Glucid

The glucids , historically called carbohydrates , are a class of Molécule S of the Organic chemistry. Their Chemical formula is based on the model C_N (H₂O) _p. However, this model is not valid for all the glucids, which are made up, for some, of atoms of Azote or Phosphore (for example).

They form part, with the Protéine S and the Lipide S, of the essential components of the living beings and their Nutrition, because they are one of the principal biological intermediaries of storage and consumption of energy. At the organizations Autotrophic S, like the Plant S, the Sucre S are converted into Amidon for storage. At the organizations Heterotrophic S, like the animal , they is used like energy source in the metabolic reactions, their oxidation during the digestion of glucids bringing approximately 17 kJ/G according to the study in the calorimetric Bombe.

Structure, classification and nomenclature

Simple oses

The oses have a whole a function Carbonyle:

Aldehyde for aldoses (example: Glucose);
Ketone for the ketoses (example: Fructose).

They are characterized by their number of Carbone:

the trioses have 3 Carbone S: Dihydroxyacetone, Glycéraldéhyde;
tetroses have 4 Carbone S: érythrose, Threose, érythrulose;
pentoses have 5 Carbone S: Ribose, Arabinose, Xylose, Lyxose, Ribulose, Xylulose;
hexoses have 6 Carbone S: Draw sheet, Altrose, Glucose, Mannose, Gulose, Idose, Galactose, Talose, Psicose, Fructose, Sorbose, Tagatose;
heptoses have 7 Carbone S; Sédoheptulose;
the octoses have 8 Carbone S.

Linear representation: model of Fischer

All the oses have a rotatory Pouvoir because of presence of a asymmetrical Carbone, the oses are known as chiral.

Two enantiomer S (antipodes optical) only have the same properties except for one: their optical Activity opposite. Figure 1 accounts for both enantiomer S of the Glucose, the form D-glucose is the natural form.

In the form D , the grouping alcohol (- OH) carried by carbon $n-1$ is with right (of representation of Fischer);
In the form L , the grouping alcohol (- OH) carried by carbon $n-1$ is with left (of representation of Fischer).

There exist also stéréoisomères which are optical isomers. All the enantiomers are stéréoisomères. There are also diastereoisomers which are stéréoisomères not enantiomers. There are épimères which are sugars which differ by the configuration from a single carbon.

Cyclic representation: model of Haworth

When one leaves a solution in the water of glucose crystallized beforehand in water, the capacity passes from +112° to +52,5°. When one leaves a solution in the water of glucose crystallized beforehand in ethanoic acid, the capacity passes from +19° to +52,5°.

This observation is due to the fact that the function Aldéhyde of glucose is hydrated to form a function alcohol which will react with another function alcohol (that of carbon 5). Glucose becomes cyclic then, it derives from the Pyrane. Carbon n°1 is a carbon anomère (the cyclic isomers are called anomères), the grouping of the function alcohol which it door can be “in top” or “bottom”. If it is in top, the grouping hydroxyl is same side that of the series, it is the form beta , if not, hydroxyl opposed to that of the series, it is the form alpha . Figure 2 explains the cyclization of glucose. In aqueous solution, the Glucose is in tautomeric balance: 65% pennies form β-D-glucopyranose, 0,1% pennies form D-glucose (linear) and 35% d'α-D-glucopyranose. The cyclization of sugars makes it possible to have twice more isomers.

The Fructose, cétohexose, cyclise by reaction between carbons 1 and 4, this form derives from the Furane, it is the furan form (figure 3). Glucose also but the D-glucofuranose is not stable and is transformed quickly into D-glucopyranose.

The Ose S being cyclisant in furan form are:

the Ribose giving ribofuranose;
2 - Désoxyribose giving 2-désoxyribofuranose;
the Fructose giving fructofuranose.

The Ose S being cyclisant in pyranic form are:

the Glucose giving glucopyranose;
the Galactose giving galactopyranose.

== optical Anomalie ==

The specific optical activity of a solution of glucose coldly prepared is not stable. Its value evolves/moves in time to lead to the value characteristic of 52.7°. It is the phenomenon of transfer-rotation. He is explained by the existance stéréoisomères alpha and beta of glucose. This rotation transferred is explained by the fact why in solution, glucose has an asymmetrical carbon moreover than in linear form.

Glycosides

The glycosides are Polymère S of Ose S bound by a osidic connection. They are hydrolisables, and also called complex glucids.

Formation of the osidic connection

A osidic Liaison is a Covalent bond produces reaction between the hemiacetalic function alcohol (- OH formed by carbon Anomère) and another molecule (glucidic or not).

Diholosides

Not-reducers

It is said that a diholoside is not reducer if bearing carbon 1 the hemiacetalic OH is engaged in a connection. In other words the final connection is of the " type; oside"

The Saccharose

It is a diholoside not reducer, its official name is l'α-D-glucopyranosyl (1→2) β-D-fructofuranoside

It is divided into 2 molecules: the D-glucopyranose and the D-fructofuranose by invertase.

The Trehalose

One finds it in mushrooms, the bacteria or in the hemolymph of the insects. Many organizations accumulate it in answer to a thermal shock. Its official name is l'α-D-glucopyranosyl (1→1) α-D-glucopyranoside

It is divided into 2 molecules: 2 molecules of D glucopyranose by invertase.

Reducers

The Lactose

It is a reducing diholoside, it is the glucid of the Lait, its official name is the β-D-galactopyranosyl (1→4) - D-glucopyranose

It is hydrolized in 2 molecules: the D glucose and the D galactose by beta galactosidase.

The Maltose

It is a diholoside homogeneous reducer, its official name is l'α-D-glucopyranosyl (1→4) D-glucopyranose

It comes from the hydrolysis partial of the Amidon. It can be in its turn hydrolized by alpha glucosidase in 2 molecules of D-glucose.

The Cellobiose

It is a diholoside homogeneous reducer, its official name is the β-D-glucopyranosyl (1→4) D-glucopyranose

It comes from the hydrolysis partial of the Cellulose and is hydrolized itself by beta glucosidase.

The Isomaltose

It is a breakdown product of the starch and glycogen. Its official name is l'α-D-glucopyranosyl (1→6) D-glucopyranose

Determination of the structure of a diholoside

It consists of 2 oses bound by osidic connection and can be reducing or not. For example: the Maltose, the Saccharose, the Lactose, the Cellobiose…

Triholosides

The Raffinose

Present in beet, it is eliminated during the refining of sugar. It is a triholoside not reducer, its official name is l'α-D-galactopyranosyl (1→6) α-D-glucopyranosyl (1→2) β-D-fructofuranoside.

The Gentianose

Present in the gentian. Its official name is l'α-D-glucopyranosyl (1→6) saccharose.

Polyholosides

Association of a very great number of molecules bound by O-glycosidiques connections. Chains linear or is ramified.

The Starch

It is a homogeneous polyholoside not reducer made up of two made up:

the amylose, (water-soluble) polymeric of glucose bound by a connection α (1→4); (20 to 30%),
the amylopectin, (insoluble) amylose ramified by a connection α (1→6). (70 to 80%)

The starch is a polymer of Maltose. And can be hydrolized by alpha glucosidase. It is the glucidic substance of reserve of the vegetable . The starchy foods are a major source of starch in the human consumption.

It is highlighted by the Lugol's solution (iodized water) which leads to a characteristic black coloring.

The Glycogen

On the level of its structure, it is practically identical to the Amidon: it has more ramifications than the Amidon (a ramification all the 10 residues glucose), all the remainder of the structure is identical to the starch. Its molar Masse is higher (about 10⁶ g.mol^-1). It is the glucidic substance of reserve of the animals. Contrary to the Starch, it is soluble in cool water.

The Glycogen at summer extracted by Claude Bernard in 1856 in the liver. It is the shape of storage of surplus glucose at the animals and mushrooms. It is also about a polymer of ramified glucose (a connection all 6 to 8 units glucose). The glycogen is formed by the Glycogénogenèse and is broken up by the Glycogénolyse.

It is found of surplus in the case of the glycogénose of the type II; disease in which the acid absence of maltase of the lysosomes, prevents its degradation.

The glycogen is highlighted by the Lugol's solution (iodized water) which leads to a coloring brown mahogany tree.

The Cellulose

It is a homogeneous polyholoside of glucose bound by a laison β (1→4).

The principal molecule structural of the plants is the Cellulose. Wood is partly composed of cellulose, while paper and cotton are almost pure cellulose. The cellulose is a polymer of glucose. It is a very long and rigid molecule, of which the structure confers its mechanical properties to him as observed at the plants.

It cannot be comparable by the human being but its presence in the food supports the intestinal transit time and protects the organization against cancer from the colonist.

The Agar-agar

It is a mixed polyholoside made up of D- and L-galactose esterified by sulphuric Acid . Extracted from algae, it is used in Bactériologie and in food industry like gelling.

The Hyaluronic acid

It is a mucopolysaccharide and made up of acid-β-glucuronique and NR - acétylglucosamine connected by a connection β (1→3). It is one of the principal components of the fundamental Substance of the conjunctive Tissu on which it confers its viscosity.

The Heteroside S

The Nucleotide S

They consist of a nitrogenized Base, of a Ose (Ribose, 2 - Désoxyribose)) and of a grouping Phosphate. They are N-heterosides

Let us quote the Adénosine monophosphate, and its derivatives used in the transport of intracellular energy, the ribonucleic acid (ARN) charged to guide the cellular synthesis of the Protéine S, the desoxyribonucleic acid (DNA) carrying heredity…

ONPG

Used in systematic bacteriology, the orthonitrophényl-β-D-galactopyranoside is hydrolized by a β-galactosidase.

It is a O-heteroside .

Chemical properties

Reducing properties

The simple oses and the diholosides having free their carbon Hémiacétalique are reducing from their function Aldéhyde. The function Aldéhyde is oxidized in function Carboxylic acid. One of the functions primary alcohol can be oxidized in function Carboxylic acid.

The diholosides not reducers are those whose no carbon Hémiacétalique is free, it is concerned in the osidic Liaison.

Reduction of the metal ions

Reduction of the ion Copper II (Cu²⁺) out of copper oxide I (Cu₂O) (Fehling's solution). This property is used in the Méthode of Bertrand for the proportioning of the Glucide S.

Réduction of organic compounds

For example 3,5-dinitrosalicylate (DNS) is tiny room in 3-amino-5-nitrosalicylate, made up brown red making it possible to proportion the reducing oses by Colorimétrie.

Oxidation by enzymatic way

In the presence of Dioxygène, glucose Oxydase oxide the Glucose in gluconolactone (then gluconic acid) with release of Hydrogen peroxide. The oxidation of the carbon 1 (carrying the pseudo-aldehydique function) led to the gluconolactone (not reducer), that in C6 led to the glucuronic acid (reducing). A double glucosaccharidic oxidation in C1 and C6 led to the acid (not reducer). Glucose oxydase is used to measure the Glycémie.

Hydrolize osidic connection

Chemical hydrolysis

The chemical hydrolysis is not specific, it leads to the smallest sub-unit of the Glucide S: the Ose S. It is carried out in the presence of Hydrochloric acid

Enzymatic hydrolysis

The enzymatic hydrolysis, contrary to the chemical hydrolysis, is specific. These Enzyme S is Hydrolase S.

the β-glucosidase hydrolysis the osidic connections bringing into play a glucose whose l'-OH hemiacetalic is in position β;
L'α-amylase breaks the osidic connections inside the amylose chain;
the β-amylase hydrolizes the osidic connections starting from the ends.

See too

glucid

Metabolism of the glucids
Biochemistry
Macromolecules
Glucose

External bonds

Course on glucids (pdf)
the Glucids
Nomenclature of sugars

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