A protein may consist of a single polypeptide or sever-
al intertwined polypeptides. A given type of protein has a
unique three-dimensional shape because of the ways that
each polypeptide twists and folds as associated polypep-
tides come together (Figure 2.12). The twists and folds,
which form the secondary and tertiary structures of a pro-
tein, are due to hydrogen bonds between nearby amino ac-
ids within the polypeptide chain. In addition to hydrogen
bonds, disulfide bonds, which are bonds between sulfur-
containing groups of nearby amino acids, also contribute
to secondary and tertiary structures. For example, a hair
stylist uses chemicals to give a client a “permanent wave.”
These chemicals cause the proteins in hair to form disul-
fide bonds, which cause the hair to curl. Other chemicals
break these disulfide bonds to reverse a permanent wave.
The sequence of amino acids in a protein is critical to
its function. A mistake of even one amino acid can have se-
rious consequences. For example, a single substitution of
an amino acid in hemoglobin, a blood protein, can result in
a deformed molecule that produces sickle-cell disease. The
defective protein causes the cells to become sickle-shaped,
which interferes with oxygen transport and blood flow.
The shape of a protein is also critical to its function.
Changes in temperature, radiation, pH, and ion concentra-
tions can break the hydrogen and disulfide bonds within a
protein, thereby making it dysfunctional. This nonrevers-
ible process is called denaturation (de-na'-chur-A-shun).
A common example of denaturation is frying an egg. In a
raw egg, the egg-white protein (albumin) is soluble, and
the egg white appears as a clear, viscous fluid. When heat
is applied to the egg, the albumin denatures; it changes
shape, becomes insoluble, and turns white.
Enzymes Speed Up Chemical Reactions
When atoms and molecules collide with sufficient energy,
chemical bonds are made and broken. However, at room
temperature, this does not happen rapidly enough to
maintain life. So, all cells have a class of proteins called
enzymes (EN-zims) that speed up chemical reactions by
bringing together and properly orienting the reacting
molecules (called substrates) to form or break chemical
bonds. Enzymes are biological catalysts (KAT-a-lists); they
speed up a chemical reaction without being consumed.
The names of enzymes usually end in
can be grouped according to the types of chemical reac-
tions they catalyze. For example,
add phosphate, and
Enzymes have three important properties: •
—Each enzyme catalyzes a particular chemical
reaction with a specific reactant.
together on the enzyme’s surface, where they react. The
enzyme fits the substrate like a lock accepts a key, so the
shape of the enzyme is critical to its function.
—Enzymes catalyze reactions at high rates,
millions to billions of times faster than the reaction
would occur alone. One enzyme molecule can catalyze
a reaction up to 600,000 times per second.
—Enzymes are controlled by many factors. For
example, genes control the rates at which enzymes
are made. Some substances can bind to an enzyme and
inhibit or enhance its activity. Some enzymes require
non-protein substances called cofactors or coenzymes
for activity. Cofactors can be minerals or vitamins, while
coenzymes are vitamins.
During an enzymatic reaction (Figure 2.13), substrates
bind to the surface of the enzyme at particular sites
called active sites. Once bound, the compound is called
the enzym e-substrate complex.
How an enzyme
works • Figure 2.13
speed up chemical reactions by bringing together
and properly orienting the reacting molecules to form or break
chemical bonds without being consumed in the process.
At active site,
enzyme and substrate
come together in
enzyme is unchanged
and free to catalyze
the reaction again.
reaction and transforms
substrate into products.
Life Uses Important Chemicals 39