Chemical Composition of the Body
33
down by hydrolysis to yield individual amino acids, as happens
in the stomach and intestines when we digest protein in our
diet.
Note that when two amino acids are linked together, one
end of the resulting molecule has a free amino group, and the
other has a free carboxyl group. Additional amino acids can
be linked by peptide bonds to these free ends. A sequence of
amino acids linked by peptide bonds is known as a
polypep-
tide.
The peptide bonds form the backbone of the polypeptide,
and the side chain of each amino acid sticks out from the chain.
By convention, if the number of amino acids in a polypeptide is
50 or less, the molecule is known as a
peptide;
if the sequence
is more than 50 amino acid units, it is known as a protein. The
number 50 is somewhat arbitrary but is useful in distinguish-
ing among large and small polypeptides. Small peptides have
certain chemical properties that differ from proteins (e.g.,
peptides are generally soluble in acid, while proteins generally
are not).
When one or more monosaccharides are covalently
attached to the side chains of specifi c amino acids, the proteins
are known as
glycoproteins.
These proteins are major com-
ponents of connective tissue, and are also abundant in fl
uids
like mucus, where they play a protective or lubricating role.
All proteins have multiple levels of structure that give
each protein a unique shape. The shape of the protein deter-
mines its biological activity. In all cases, a protein’s shape
depends on its amino acid sequence, known as the primary
structure of the protein.
Primary Protein Structure
Two variables determine the
primary structure
of a polypep-
tide: (1) the number of amino acids in the chain, and (2) the
specifi c type of amino acid at each position along the chain
(
Figure 2–15
). Each position along the chain can be occu-
pied by any one of the 20 different amino acids. Consider
the number of different peptides that can form that have a
sequence of just three amino acids. Any one of the 20 differ-
ent amino acids may occupy the fi rst position in the sequence,
any one of the 20 the second position, and any one of the 20
the third position, for a total of 20
×
20
×
20 = 20
3
= 8000
possible sequences of three amino acids. If the peptide is six
amino acids in length, 20
6
= 64,000,000 possible combina-
tions. Peptides that are only six amino acids long are still very
small compared to proteins, which may have sequences of 1000
or more amino acids. Thus, with 20 different amino acids,
an almost unlimited variety of polypeptides can be formed by
altering both the amino acid sequence and the total number
of amino acids in the chain. Only a fraction of these potential
proteins is found in nature, however.
Secondary Protein Structure
A polypeptide is analogous to a string of beads, each bead
representing one amino acid (see Figure 2–15). Moreover,
because amino acids can rotate around bonds within a poly-
peptide chain, the chain is fl exible and can bend into a number
of shapes, just as a string of beads can be twisted into many
confi gurations. The three-dimensional shape of a molecule
is known as its
conformation
(
Figure 2–16
). The confor-
mations of peptides and proteins play a major role in their
functioning.
COOH
NH
2
1
223
Figure 2–15
The position of each type of amino acid in a polypeptide chain
and the total number of amino acids in the chain distinguish one
polypeptide from another. The polypeptide illustrated contains 223
amino acids. Different amino acids are represented by different-
colored circles. The bonds between various regions of the chain
(red to red) represent covalent disulfi de bonds between cysteine side
chains.
COOH
NH
2
Figure 2–16
Conformation (shape) of the protein molecule myoglobin. Each dot
corresponds to the location of a single amino acid.
Adapted from Albert L. Lehninger.
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