48
UNIT 1
Organization of the Body
2
four amino acids apart. Hydrogen bonds in
a
-helices always
link different parts of the
same
chain together.
In another type of secondary structure, the
beta (β)-pleated
sheet
, the primary polypeptide chains do not coil, but are linked
side by side by hydrogen bonds to form a pleated, ribbonlike
structure that resembles an accordion’s bellows (Figure 2.19b).
Notice that in this type of secondary structure, the hydrogen
bonds may link together
different polypeptide chains
as well as
different parts
of the same chain that has folded back on itself.
A single polypeptide chain may exhibit both types of secondary
structure at various places along its length.
Many proteins have
tertiary structure
(ter
9
she-a
0
re), the next
higher level of complexity, which is superimposed on second-
ary structure. Tertiary structure is achieved when
a
-helical or
Structural Levels of Proteins
Proteins can be described in terms of four structural levels.
±e linear sequence of amino acids composing the polypeptide
chain is called the
primary structure
of a protein. ±is structure,
which resembles a strand of amino acid “beads,” is the backbone
of the protein molecule
(Figure 2.19a)
.
Proteins do not normally exist as simple, linear chains of
amino acids. Instead, they twist or bend upon themselves to
form a more complex
secondary structure
. ±e most common
type of secondary structure is the
alpha (
)-helix
, which resem-
bles a Slinky toy or the coils of a telephone cord (Figure 2.19b).
±e
a
-helix is formed by coiling of the primary chain and is sta-
bilized by hydrogen bonds formed between NH and CO groups
in amino acids in the primary chain which are approximately
(a) Generalized
structure of all
amino acids.
(b) Glycine
is the simplest
amino acid.
R
C
C
N
OH
H
H
H
O
H
(c) Aspartic acid
(an acidic amino acid)
has an acid group
(—COOH) in the
R group.
(d) Lysine
(a basic amino acid)
has an amine group
(
NH
2
) in the R group.
(e) Cysteine
(a basic amino acid)
has a sulfhydryl (
SH)
group in the R group, which
suggests that this amino acid
is likely to participate in
intramolecular bonding.
C
C
N
OH
H
H
H
O
C
CH
2
COOH
C
N
OH
H
H
H
O
C
CH
2
C
N
OH
H
H
H
O
NH
2
H
CH
2
CH
2
C
H
C
CH
2
C
N
OH
H
H
H
O
SH
Amine
group
Acid
group
Figure 2.17
Amino acid structures.
All amino acids have both an amine group (
¬
NH
2
) and
an acid group (
¬
COOH). They differ only in their R groups (green). Differences in their R groups
allow them to function differently in the body.
R
C
C
N
OH
H
H
H
O
R
C
C
N
OH
H
H
H
2
O
H
2
O
H
O
R
C
C
O
H
N
H
H
R
C
H
H
C
N
OH
O
Amino acid
Amino acid
Dipeptide
Dehydration synthesis:
The acid group of one amino
acid is bonded to the amine
group of the next, with loss of
a water molecule.
Hydrolysis:
Peptide bonds
linking amino acids together
are broken when water is
added to the bond.
+
Peptide
bond
Figure 2.18
Amino acids are linked together by peptide bonds.
Peptide bonds are formed
by dehydration synthesis and broken by hydrolysis reactions.
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