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Файл:Carbohydrates. Bioenergetics. Study manual
.pdf
When replacing the hydroxyl group to the amino group of amino sugar is
formed. The most important representatives of amino sugars are aminoanalogues of
D-glucose and D-galactose:
Chemical properties of monosaccharaides
Monosaccharaides constitute multifunctional compounds. In this regard, their
chemical properties are due to the presence of both carbonyl and hydroxyl groups.
I. Reactions involving hydroxyl groups.
Monosaccharide is inherent in all reactions, characteristic of hydroxyl compounds: they form simple and complex esters, hemiacetals and ketones. Ester groups
are easy to put and also easy tiny, which allows their use as protective groups in the
synthesis of carbohydrates.
1. Reaction with alcohols the formation of glycosides (pyranosides, fu-
ranosides):
Semi-acetal hydroxyl is significantly different in its properties from other hy-
O
OH
H
H
H
OH
OH
H OH
H
CH2OH
CH3I, KOH
O
OH
H
H
H
OH
OH
H OH
H
CH2OH
OH
K
...
OH
K
...
CH
3
I
+
CH
3
I
+
KI,
H2O
O
OCH
3
H
H
H
H3CO
OCH
3
H OCH
3
H
CH2OCH
3
droxyl groups in the molecule of the monosaccharide. It can easily be replaced with
other nucleophilic groups, resulting in formation of various derivatives of sugars at
C-1 (including only 1-O-acyl derivatives).
Solutions of glycosides do not mutate, due to the lack of a glycoside hydroxyl. Glycosides easily hydrolyzed in acidic conditions, but stable in alkaline medium, as they are the Bronsted bases.
2. The formation of ethers.
Come into the reaction of all hydroxyl groups of carbohydrates:
Strong alkaline environment weakens the interaction of protons with oxygen atoms of all the available hydroxyl groups and facilitates the attack of an electrophilic
group, such as carbocationCH
+
which is formed by the interaction of the
3
cation K+with CH3I:
In the hydrolysis of ethers hydrolytic cleavage is exposed only to the glycosidic bond:
This is because the glycosidic bond is formed with the participation of acetyl
hydroxyl group having a higher reactivity.
3. The formation of esters.
When acylation of monosaccharaides participate in the reaction of all hydroxyl
groups due to the nature an acylating agent capable of acylating hydroxyl groups of
any activity:
Esters of the monosaccharaides are hydrolyzed in both acidic and in alkaline
media.
II. Reactions involving the carbonyl group.
1. Oxidation and reduction.
a) Oxidation with a strong oxidant:
b) Oxidation with a weak oxidizing agent:
c) Oxidation in an alkaline medium:
Monosaccharaides are oxidized by Benedict's reagent (B), Felling reagent (F)
and Tollens reagent. The principle of action of reagents are based on the
restoration Cu2+to Cu+by deposition of copper oxide Cu2O (red brick precipitate):
These reactions are used in biochemical laboratories for detecting aldoses and
ketoses in biological fluids (blood, urine).
g) The reduction reaction:
The reaction has found application for obtaining the sweetener that is used by
people who have diabetes.
2. Reactions of electrophilic accession.
a) reaction with hydrazine:
b) reaction with hydroxylamine:
Mutual transformation of aldoses and ketoses
Dilute alkaline solutions of monosaccharaides capable of isomerization from one
monosaccharide is formed an equilibrium mixture of different monosaccharaides:
Weakly alkaline medium catalyzes the keto-enol transformation of D-glucose in
the enol form, which is caused by the mobility of the hydrogen atom at C-2 due to
the proximity of two electron-deficient groups aldehyde and hydroxyl:
1.2. The oligosaccharides
Oligosaccharides are carbohydrates containing in their molecule two to ten
residues of monosaccharaides connected by glycoside linkages.
The simplest oligosaccharides are the disaccharides. The composition of the
disaccharide may enter the remains of the monosaccharaides of the same nature or
different. A required component is disaccharide α-D-glucose.
The most important disaccharides are maltose, lactose, sucrose and cellobiose.
Maltose consists of two residues of α-D-glucose:
Lactose is composed of residues of D-galactose and D-glucose:
Cellobiose consists of two D-glucopyranose residues connected by β-glycoside bond:
Maltose, lactose and cellobiose are reducing disaccharides, which are associated with the presence of free semi-acetyl hydroxyl group, resulting in preserved the
ability to disclose a cycle (takes place cyclo-oxo-tautomerism):
The solutions of these disaccharides mutarotate.
Sucrose is a non-reducing disaccharide due to the absence of a free semi-acetyl
hydroxyl group. Sucrose consists of D-glucose in pyranose and D-fructose in furanose forms:
In the hydrolysis of sucrose, a mixture of glucose and fructose called invert
sugar.
1.3. Polysaccharides
Polysaccharides are a high-molecular carbohydrates consisting of many
monosaccharide residues connected by α- or β-glycosidic bonds. Macromoleculesof
polysaccharides can be linear or branched.
The monosaccharaides contained in the composition of the polysaccharides can
be in the pyranose and furanose forms.
A distinction is made between homopolysaccharides consisting of the residues
of the same type of monosaccharaides and heteropolysaccharides consisting of the
residues of two or more types of monosaccharaides. To homopolysaccharides polysaccharides are of plant (starch, cellulose, pectin), animal (glycogen, chitin) and bacterial (dextrins) origin.
The heteropolysaccharides are part of the connective tissues (skin, cartilage,
tendons, etc.).
Polysaccharides are stable in an alkaline environment and are hydrolyzed in an
OH
n
O
O
H
H
H
OH
OH
H OH
H
CH2OH
O
O
H
H
H
OH
H OH
H
CH2OH
O
H
H
H
OH
H OH
CH2OH
acidic environment. Complete hydrolysis of polysaccharides leads to the formation
of monosaccharides.
a) the Homopolysaccharides.
The homopolysaccharides are divided into structural and reserve.
To reserve include starch, glycogen; structural cellulose (cellulose, chitin,
glycogen).
Starch is a mixture of two polysaccharides (amylose and amylopectin), com-
posed of residues of D-glucopyranose. Formed in plants during photosynthesis and
is contained in the tubers, roots, seeds. The chain of amylose is linear and comprises
200-1000 glucose residues. Its molecular weight is 160000.
In an aqueous solution of a macromolecular chain amylose is rolled into a spiral. When rapidly heated, the starch undergoes hydrolysis (of dextrinization), which
is accompanied by the formation of oligosaccharides.
Amylopectin has a branched structure (star-shaped). The molecular weight is
hundreds of times greater than the molecular weight of amylose and is 1–6 million
Glycogen in animal organisms is the functional analogue of the plant starch (reserve carbohydrate). A molecule of glycogen has considerable branching making it
easier to implement the energy function (quick regeneration of glucose in the human
body in case of stress, when physical and mental stress). The molecular weight of
n
OH
O
H
H
H
OH
H OH
CH2OH
O
H
H
H
H
OH
OH
H OH
CH2OH
O
H
H
H
H
OH
H OH
CH2OH
O
O
the carbohydrate is 100 million
Cellulose is a plant structural polysaccharide type. Structural unit of cellulose
is D-glucopyranose:
The molecular weight of the carbohydrate is from 400 000 to million 1–2 million Da/ Molecule of cellulosw is linear.
Cellulose is not broken down by enzymes of the gastrointestinal tract, but it is a
necessary ballast substance for normal human nutrition.
Great practical importance are the ether derivatives of cellulose acetate (artificial silk), xanthogenates (rayon, cellophane), nitrates (explosives).
The heteropolysaccharides in the body of the person performing the role of
connective tissue: skin, cartilage, tendon, ligament, joint fluid, cornea,
bones. Polysaccharides of connective tissue associated with proteins.
1.4. Individual members of carbohydrates
D-glucose (grape sugar) the most common monosaccharide. In free form is
found in plants (especially fruits) in the blood and lymph of humans and animals.
D-fructose (fruit sugar) contained in fruits and honey.
Glycosides (derivatives of monosaccharides only by the glycoside OH group) is
widely represented in plants: part of color pigments, aromatic substances, natural
dyes, substances that stimulate the heart activity of a person. The latter has found
wide application in the manufacture of drugs.
Xylitol easily soluble substance with a sweet taste. Used as sugar substitute
for diabetes patients. Xylitol is synthesized by recovering D-xylose with hydrogen in
the presence of a catalyst.
Aminosugars. The most important representatives of amino sugars are 2aminoanaloges of D-glucose and D-galactose.
Esters. Phosphates esters of monosaccharaides and phosphoric acid are con-
tained in all vegetable and animal organisms and constitute a metabolically active
form of monosaccharaides. The most important phosphates are the phosphates of Dglucose and D-fructose. The phosphates of D-ribose and 2-deoxy-D-ribose serve as
structural components of nucleic acids and several coenzymes.
Esters of monosaccharaides and sulfuric acid sulfates part of the polysaccharides of connective tissue.
Oligosaccharides in the form of sucrose are present in almost all plants: seeds,
leaves, fruits, roots. The content of sucrose in sugar beets makes up 17-19%.
The second major group of natural oligosaccharides is oligosaccharides of human milk that play an important role in the formation of the intestinal flora of neonates, is necessary for normal digestion.
Cellulose is the most abundant polysaccharide. Its main source wood con-
tains about 50 % cellulose. Cotton represents almost pure cellulose. On the basis of
cellulose and its derivatives to produce plastics, explosives, emulsifiers.
A rich source of polysaccharides are sea water were growing up, where their
content reaches approximately 80%. On an industrial scale from red algae get agar,
which is a mixture of sulfated polysaccharides. Agar has found wide application in
biochemical research.
Proteoglycans – complex carbohydrates of high molecular weight compounds
consisting of a protein portion (about 5–10 %) and carbohydrate glycosaminoglycans
(about 90–95 %). They are the main component of the extracellular matrix of connective tissue and can be up to 30 % of dry weight fabric. Proteins in proteoglycans is represented by a single polypeptide chain of different molecular weight. The polysaccharide components of proteoglycans differ from a large group of proteins that are
called glycoproteins. It is the complex proteins that contain oligosacharide chains of
different lengths covalently attached to the polypeptide basis. The carbohydrate component of glycoproteins is much smaller in mass than that of proteoglycans, and is not
more than 40 % of the total weight. Glycoproteins perform in the human body a variety of functions and are present in all classes of proteins – enzymes, hormones,
transport, structural proteins and other representatives of glycoproteins – collagen
and elastin, immunoglobulins, angiotensinogen, transferrin, ceruloplasmin, Castle's
intrinsic factor, thyroid stimulating hormone.
Glycosaminoglycans – linear negatively charged heteropolysaccharides. They
are also called mucopolysaccharides because they are detected in the mucous secrets
(mucosa), giving them a viscosity and lubricating properties. Due to its hydrophilic
glycosaminoglycans can bind significant amounts of water, resulting in extracellular
matrix becomes a gelatinous character.
Glycosaminoglycans are long unbranched chain of heteropolysaccharides. They
are built from repeating disaccharide units. One monomer of the disaccharide is
hexuronic acid (D-glucuronic acid or L-iduronic acid), the second monomer is an
aminosugar derivative (glucose or galactosamine). NH2-group of aminosugars usually acetylated, which leads to the disappearance of their inherent positive
charge. Exept the hyaluronic acid, all glycosaminoglycans contain sulfate groups in
the form of O-esters or N-sulfate.
Glycosaminoglycans and proteoglycans are essential components of the extracellular matrix, play an important role in intercellular interactions, the formation
and maintenance of cells and organs, the formation of the frame during tissue formation. Due to the peculiarities of its structure and physico-chemical properties of pro-
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