Chemistry Reference
In-Depth Information
N
-terminus
α
-helix
H
C
N
R
R
O
H
C
H-bonding in α-helix
H
N
C
H
H
C
N
C
R
2
R
4
H
O
H
O
H
O
R
H
O
R
N
N
N
O
C
N
N
H
N
C
H
C
R
1
R
3
R
5
H
O
H
O
H
C
N
O
H
N
-terminus
C
-terminus
O
R
R
N
C
C
C
H
H
H
O
N
C
O
C
-terminus
Figure 13.2
Secondary structure of proteins: hydrogen bonding in
α
-helix
-pleated sheet provides a particularly
nice and easily appreciated example of regular
hydrogen bonding in polypeptide chains, the most
common arrangement found in proteins is actually
the
Whereas the
β
properties. This elasticity can be traced back to the
α
-helix structure, in which weak hydrogen bonds
are parallel to the direction of stretching, i.e. a
spring-like structure. On the other hand, proteins
such as
α
-fibroin in silk are relatively inelastic since
α
-helix (Figure 13.2). Do not worry about the
α
they
-pleated sheet structure, where
extension is resisted by the full strength of covalent
bonds. In the
contain
the
β
or
used in the nomenclature; this merely signifies
that the helical structure (
β
α
) was deduced earlier than
-pleated sheet, the weaker hydrogen
bonds would be perpendicular to the direction of
stretching. However, most proteins actually have
a roughly spherical shape and are thus termed
globular proteins
. Globular proteins are likely to
contain portions of the polypeptide chain that adopt
both helical and sheet structures. In contrast to the
structural proteins like
α
-keratin or
α
-fibroin, the
helical or sheet fragments in globular proteins are
rather short and do not extend far without a change in
direction. The overall folding of the polypeptide chain
and the three-dimensional arrangement produced
provide the
tertiary structure
of the protein. The
globular shape is facilitated, however, by a number
of other non-covalent interactions.
β
that of the pleated sheet (
).
The
α-helix
is a right-handed helix, an ordered
coil array stabilized by hydrogen bonding between
carbonyl and N-H groups in the same chain. In
a right-handed helix, movement along the chain
involves a clockwise or right-handed twist, just
like an ordinary screw - you turn the screwdriver
clockwise. Hydrogen bonds link carbonyl and N-H
bonds in amino acids that are separated by three other
residues, and each turn of the helix is found to take up
3.6 amino acid residues. Note that all of the R groups,
which in the majority of amino acids are quite bulky,
are accommodated on the outside of the helix. Only
the imino acid proline cannot fit into the regular array
of the
β
-helix. We have just seen that
proline
must
distort the regular array and introduce a bend into the
chain, and that there is no N-H for hydrogen bonding.
The secondary structure is responsible for some
of the physical properties of proteins. For example,
structural proteins such as
α
13.3.1 Tertiary structure: intramolecular
interactions
-kera insinskinand
hair are fibrous in nature, and have good elastic
α
The conformation of a protein is determined and
maintained by a range of intramolecular interactions
