Alkaline phosphatase (bone) Removal of

phosphate group

9.0

Trypsin (pancreatic juice) Digestion of protein 9.5

Monoamine oxidase

(nerve endings)

Removal of amine

group from

norepinephrine

9.8