Human Physiology, 14th edition (2016)

Enzymes and Energy 91

Naming of Enzymes

In the past, enzymes were given names that were somewhat
arbitrary. The modern system for naming enzymes, established
by an international committee, is more orderly and informative.
With the exception of some older enzyme names (such as pepsin,
trypsin, and renin ), all enzyme names end with the suffix - ase
( table 4.2 ), and classes of enzymes are named according to their
activity, or “job category.” Hydrolases, for example, promote
hydrolysis reactions. Other enzyme categories include phospha-
tases, which catalyze the removal of phosphate groups; synthases
and synthetases, which catalyze dehydration synthesis reactions;

dehydrogenases, which remove hydrogen atoms from their sub-

strates; and kinases, which add a phosphate group to (phosphory-

late) particular molecules. Enzymes called isomerases rearrange

atoms within their substrate molecules to form structural isomers,

such as glucose and fructose (chapter 2; see fig. 2.13).

The names of many enzymes specify both the substrate of

the enzyme and the job category of the enzyme. Lactic acid dehy-

drogenase, for example, removes hydrogens from lactic acid.

Enzymes that do exactly the same job (that catalyze the same

reaction) in different organs have the same name, since the name

describes the activity of the enzyme. Different organs, however,

may make slightly different “models” of the enzyme that differ

in one or a few amino acids. These different models of the same

enzyme are called isoenzymes. The differences in structure do not

affect the active sites (otherwise the enzymes would not catalyze

the same reaction), but they do alter the structure of the enzymes

at other locations so that the different isoenzymatic forms can be

separated by standard biochemical procedures. These techniques

are useful in the diagnosis of diseases.

are more easily formed as substrates are brought close together
in the proper orientation. This model of enzyme activity, in
which the enzyme undergoes a slight structural change to bet-
ter fit the substrate, is called the induced-fit model. This has
been likened to putting on a thin leather glove. As your hand
enters it, the glove is induced to fit the contours of your hand.
The enzyme-substrate complex, formed temporarily in the
course of the reaction, then dissociates to yield products and
the free unaltered enzyme.
Because enzymes are very specific as to their substrates
and activity, the concentration of a specific enzyme in a sam-
ple of fluid can be measured relatively easily. This is usually
done by measuring the rate of conversion of the enzyme’s sub-
strates into products under specified conditions. The presence
of an enzyme in a sample can thus be detected by the job it
does, and its concentration can be measured by how rapidly it
performs its job.

CLINICAL APPLICATION

When diseases damage tissues, some cells die and release

their enzymes into the blood. The activity of these enzymes,

reflecting their concentrations in the blood plasma, can be

measured in a test tube by adding their specific substrates.

Because an increase in certain enzymes in the blood can

indicate damage to specific organs, such tests may aid the

diagnosis of diseases. An increase in a man’s blood levels

of the acid, phosphatase, for example, may result from dis-

ease of the prostate ( table 4.1 ).

Clinical Investigation CLUES

Sheryl’s blood tests reveal elevated levels of CPK, LDH,

AST, and ALT.

• What enzymes do these letters indicate, and what
  diseases do elevated blood levels of these enzymes
  suggest?


• How might these test results relate to Sheryl’s
  symptoms?

Table 4.1 | Examples of the Diagnostic

Value of Some Enzymes Found in Plasma

Enzyme

Diseases Associated

with Abnormal Plasma

Enzyme Concentrations

Alkaline phosphatase Obstructive jaundice, Paget’s

disease (osteitis deformans),

carcinoma of bone

Acid phosphatase Benign hypertrophy of prostate,

cancer of prostate

Amylase Pancreatitis, perforated peptic

ulcer

Aldolase Muscular dystrophy

Creatine kinase (or creatine

phosphokinase-CPK)

Muscular dystrophy, myocardial

infarction

Lactate dehydrogenase

(LDH)

Myocardial infarction, liver disease,

renal disease, pernicious anemia

Transaminases

(AST and ALT)

Myocardial infarction, hepatitis,

muscular dystrophy

Table 4.2 | Selected Enzymes

and the Reactions They Catalyze

Enzyme Reaction Catalyzed

Catalase 2 H^2 O^2 → 2 H^2 O^1 O^2

Carbonic anhydrase H 2 CO 3 → H 2 O^1 CO 2

Amylase starch^1 H 2 O → maltose

Lactate dehydrogenase lactic acid → pyruvic acid^1 NADH^1 H

1

Ribonuclease RNA^1 H^2 O → ribonucleotides