Chemistry Reference
In-Depth Information
-amino acids are
possible and the proteins are very complex molecules. The arrangement or sequence of amino acids in
proteins is referred to as their
primary structure
. The amide linkage is referred to in biochemistry as
the
peptide linkage
or
Because R represents many different groups, many different combinations of
a
the
peptide group
.
A dipeptide then is a compound consisting of two amino
acids, a tripeptide of three, etc.
refers to proteins, though the term is often reserved
for lower molecular weight fractions, usually less than 10,000. Many proteins are monodisperse.
This distinguishes them from many other naturally occurring polymers, such as polysaccharides, that
are polydisperse.
Polypeptide
8.5.1
-Amino Acids
a
Twenty-five known naturally occurring amino acids were isolated from various proteins by hydrolysis.
All but one of them, glycine, possess an asymmetric carbon. Table
8.1
lists the naturally occurring
amino acids and gives their structures [
26
,
28
,
31
].
Among the above shown amino acids, a certain number are known as
essential amino acids
.
They are not synthesized by human bodies and must be ingested for human metabolism.
All the optically active amino acids (that means all except glycine) have an
L
configuration.
In addition, all amino acids exist as zwitterions.
8.5.2 Structures and Chemistry of Proteins
Proteins can be separated into two major groups,
fibrous proteins
and
globular proteins
, depending
upon their shapes. The fibrous proteins are long molecules that function as structural materials in
animal tissues. Hydrogen bonding holds these water insoluble molecules together to form extended
coiled chains. To this group belong
, protein of the connecting tissues;
myosin
, protein of the
muscles;
keratin
, protein found in hair, nails, horns, and feathers; and
fibroin
, protein of silk fibers.
Globular proteins are held by strong intramolecular hydrogen bonds in spherical or elliptical
forms. Their intermolecular forces are weak and they are soluble in water and in dilute salt solutions.
To this group of proteins belong enzymes, many hormones, egg albumin, and hemoglobin.
Some proteins also contain a non-peptide portion that is attached chemically to the polyamide
chain. The non-peptide moieties are called
prosthetic groups
, and the proteins with such groups are
called
conjugated proteins
. Examples are hemoglobin and myoglobin that consist of polypeptide
portions with iron-porphyrin prosthetic groups attached. This particular prosthetic group, called
heme
, is illustrated in Fig.
8.2
. There are also a number of proteins that are associated with a nucleic
acid. They are known as
nucleoproteins
.
Numerous studies of protein structures have shown that the common conformations of the protein
chains (fibrous) can be either as an
collagen
-sheets, or random coils [
26
]. The steric arrangement or the
conformations of the proteins are referred to as the
-helix,
a
b
-
amino acids in the polypeptide chains is called the
primary structure
[
26
]. Based on X-ray crystallogra-
phy data, Pauling et al. [
24
] deduced that an
secondary structure
, while the composition of
a
type configuration is formed because it
accommodates hydrogen bonding of each nitrogen to a carbonyl oxygen (see Fig.
8.3
). It allows space
for all bulky substituents in amino acids and stabilizes the structure. The
a-helix
-helix is probably the most
a
important secondary structure in proteins [
26
]. The two
-helix illustrations are after Pauling et al. [
24
].
The one on the left shows right-handed helix. It is interesting to note that an
a
-helix conformation may
a
