Chemistry Reference
In-Depth Information
Fig. 8.3
-Helix structure
a
of proteins
where the folding of the helix is such that it allows hydrogen bonding between distant sites.
The overall, three-dimensional picture of a protein structure is referred to as the
tertiary structure
.
Disruption of the tertiary structure in proteins is called
denaturation
. When the protein is composed
of more than a single peptide chain, the arrangement is called a
quaternary structure
. This associa-
tion results from non-covalent interactions.
There is a relationship between the primary structures, or the amino acid content of many proteins,
and the secondary structures [
27
]. The helical contents are inversely proportional to the amount of
serine, threonine, valine, cysteine, and proline in the molecule. Conversely [
28
], valine, isoleucine,
serine, cysteine, and threonine are non-helix-forming amino acids. Proline, due to its specific
configuration, actually disrupts the helical structure when it is present in the polypeptide [
29
].
In addition, proteins that are composed of low ratios of polar to nonpolar amino acids have a tendency
to aggregate [
30
]. Also, the globular protein, will, in an aqueous environment, tend to form shapes
with nonpolar groups located inside the structure. This is due to the thermodynamic nature of the
hydrophobic side chains. The polar ones, on the other hand, tend to be located outside, toward the
water [
31
].
To date, much more information is available on some proteins than on others. Some of the more
thoroughly explored proteins will be mentioned below.
Keratins
are proteins that are found in wool, hair, fur, skin, nails, horns, scales, feathers, etc.
They are insoluble because the peptide chains are linked by disulfide bonds [
32
,
33
]. Many keratins
contain coils of
-helixes [
34
-
36
]. Some keratins, however, were found to consist of complicated
a
-helical structures. This apparently has not been fully explained. Wool keratin was shown to range in
molecular weight from 8,000 to 80,000 [
37
]. The extensibility of
b
-keratins is believed to be due to
the helical structures. The extent of keratin hardness (in claws, horns, and nails) is believed to be due
to the amount of sulfur links.
Silk fibers, which are obtained from the secretion of the silkworm, are double filaments that are
enclosed by a coating of a gum (sericin) as they are secreted [
40
]. The amino acid sequence of the silk
protein was shown to be (glycine-serine-glycine-alanine-glycine-alanine)
n
. The polypeptide chains
are bound into antiparallel pleated
a
-sheet structures by hydrogen bonding [
31
,
39
,
42
]. The structures
are also held together by van der Waal forces [
31
,
38
].
b
