Nutrient chemistry

Content of the subsection

 * 1) Nutrient overview – sacharides
 * 2) Nutrient overview – lipids
 * 3) Nutrient overview – proteins

Clasification and structure
Sacharides, also called carbohydrates or glycids, are the most abundant organic substances on Earth. Their molecules are made up of oxygen, carbon and hydrogen atoms. From a chemical point of view, these are polyhydroxyaldehydes and polyhydroxyketones. They contain functional aldehyde or keto groups in their molecule, as well as a larger number of hydroxyl groups.

Clasification of carbohydrates
According to the number of units in the molecule, we distinguish:


 * monosaccharides – cannot be further hydrolyzed into simpler units;
 * oligosaccharides – they form 2–10 units of monosaccharides by hydrolysis;
 * polysaccharides – hydrolyzing into more than 10 monosaccharides.


 * Monosaccharides and oligosaccharides are generally called sugars. A synonym for polysaccharide is the word glycan.

We divide monosaccharides according to:


 * Number of C-atoms: trioses, tetroses, pentoses, hexoses.
 * Functional groups: aldoses and ketoses.

We divide polysaccharides into:


 * Homopolysaccharides: these are polymers made up of the same type of monosaccharide. Examples are starch, glycogen or cellulose.
 * Heteropolysaccharides: they are polymers made up of more than one type of monosaccharide. An example is hemicellulose.

Structure of saccharides
The structure of a saccharide molecule can be expressed in different formulas:
 * Linear (Fischer) formula;
 * Cyclic (Haworth) formula, which results from the formation of a heterocyclic structure.


 * The cycle can contain:
 * six atoms - pyranose - similar to six-carbon pyran;
 * five atoms - furanose - similar to five-carbon furan.


 * Tollens' formula describes the creation of a cyclic structure from a linear formula. It shows the reaction of hydroxyl with a carbonyl group to form a so-called hemiacetal structure.

Isomerism
It is a state where compounds with the same general formula have a different structural arrangement of atoms in the molecule. The following types of isomerism are found in carbohydrate molecules.


 * D- and L- prefixes
 * It is determined by the position of the −OH group on the last chiral carbon. Assignment of the prefix is based on similarity with the original compound of the carbohydrate series – glyceraldehyde. The −OH group is located on the right for D- and on the left for L- isomers in the Fischer formula.


 * D- and L- isomers are mirror images - so-called enantiomers - optical isomers. They differ in the sign of optical rotation, or the direction in which they rotate the plane of polarized light.


 * However, it is not generally the case that the D-isomers rotate light to the right and the L-isomers rotate light to the left.
 * An equimolar mixture of enantiomers is called a racemic mixture, or a DL mixture, and does not exhibit optical activity.
 * D-isomers are more common in nature.




 * Pyranoses and furanoses
 * They are labeled according to the similarity of the cyclic form of the respective monosaccharide with the pyran or furan cycle. Glucose in solution occurs more than 99% in the form of gluco-pyranose, the rest of the molecules, less than 1%, then appears in the form of gluco-furanose.


 * α- a β- anomers
 * They are labeled according to the position of the hemiacetal or hemiketal −OH in the cycle. Hemiacetals are formed by the reaction of aldehyde and alcohol groups, hemiketals by the reaction of keto and alcohol groups.


 * If the −OH group is oriented to the same side as the −OH group indicating belonging to the D- or L- group, it is an α-anomer. If the −OH group is oriented to the opposite side, it is a β-anomer.

Anomers differ in optical rotation. Examples are glucose and mannose.
 * Epimers
 * hey differ from each other by the position of one −OH group in the molecule.




 * Aldoses a ketoses
 * They are labeled according to the functional group on the 1st and 2nd carbons of the molecule.

Carbohydrates are not essential for the body and are normally synthesized in it, e.g. from amino acids or glycerol.


 * Monosaccharides and disaccharides represent an important source of energy. They are especially necessary for brain cells and erythrocytes.


 * Polysaccharides serve as energy storage - glycogen in animals.

Carbohydrates also perform structural functions, for example as part of glycoproteins and glycolipids in membranes.

They also play a key role in the synthesis of nucleic acids or coenzymes. They are also part of the intercellular mass, for example in proteoglycan molecules.

Monosaccharides and disaccharides

 * Monosaccharides and disaccharides are white crystalline substances soluble in water, of a neutral nature, which do not dissociate in aqueous solutions. They have a polar character and the −OH groups cause their sweet taste and strong hydration in solution.


 * The most important monosaccharides in food are glucose, fructose and galactose. Of the disaccharides, sucrose α-Glc (1→2) β-Fru used as a sweetener, beet sugar, lactose, β-Gal (1→4) β-Glc, present in milk and maltose, α-Glc (1→4) β -Glc, present in malt.

Sugar alcohols

 * Sugar alcohols are formed by the reduction of a carbonyl group to a hydroxyl group. For example, glucitol a.k.a. sorbitol, which is produced by the reduction of glucose or fructose.

Polyhydroxy derivatives of carboxylic acids

 * They are formed by the oxidation of monosaccharides. When oxidized with a weak reagent, the aldehyde group is oxidized and aldonic acids are formed. Stronger reagents oxidize not only the aldehyde group, but also the primary −OH groups at the end of the molecule, so that dicarboxylic aldaric acids are formed. Oxidation of only the primary −OH group of aldoses in the body takes place enzymatically to form uronic acids. For example, glucose produces glucuronic acid, an important conjugation agent in the liver that aids in the excretion of poorly water-soluble substances.

Deoxysugars

 * They are formed by the reduction of the hydroxyl group of a saccharide. An example is deoxyribose, an important component of nucleic acids.

Aminosugars

 * They are formed by replacing the hydroxyl group with the −NH2 group. Important amino sugars in the body include, for example, D-glucosamine, a component of intercellular mass molecules.

Esters

 * They are formed by esterification of the hydroxyl group of H 3 PO 4 . For example, the formation of glucose-6-phosphate from a glucose molecule. Or H 2 SO 4 components of proteoglycans.

Glycosides
They are formed by the reaction of hydroxyl groups with:
 * 1) Alcohol – formation of an O-glycosidic bond. For example, the formation of di- and polysaccharides, or the binding of monosaccharides to proteins via the amino acids serine and threonine.
 * 2) Amine - formation of an N-glycosidic bond. An example is the binding to proteins via aspartate or the binding of ribose in nucleotides.


 * The most reactive group in the monosaccharide molecule is the anomeric group −OH.


 * Non-reducing disaccharides are formed when a glycosidic bond is formed between the anomeric hydroxyls of both monosaccharides, as for example with sucrose. The disaccharide does not react with the oxidizing agent.




 * A reducing disaccharide is formed when the anomeric hydroxyl of one monosaccharide reacts with a non-anomeric hydroxyl of another monosaccharide. Free aldoses, monosaccharides, are all reducing, some examples of reducing disaccharides are lactose or maltose.

Polysacharidy a vláknina

 * Polysacharidy bývají látky amorfní a jsou buď ve vodě nerozpustné, nebo tvoří koloidní roztoky. Obecně se označují jako glykany. Mohou být tvořeny jen jedním typem monosacharidu, například glukózou jako u škrobu a glykogenu. Tyto polysacharidy pak označujeme jako glukany. Pokud monosacharid bude fruktóza, nazýváme tento polysacharid fruktan. Nebo jsou tvořeny různými monosacharidy a jejich deriváty, jako jsou glykosaminoglykany.


 * Zásobní polysacharidy jako škrob či glykogen jsou ve vodě částečně rozpustné, zatímco strukturní polysacharidy jako celulóza mají v struktuře mnoho intra- a intermolekulárních vodíkových můstků a jsou ve vodě nerozpustné.

Vláknina

 * Tvoří ji heterogenní skupina strukturních polysacharidů, které lidské enzymy nedokáží rozštěpit, a proto je nevstřebatelnou součástí potravy. Pro trávení je ovšem velmi důležitá – zvyšuje objem tráveniny, což urychluje střevní peristaltiku a škodlivé látky tak zůstávají v trávicím traktu kratší dobu. Současně na sebe váže některé cizorodé i endogenní látky, čímž zvyšuje jejich vylučování z organismu. Toto platí například pro žlučové kyseliny tvořené z cholesterolu – konzumace vlákniny tedy snižuje množství cholesterolu v organismu.

Vlákninu dělíme:
 * 1) Rozpustnou vlákninu (hemicelulóza, pektiny).
 * Je štěpena bakteriemi tlustého střeva na mastné kyseliny s krátkým řetězcem, kyselina octová, propionová, máselná, jež jsou významným zdrojem energie pro kolonocyty.
 * 1) Nerozpustnou vlákninu (celulóza).
 * Celulózu nedokážou rozštěpit ani bakteriální enzymy a z těla odchází nestrávená. Její význam spočívá ve zvyšování objemu tráveniny a podpoře peristaltických pohybů.

Heteroglykosidy

 * Látky obsahující kromě sacharidové části i jiný typ sloučenin se nazývají aglykony. Patří sem:


 * 1) Proteoglykany
 * Obsahují lineární dlouhé polysacharidové řetězce vázané na protein. Řetězce jsou tvořeny opakujícími se dimery aminocukr-uronová kyselina – ty se označují jako glykosaminoglykany (GAG).
 * 1) Glykoproteiny
 * Glykoproteiny, čili proteiny na různých místech glykosylováné (O– nebo N– glykosidickou vazbou) krátkými větvenými molekulami oligosacharidů, na rozdíl od proteoglykanů neobsahují uronové kyseliny.
 * 1) Glykolipidy
 * Glykolipidy, látky lipidové povahy mají v molekule jednu nebo několik monosacharidových jednotek.