50
UNIT 1
Organization of the Body
2
chemical and physical factors, such as excessive acidity or tem-
perature. Although individual proteins vary in their sensitivity
to environmental conditions, hydrogen bonds begin to break
when the pH drops or the temperature rises above normal
(physiological) levels, causing proteins to unfold and lose their
specific three-dimensional shape. In this condition, a protein is
said to be
denatured
.
Fortunately, the disruption is reversible in most cases, and
the “scrambled” protein regains its native structure when desir-
able conditions are restored. However, if the temperature or pH
change is so extreme that protein structure is damaged beyond
repair, the protein is
irreversibly denatured
. Te coagulation of
egg white (primarily albumin protein) that occurs when you
boil or fry an egg is an example of irreversible protein denatura-
tion. Tere is no way to restore the white, rubbery protein to its
original translucent form.
When globular proteins are denatured, they can no longer
perform their physiological roles because their function de-
pends on the presence of specific arrangements of atoms, called
active sites
, on their surfaces. Te active sites are regions that fit
and interact chemically with other molecules of complementary
shape and charge. Because atoms contributing to an active site
may actually be far apart in the primary chain, disruption of
intramolecular bonds separates them and destroys the active
site. For example, hemoglobin becomes totally unable to bind
and transport oxygen when blood pH is too acidic, because the
structure needed for its function has been destroyed.
We will describe most types of body proteins in conjunction
with the organ systems or functional processes to which they
are closely related. However, two groups of proteins—
molecular
chaperones
and
enzymes
—are intimately involved in the nor-
mal functioning of all cells, so we will consider these incredibly
complex molecules here.
Check Your Understanding
24.
What does the name “amino acid” tell you about the
structure of this molecule?
25.
What is the primary structure of proteins?
26.
What are the two types of secondary structure in proteins?
For answers, see Appendix H.
Molecular Chaperones
In addition to enzymes, all cells contain a class of unrelated
globular proteins called
molecular chaperones
which, among
other things, help proteins to achieve their functional three-
dimensional structure. Although its amino acid sequence de-
termines the precise way a protein folds, the folding process
also requires the help of molecular chaperones to ensure that
the folding is quick and accurate. Molecular chaperones can as-
sociate with a broad range of “client” proteins, allowing them to
perform a dizzying array of jobs. For example, specific molecu-
lar chaperones
Prevent accidental, premature, or incorrect folding of polypep-
tide chains or their association with other polypeptides
Aid the desired folding and association process
β-pleated regions of the polypeptide chain fold upon one an-
other to produce a compact ball-like, or
globular
, molecule (Fig-
ure 2.19c). Te unique structure is maintained by both covalent
and hydrogen bonds between amino acids that are o±en far
apart in the primary chain.
When two or more polypeptide chains aggregate in a regular
manner to form a complex protein, the protein has
quaternary
structure
(kwah
9
ter-na
0
re). Prealbumin, a protein that trans-
ports thyroid hormone in the blood, exhibits this structural
level (Figure 2.19d).
How do these different levels of structure arise? Although a
protein with tertiary or quaternary structure looks a bit like a
clump of congealed pasta, the ultimate overall structure of any
protein is very specific and is dictated by its primary structure.
In other words, the types and relative positions of amino ac-
ids in the protein backbone determine where bonds can form
to produce the complex coiled or folded structures that keep
water-loving amino acids near the surface and water-fleeing
amino acids buried in the protein’s core. In addition, cells deco-
rate many proteins by attaching sugars or fatty acids to them in
ways that are difficult to imagine or predict.
Fibrous and Globular Proteins
Te overall structure of a protein determines its biological func-
tion. In general, proteins are classified according to their overall
appearance and shape as either fibrous or globular.
Fibrous proteins
are extended and strandlike. Some exhibit
only secondary structure, but most have tertiary or even qua-
ternary structure as well. For example,
collagen
(kol
9
ah-jen) is
a composite of the helical tropocollagen molecules packed to-
gether side by side to form a strong ropelike structure. Fibrous
proteins are insoluble in water, and very stable—qualities ideal
for providing mechanical support and tensile strength to the
body’s tissues. Besides collagen, which is the single most abun-
dant protein in the body, the fibrous proteins include keratin,
elastin, and certain contractile proteins of muscle
(Table 2.3)
.
Because fibrous proteins are the chief building materials of the
body, they are also known as
structural proteins
.
Globular proteins
are compact, spherical proteins that have
at least tertiary structure. Some also exhibit quaternary struc-
ture. Te globular proteins are water-soluble, chemically active
molecules, and they play crucial roles in virtually all biological
processes. Consequently, this group is also called
functional
proteins
. Some (antibodies) help to provide immunity, others
(protein-based hormones) regulate growth and development,
and still others (enzymes) are catalysts that oversee just about
every chemical reaction in the body. Te roles of these and se-
lected other proteins found in the body are summarized in
²able 2.3.
Protein Denaturation
Fibrous proteins are stable, but globular proteins are quite the
opposite. Te activity of a protein depends on its specific three-
dimensional structure, and intramolecular bonds, particularly
hydrogen bonds, are important in maintaining that structure.
However, hydrogen bonds are fragile and easily broken by many
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