PROCESS DIAGRAM
Amino acids bind together to form proteins • Figure 2.12
Amino acids combine to form peptides and proteins. The amino acid sequence, called the
p rim a ry structure,
determines the higher-order twists and folds, called the secondary and
tertiary structures. When two or more polypeptide chains bind together, the protein has
quaternary structure. The protein's function depends on its three-dimensional structure
(both secondary and tertiary).
Peptide bond
Amino acid
Amino acid
Dipeptide
+
H2O
^
Peptide bond
C
C R
H
N
O
R
C
N " C
»- C 'uJ
H
R
O
C
N
C
R
C
- O
3 R
C "
C
H
O
C
H
H R
-
^
N O
C
;
N
R
O
C
C
H
N
O
C
R
C
O
H
c N
4
R
*trn
c ^ h
N
R
C = O
C
C =
O
N -H
C = O
H- N
Q Primary
structure
(amino acid sequence)
Q
Q uaternary structure
(arrangement of two or
more polypeptide chains)
!— Hydrogen
H
bond
«
' %
\ , R
&
^
S e co n d ary structure
(twisting and folding of
neighboring amino acids,
stabilized by hydrogen
bonds)
O
Alpha helix
Q
Tertiary
structure
(three-dimensional shape of
polypeptide chain)
Amino
acids
Peptide
bond
Beta pleated sheet
H
H- N
H-
C -
R
H- N
N -H
Polypeptide chain
acid links to the carboxyl group of another by a dehydra-
tion synthesis reaction to form a covalent bond called a
peptide bond (Figure 2.12). When amino acids link to-
gether, they form
peptides
(PEP-tidz):
Dipeptide
—Two amino acids linked together
Tripeptide
—Three amino acids linked together
Polypeptide
—Four or more amino acids linked together
Proteins are polypeptides that contain 50 to 2,000 amino
acids. Because each variation in the number and sequence
of amino acids produces a different protein, numerous
proteins are possible. The situation is similar to using an
alphabet of 20 letters to form words. Each letter would be
equivalent to an amino acid, and each word would be a dif-
ferent protein. The sequence of amino acids in a protein is
referred to as the
primary structure
of the protein.
38 CHAPTER 2
Introductory Chemistry
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