Monosaccharide

(Also called simple sugars ) monosaccharides are a group of organic chemical compounds. They are the products of partial oxidation of polyhydric alcohols. All monosaccharides have a chain of at least three carbon atoms as the basic structure and have a carbonyl group and at least one hydroxyl. They are the building blocks of all carbohydrates and may be subject to disaccharides ( double sugars ), oligosaccharides (several sugars ) or polysaccharides connect ( multiple sugars ).

The monosaccharides glucose, fructose, and galactose are the most important sugar metabolism. They are energy sources and also serve as cellular components.

Construction

Each simple sugar consists of a chain of carbon atoms. According to the number of carbon atoms is referred to as trioses (3), tetroses (4), pentoses (5), hexoses (6), heptoses (7 ), etc. The smallest monosaccharides, triose, so three carbon atoms. In principle, the length of the carbon chain is unlimited in nature only simple sugars have been observed but with a maximum of nine carbon atoms, where hexoses and pentoses are most commonly found.

Furthermore, located at one of the carbon atoms of the acyclic ( open chain ) form a double-bonded oxygen atom, that is a carbonyl group. The carbonyl group is at the end of the carbon chain, refers to the group as aldehyde and aldose sugar, at a carbonyl group within the chain is called a keto group and the sugar of ketoses.

Both nomenclatures can be applied together, so that one speaks in a simple sugars with six carbon atoms and an aldehyde group of an aldohexose.

Another classification ability of the monosaccharides is to distinguish between open-chain sugars and cyclic sugars ( aldose or ketose ). The cyclic monosaccharides are aldoses or ketoses from the corresponding derived hemiacetals or hemiketals. A distinction furanose ( five-membered rings ) and pyranose ( six-membered rings ) with one oxygen atom in the ring.

In the simplest representatives of the other monosaccharides without carbonyl carbon atoms each carry a hydroxyl group ( OH group) and otherwise hydrogen atoms. For these compounds, the empirical formula is: CnH2nOn.

However, also includes derivatives of these simple connections to the monosaccharides, such as amino sugars (eg, glucosamine ) and deoxy sugars (eg, deoxyribose ). Do not conform to this general empirical formula.

Spatial structure

In addition to the position of the carbonyl group ( oxo group ) in the carbon chain and the spatial arrangement of the OH groups has an important role. For example an aldohexose, four of the carbon atoms depending on the ' right ' and ' left ' attached OH group distinguishable. There are thus a total of 16 (24 = 16) different stereoisomers of an aldohexose, which differ in metabolism and in the optical activity. Furthermore, the number of theoretically possible stereoisomers fivefold or as addition to the open-chain form of the furanose or pyranose form can arise with α - or β - configuration, by intramolecular formation of a cyclic hemiacetal.

The stereochemical representation can be made in three equal ways. The oldest representation of the Fischer projection in which all carbon-carbon bonds in an imaginary ( thermodynamically unfavorable ) eclipsed position are written unrolled vertically above one another and to the plane of the paper. The substituents, here hydrogen atoms and hydroxyl groups are listed depending on the configuration on the right or left and are above the plane of the paper, thus resulting in a unique configuration. The hydroxy group of the farthest from the anomeric carbon atom remote chiral C atom is in the right position, the D- configuration in the left position, the L- configuration (see the example of D- and L- ribose above).

The cyclic hemiacetal is also Tollens ring formula given representation ( 1) confusing and there are long bonds required. Therefore, further representations were developed. The Haworth representation ( 2) corresponds to an "on the side down " and rolled Fischer projection. All ring atoms are located on one level, the spatial impression can be reinforced by ties perspective. The bond of a substituent "up" to identify that this is above the ring plane. The in the Fischer projection to the left ( or right) looking at Haworth groups show ring up ( or down ).

Even more realistic is the Konformationsformel ( 3), since in this case the angled arrangement of the carbon chain is recognizable. The stereochemical representation (4 ) is common.

Important monosaccharides

Pedigree of the aldose and ketose

Trioses

• Aldotriose D- and L- glyceraldehyde ( metabolites )

• ( Dihydroxyacetone) ( Since the dihydroxyacetone has no stereogenic center, it really is not considered a monosaccharide. However, it is involved in carbohydrate metabolism and significant. )

Tetroses

• Aldotetrose D- erythrose ( metabolite )
• D- Threose ( metabolite )

• D- Erythrulose ( metabolite )

Pentoses

• Aldopentose D- ribose ( comes et al in RNA ago)
• D- and L- arabinose ( considered include vegetable oligosaccharides ago)
• D-xylose ( including wood sugar, a main component of hemicellulose )
• D- lyxose ( is not found in nature )
• D -deoxyribose (coming inter alia, in the DNA ago)

• Ketopentose D -ribulose ( metabolite )
• D-and L -xylulose (the former is part of the pentose phosphate pathway, the latter of the glucuronate metabolism )

Hexoses

• Aldohexoses D- allose ( is not found in nature )
• D- altrose (very rare? )
• D- glucose ( dextrose also, most abundant monosaccharide )
• D -mannose ( common monosaccharide )
• D- Gulose ( is not found in nature )
• D- Idose ( is not found in nature )
• D- galactose ( mucus sugar, frequent monosaccharide )
• D- Talose (very rare, part of the formed by Streptomyces antibiotic effect Hygromycine )

• More physiologically important hexoses D-glucuronic acid (6 -carboxy -D-glucose, commonly, is usually as glucuronate or esterified before )
• D-galacturonic acid (6 -carboxy -D-galactose, is usually as uronate or esterified before )
• N-acetyl -D-glucosamine ( N- Acetylchitosamin, monomer of the chitin is widely available)
• D -glucosamine (also chitosamine, monomer of chitosan )
• N -acetyl -D-galactosamine ( N- Acetylchondrosamin, is widely distributed )
• D and especially L - fucose (6- deoxy-D- and L-galactose, the latter is widely distributed )
• L- rhamnose (6- deoxy -L -mannose, comes in vegetable oligosaccharides ago)
• D- quinovose (6 -deoxy -D-glucose, is used, for example, in vegetable oligosaccharides before )

• Ketohexoses D- fructose ( fruit sugar also, frequent monosaccharide )

Higher monosaccharides

• D- sedoheptulose ( C7 - keto sugars, is implicated as a 7- phosphate in the pentose phosphate pathway )
• 3-deoxy- D- manno - oct-2- ulosonsäure (also 2-keto -3- deoxyoctonate, KDO, a C8 - sugar, important component of lipopolysaccharides of the cell surface of certain bacteria)
• D- sialic acid ( N- acetylneuraminic acid, a C9 ​​keto sugars in the cell -cell recognition in glycoconjugates play a role )

Photosynthesis

The starting point of most simple sugars in living things is the oxygenic photosynthesis. During this process of CO2 ( carbon dioxide ) and H2O (water) hydrogen atoms contained sugar built using solar energy. In the water splitting requisite oxygen is released as a waste product.

Simple sugars in food

Simple sugars are as glucose (dextrose ) and fructose (fruit sugar) in foods such as fruits, honey and sweets. The galactose, the so-called mucilage sugar in milk, is a simple sugar. In contrast, sucrose, lactose or maltose disaccharide; Starch and glycogen are multiple sugar. All higher sugar must first be degraded to water-soluble di-or monosaccharides to about transport proteins - such as the glucose transporter - to be included or simple diffusion into the blood and transported to the liver can.

Oral intake of the monosaccharides glucose and galactose results in a rapid rise in blood glucose levels; all other simple sugars are metabolized primarily in the liver and have no direct effect on the glucose levels in the blood. Since the blood glucose level needs to have a narrow range of fluctuation, it is necessary for the body to counteract the rapid rise due to rapid processing. The insulin level rises and the blood sugar in the liver glycogen - a polysaccharide of glucose units - converted. This produces a fast energy storage in the liver, as glycogen, if necessary can be rapidly broken down into glucose again. Excess glucose that can not be stored as glycogen, is in adipose tissue and liver in triacylglycerols (fats) converted to serve in the liver, skeletal muscle and fat cells as an energy depot.

Nutritionists recommend recording a maximum of 10 % of the total amount of energy by single and double sugar. Multiple sugars such as va strength are considered better suited to meet the carbohydrate needs, as they must first be converted into simple sugars in the gastrointestinal tract, resulting in a much slower absorption of carbohydrates.