Enzymes and Energy 89The ability of yeast cells to make alcohol from glucose (a pro-
cess called fermentation ) had been known since antiquity, yet
even as late as the mid-nineteenth century no scientist had been
able to duplicate this process in the absence of living yeast.
Also, a vast array of chemical reactions occurred in yeast and
other living cells at body temperature that could not be dupli-
cated in the chemistry laboratory without adding substantial
amounts of heat energy. These observations led many mid-
nineteenth-century scientists to believe that chemical reac-
tions in living cells were aided by a “vital force” that operated
beyond the laws of the physical world. This vitalist concept
was squashed along with the yeast cells when a pioneering
biochemist, Eduard Buchner, demonstrated that juice obtained
from yeast could ferment glucose to alcohol. The yeast juice
was not alive—evidently some chemicals in the cells were
responsible for fermentation. Buchner didn’t know what these
chemicals were, so he simply named them enzymes (Greek for
“in yeast”).
Chemically, enzymes are a subclass of proteins. The
only known exceptions are the few special cases in which
RNA demonstrates enzymatic activity; in these cases they are
called ribozymes. Ribozymes function as enzymes in reactions
involving remodeling of the RNA molecules themselves, and
in the formation of a growing polypeptide in ribosomes.
4.1 Enzymes as Catalysts
Enzymes are biological catalysts that increase the rate of
chemical reactions. Most enzymes are proteins, and their
catalytic action results from their complex structure. The
great diversity of protein structure allows different enzymes
to be very specific in the reactions they catalyze.
Functionally, enzymes (and ribozymes) are biological
catalysts. A catalyst is a chemical that (1) increases the rate of
a reaction, (2) is not itself changed at the end of the reaction,
and (3) does not change the nature of the reaction or its final
result. The same reaction would have occurred to the same
degree in the absence of the catalyst, but it would have pro-
gressed at a much slower rate.
In order for a given reaction to occur, the reactants must
have sufficient energy. The amount of energy required for a
reaction to proceed is called the activation energy. By anal-
ogy, a match will not burn and release heat energy unless it
is first “activated” by striking the match or by placing it in a
flame.
In a large population of molecules, only a small frac-
tion will possess sufficient energy for a reaction. Adding
heat will raise the energy level of all the reactant molecules,
thus increasing the percentage of the population that has the
activation energy. Heat makes reactions go faster, but it also
produces undesirable side effects in cells. Catalysts make reac-
tions go faster at lower temperatures by lowering the activation
energy required, thus ensuring that a larger percentage of the
population of reactant molecules will have sufficient energy to
participate in the reaction ( fig. 4.1 ).
Because a small fraction of the reactants will have the
activation energy required for a reaction even in the absence
of a catalyst, the reaction could theoretically occur spontane-
ously at a slow rate. This rate, however, would be much too
slow for the needs of a cell. So, from a biological standpoint,
the presence or absence of a specific enzyme catalyst acts as
a switch—the reaction will occur if the enzyme is present and
will not occur if the enzyme is absent.Mechanism of Enzyme Action
The ability of enzymes to lower the activation energy of a
reaction is a result of their structure. Enzymes are large pro-
teins with complex, highly ordered, three-dimensional shapes
produced by physical and chemical interactions between their
amino acid subunits. Each type of enzyme has a characteris-
tic three-dimensional shape, or conformation, with ridges,
grooves, and pockets lined with specific amino acids. The par-
ticular pockets that are active in catalyzing a reaction are called
the active sites of the enzyme.
The reactant molecules, which are called the substrates
of the enzyme, have specific shapes that allow them to fit into
the active sites. The enzyme can thus be thought of as a lock
into which only a specifically shaped key—the substrate—can
fit. This lock-and-key model of enzyme activity is illustrated
in figure 4.2.
In some cases, the fit between an enzyme and its substrate
may not be perfect at first. A perfect fit may be induced, how-
ever, as the substrate gradually slips into the active site. This
induced fit, together with temporary bonds that form between
the substrate and the amino acids lining the active sites of the
enzyme, weakens the existing bonds within the substrate mol-
ecules and allows them to be more easily broken. New bondsLEARNING OUTCOMES
After studying this section, you should be able to:- Explain the properties of a catalyst and how
enzymes function as catalysts. - Describe how enzymes are named.
Sheryl, an active 78-year-old, suddenly became greatly
fatigued and disoriented while skiing. When she was
brought to the hospital, blood tests revealed elevated
levels of LDH, AST, ALT, and the MB isoform of CK.
Some of the new terms and concepts you will encoun-
ter include:- Enzymes, isoenzymes, coenzymes, and cofactors
- LDH, AST, ALT, and CK
Clinical Investigation