66
UNIT ONE
structure may range from fewer than 100 to more than 5,000
amino acids. The amino acid sequence is characteristic of a
particular protein. Hemoglobin, actin, and an antibody pro-
tein have very different amino acid sequences.
In the
secondary structure
(fig. 2.19
b
), the polypeptide
chain either forms a springlike coil (alpha helix) or folds back
and forth on itself (beta-pleated sheet) or folds into other
shapes. Secondary structure arises from hydrogen bonding.
Recall that polar molecules result when electrons are not shared
equally in certain covalent bonds. In amino acids, this results in
slightly negative oxygen and nitrogen atoms and slightly posi-
tive hydrogen atoms. Hydrogen bonding between oxygen and
hydrogen atoms in different parts of the molecule determines
the secondary structure. A single polypeptide may include heli-
ces; sheets; and other localized shapes, called motifs.
Hydrogen bonding and even covalent bonding between
atoms in different parts of a polypeptide can impart another,
larger level of folding, the
tertiary structure.
Altogether, the pri-
mary, secondary, and tertiary structures contribute to a protein’s
distinct conformation (F g. 2.19
c
), which determines its func-
tion. Some proteins are long and F brous, such as the keratins
that form hair and the threads of F brin that knit a blood clot.
Myoglobin and hemoglobin are globular, as are many enzymes.
In many cases, slight, reversible changes in conformation
may be part of the protein’s normal function. ±or example,
some of the proteins involved in muscle contraction exert a
pulling force as a result of such a shape change, leading to
movement. Such changes in shape are reversible, enabling
the protein to function repeatedly.
FIGURE 2.17
Amino acid
structure. (
a
) An amino acid
has an amino group, a carboxyl
group, and a hydrogen atom
that are common to all amino
acid molecules, and a specif
c R
group. (
b
) Some representative
amino acids and their structural
Formulas. Each type oF amino
acid molecule has a particular
shape due to its di±
erent R
group.
H
N
H
C
H
C
O
OH
S
C
H
H
H
H
N
H
C
H
C
O
OH
C
C
C
H
H
C
H
C
H
H
C
H
C
H
H
N
H
C
H
C
O
OH
R
(a)
General structure of an
amino acid. The portion
common to all amino acids
is within the oval. It includes
the amino group (—NH
2
)
and the carboxyl group
(—COOH). The "R" group,
or the "rest of the molecule,"
is what makes each amino
acid unique.
(b)
Cysteine. Cysteine has an
R group that contains sulfur.
Phenylalanine. Phenylalanine
has a complex R group.
Improper metabolism of
phenylalanine occurs in the
disease
phenylketonuria
.
O
OH
C
N
C
C
NC
R
O
H
H
H
H
H
R
FIGURE 2.18
A peptide bond (red) joins two amino acids.
Protein misFolding can cause disease. In cystic f
brosis, For example,
a protein cannot Fold into its f
nal Form. It cannot anchor in the cell
membrane, where it would normally control the ²
ow oF chloride ions.
Certain body ²
uids dry up, which impairs respiration and digestion. A
class oF illnesses called transmissible spongiForm encephalopathies,
which includes “mad cow disease,” results when a type oF protein
called a prion Folds into an abnormal, inFectious Form—that is, it con-
verts normal prion protein into the pathological Form, which riddles
the brain with holes. Alzheimer disease results From the cutting oF a
protein called beta amyloid into pieces oF a certain size. The proteins
misFold, attach, and accumulate, Forming structures called plaques in
parts oF the brain controlling memory and cognition.
Various treatments can more dramatically change or
denature
the secondary and tertiary structures of a pro-
tein’s conformation. Because the primary structure (amino
acid sequence) remains, sometimes the protein can regain
its shape when normal conditions return. High temperature,
radiation, pH changes, and certain chemicals (such as urea)
can denature proteins.
A familiar example of irreversible protein denaturation
is the response of the protein albumin to heat (for example,
cooking an egg white). A permanent wave that curls hair
also results from protein denaturation. Chemicals F rst break
apart the tertiary structure formed when sulfur-containing
amino acids attract each other within keratin molecules. This
relaxes the hair. When the chemicals are washed out of the
set hair, the sulfur bonds reform, but in different places. The
appearance of the hair changes.
Not all proteins are single polypeptide chains. In some
proteins, several polypeptide chains are connected in a fourth
level, or
quaternary structure,
to form a very large structure
(F g. 2.19
d
). Hemoglobin is a quaternary protein made up of
four separate polypeptide chains.
A protein’s conformation determines its function. The
amino acid sequence and interactions among the amino
acids in a protein determine the conformation. Thus, it is the
amino acid sequence of a protein that determines its func-
tion in the body.
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