Chemistry Reference
In-Depth Information
. The polymer is
rigid and cross-linked. Mild hydrolysis disrupts the rigid secondary valence forces and produces
gelatin [
26
]. The fundamental unit of collagen exists as a triple helix [
41
]. Three left-handed helices
twist together to form a right-handed threefold super helix [
31
]. Collagen is composed mainly of
glycine, proline, and hydroxyproline. Some other amino acids are also present in minor amounts.
A protein that is similar to collagen is
elastin
, which is present in elastic tissues, such as tendons
and arteries. Hydrolyses of elastin, which has rubber-like properties, however, do not yield gelatin.
Mildly hydrolyzed elastin can be fractionated into two proteins [
26
].
Among the most studied globular proteins
are myoglobin
and
hemoglobin
. Myoglobin consists of
a single chain of 153 amino acid residues and a prosthetic group that contains iron, called
The protein of skin and extracellular connective tissues in animals is
collagen
heme
.
Myoglobin polypeptides have eight helical segments that consist of right-handed
-helices that are
interrupted by corners and non-helical regions. The overall shape resembles a pocket into which the
heme group just fits. The pocket is hydrophobic because all but two side groups are nonpolar.
The heme group's two carboxylic acids protrude at the surface and are in contact with water [
43
].
The hemoglobin is similar to myoglobin but more complex [
44
]. There are four heme groups enclosed
in the hemoglobin structure. Detailed conformational analysis has shown that hemoglobin is build up
from 2
a
2 myoglobin-like subunits,
a
2
and
b
2
[
45
,
46
].
is present in several animal and vegetable sources. Commercially, however, casein is
primarily obtained from milk that contains about 3% of this protein. The polymer is isolated either by
acid coagulation or with the help of enzymes obtained from animal stomachs. It is very heteroge-
neous. The molecular weight of a large portion of bovine casein is between 75,000 and 100,000.
It consists of two components,
Casein
. Casein belongs to groups of proteins that are identified as
phosphoproteins
because the hydroxyl residues of the hydroxy amino acids are esterified with
phosphoric acid.
One other group of proteins that has so far not been fully identified is
glycoprotein
. This group of
proteins contains a prosthetic group that is either a carbohydrate or a derivative of a carbohydrate.
Glycoproteins are found in mucous secretions.
Very special proteins are called
enzymes
. These are biological catalysts. Their primary function is
to increase the rate of reactions in organisms and they are found in all living systems. Many enzymes,
like
and
a
b
or
trypsin
, are relatively simple proteins. Others are conjugated proteins containing
prosthetic groups often known as
coenzymes
. Because of their extreme importance to biochemists,
enzymes and their actions are being investigated extensively. The full structures of several enzymes
have been determined. One such enzyme is
lysozyme
.
Lysozyme enzymes occur in many species of plants and animals and the chemical behavior may
differ. The enzyme found in egg white has a peptide chain consisting of two sections, approximately
equal in size. The two sections are separated by a deep cleft. This enzyme performs its function
by binding the substrates within this cleft with hydrogen bonds. The substrate is then hydrolyzed with
the aid of glutamine (35th amino acid) and aspertine (52nd amino acid). Egg lysozymes primary
structure contains 129 amino acid residues. The polymer is a single polypeptide chain that is cross-
linked at four places by disulfide bonds [
47
].
In addition, it was demonstrated [
72
] that the secondary structure of an enzymic protein is essential
to protein's catalytic activity. Also, it was shown that this structure remains intact in neat organic
solvents [
72
]. The molecules, however, are denatured in water-organic solvent mixtures. The
pepsin
-helix
of lysozyme, for instance, when the enzyme is crystalline or dissolved in neat acetonitrile, 35% of it is
an
a
-helix, but in pure water that value is 23%. In a 60:30 mixture by volume of acetonitrile and
water, it is reduced to 13% [
72
].
Some of the uncertainty about
a
the transition state of the reaction of some enzymes,
like
-phosphoglucomutase-catalyzed transfer of a phosphoryl group to a substrate in sugar metabolism,
was resolved recently. Allan and Dunaway demonstrated that by means of
19
F nuclear magnetic
resonance that the transition state involved a bipyramidal oxyphosphorane intermediate [
72
].
b
