Nutrient chemistry

Content of the subsection

1. Nutrient overview – sacharides
2. Nutrient overview – lipids
3. Nutrient overview – proteins

Nutrient overview - sacharides

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.

File:Chitosan Haworth.gif

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.

Monosaccharides that normally occur in the body are D- isomers, and the enzymes that catalyze their transformations are "stereospecific" for these isomers.

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.

• 

  Example of pyranose

• 

  Example of 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.

In the case of the D-isomers, the α-anomer has the -OH group oriented below the plane of the ring.

Anomers differ in optical rotation.

For example, when crystalline sugar is dissolved, the balance between the two anomers is stabilized and. the optical rotation of the solution changes. This phenomenon called mutarotation.

Epimers

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

Aldoses a ketoses

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

Struktura glykogenu

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.

Cataracts in diabetics is caused by a long-term increased concentration of glucose, which is reduced to glucitol in the lens. Its removal is slow, and since it is strongly osmotically active, it changes the osmolarity of the lens. Lens proteins (crystallins) precipitate in such an environment and form strongly light-scattering deposits.

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 H3PO4. For example, the formation of glucose-6-phosphate from a glucose molecule. Or H2SO4 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.

Saccharosis

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.

Aglycones are non-carbohydrate substances that bind to monosaccharides via a glycosidic bond.

• 

  Laktóza
• 

  Maltóza

Polysaccharides and fiber

Polysaccharides tend to be amorphous substances and are either insoluble in water or form colloidal solutions. They are generally referred to as glycans. They can be made of only one type of monosaccharide, for example glucose as in starch and glycogen. We refer to these polysaccharides as glucans. If the monosaccharide is fructose, we call this polysaccharide fructan. They can also be made up of various monosaccharides and their derivatives, such as glycosaminoglycans.

Storage polysaccharides such as starch or glycogen are partially soluble in water, while structural polysaccharides such as cellulose have many intra- and intermolecular hydrogen bonds in their structure and are insoluble in water.

Fiber

It consists of a heterogeneous group of structural polysaccharides that cannot be broken down by human enzymes, which is why it is a non-absorbable part of food. However, it is very important for digestion - it increases the volume of digestate, which accelerates intestinal peristalsis and thus harmful substances remain in the digestive tract for a shorter time. At the same time, it binds to itself some foreign and endogenous substances, thereby increasing their excretion from the organism. This applies, for example, to bile acids formed from cholesterol - consumption of fiber therefore reduces the amount of cholesterol in the body.

We divide the fiber:

• Soluble fiber (hemicellulose, pectins).

Structure of Pectin

It is broken down by colon bacteria into short-chain fatty acids, acetic, propionic, butyric acid, which are an important source of energy for colonocytes.

• Insoluble fiber (cellulose).

Even bacterial enzymes cannot break down cellulose and it leaves the body undigested. Its importance lies in increasing the volume of digestion and supporting peristaltic movements.

Heteroglycosides

Substances containing, in addition to the carbohydrate part, another type of compound are called aglycones. Includes:

1. Proteoglycans

   They contain linear long polysaccharide chains bound to protein. The chains are made up of repeating amino sugar-uronic acid dimers – these are referred to as glycosaminoglycans (GAGs).

2. Glycoproteins

   Unlike proteoglycans, glycoproteins, i.e. proteins glycosylated in various places (O– or N– glycosidic bond) by short branched molecules of oligosaccharides, do not contain uronic acids.

3. Glycolipids

   Glycolipids, substances of a lipid nature, have one or several monosaccharide units in the molecule.

Nutrient overview - Lipids

Lipids, a group of chemically and functionally heterogeneous substances, have in common insolubility in water – hydrophobicity, excellent solubility in non-polar solvents and the presence of alcohols and fatty acids in the molecule. They are often synthesized in the body from acetyl-CoA.

Fatty acids usually mean higher monocarboxylic acids. They contain approximately 8 or more carbon atoms, with typically an even number of carbon atoms (because they are formed from acetyl-CoA). If they contain double bonds, they are usually isolated and in the cis-configuration. Acids with a length of C16 and C18 are predominant.

Clasification of lipids

Simple lipids

• Acylglyceroles
• Waxes

Complex lipids

• Phospholipids
  • Glycerolphospholipids
  • Sphingolipids
• Glycolipids
  • Cerebrosides
• Gangliosides
• Lipoproteins

Simple lipids

Simple lipids are esters of fatty acids and alcohols. They consist only of a hydrophobic part.

Acylglycerols

Acylglycerols (glycerides) are esters of higher fatty acids and glycerol. According to the number of fatty acid molecules bound to alcohol, they are divided into mono-, di- or triacylglycerols. The most important for us are triacylglycerols, which are part of fats - a mixture of solid triacylglycerols, or oils - a mixture of liquid triacylglycerols.

Triacylglycerol

Acid hydrolysis of acylglycerols produces the corresponding fatty acids and glycerol.

Alkaline hydrolysis, saponification, produces glycerol and a mixture of fatty acid salts - soap.

Waxes

Waxes are esters of higher fatty acids and higher monohydric alcohols. Examples can be cetyl alcohol with sixteen carbon atoms, ceryl alcohol with twenty-six carbon atoms or myricyl alcohol with thirty carbon atoms.

Complex lipids

Complex lipids form the basic building block of cell membranes. It includes phospholipids, glycolipids and lipoproteins. In addition to the hydrophobic part, complex lipids also contain hydrophilic components. They are therefore referred to as polar lipids and form micelles and bilayers.

Phospholipids

Phospholipids contain a residue of phosphoric acid H3PO4 in their molecule. Phospholipids include glycerol phospholipids and sphingophospholipids.

Fosfolipidová dvojvrstva

Glycerolphospholipids (phosphoacylglycerols)

The basis of glycerolphospholipids is a molecule of phosphatidic acid, which consists of glycerol esterified with two molecules of fatty acids and one molecule of H3PO4. Another component can bind to the phosphate group of phosphatidic acid, e.g. choline, serine, ethanolamine, etc. The most numerous group of glycerolphospholipids are phosphatidylcholines (lecithins) and are used as part of biological membranes.

Glycerolphospholipids also include phosphatidylethanolamines (kephalins), phosphatidylserines and phosphatidylinositols.

Sphingophospholipids

Sfingosin

Sphingophospholipids contain the alcohol sphingosine to which other components bind.

• Sphingosine with an attached fatty acid is called a ceramide.
• The most important subgroup are sphingomyelins formed by ceramide with a bound phosphoric acid residue and choline. Sphingomyelins are found, for example, in nervous tissue.

Glycolipids

Glycolipids contain one or more monosaccharides. These monosaccharides are glycosidically bound to the lipid part of the mono- or diacylglycerol or sphingosine molecule. Glycolipids include cerebrosides and gangliosides.

• Cerebrosides are formed by a ceramide molecule with bound galactose. They occur mainly in the white matter of the CNS. H2SO4 can bind to cerebrosides. In that case, we call them sulfatides.
• Gangliosides are formed from ceramide to which an oligosaccharide, usually galactose and glucose, is attached. They are found in the ganglia of nerve cells and the gray matter of the CNS.

Lipoproteins

Lipoproteins are made up of a combination of lipids and proteins.

 For more information see [[1]].

.

Lipoprotein particle

Essential fatty acids

The human organism can desaturate a fatty acid molecule at most at the 9th carbon position.

If the double bond is located further away, the body cannot create it, and we must ingest such fatty acids in food and call them essential fatty acids.

Linoleic acid

Linoleic acid is made up of eighteen carbons and two double bonds. The cis double bond is in positions 9 and 12 and is therefore designated as ω-6.

It is mainly found in vegetable oils, for example sunflower. It has pro-inflammatory effects, like other ω-6 fatty acids, and increases plasma levels of some lipids.

Linoleic acid is used in the biosynthesis of arachidonic acid.

Arachidonic acid

Arachidonic acid is made up of twenty carbons and four double bonds, it is also an ω-6 fatty acid. It functions as an important precursor of biologically active substances - eicosanoids (prostaglandins, prostacyclins, leukotrienes and thromboxanes).

Arachidonic acid

α-linoleic acid

Alpha-linolenic acid consists of eighteen carbons and three double bonds. The cis double bond is located in positions 9, 12 and 15, therefore it is designated as ω-3. It is mainly found in fish and marine animals. It lowers the level of cholesterol and TAG in the body and thus reduces the risk of cardiovascular diseases, it also has anti-inflammatory effects.

The importance of lipids for the body

Lipids are the most reduced and therefore the most energy-rich nutrient. They are thus used as an important energy substrate. However, some tissues, such as the brain, cannot normally use them in their metabolism. Due to their hydrophobicity, which means that they do not bind water, unlike carbohydrates, they are the most efficient energy storage.

Fats make up approximately 15% of the total weight in men, in women the representation is higher around 20-25%. For a man of average weight, this represents approximately 10.5 kg of TAG, which we can obtain by oxidizing around 400,000 kJ.

Other functions

• Structural functions – for example, they form a necessary component of all cell membranes.

• Mechanical and protective function – subcutaneous fat and fat around organs isolates thermally and physically, myelin sheaths isolates neurons electrically.

A myelinated axon in an electron microscope

• Important solvents of some substances (fat-soluble vitamins) and are the starting material for the synthesis of many substances important for the body - eicosanoids, steroid hormones, bile acids, etc.

Isoprenoids

In the organism there is a group of substances whose structure is derived from the isoprene molecule - they are made up of two or more isoprene units.

Steroids, derivatives of triterpenoids, contain six isoprene units. The most important representative of steroids, cholesterol, forms part of cell membranes, and its derivatives (bile acids, steroid hormones) perform many important functions in the body.

• 

  Isoprene

• 

  Cholesterol

• 

  Cholic acid

Nutrient overview - Proteins

Proteiny jsou makromolekulární organické látky tvořené řetězcem aminokyselin vzájemně spojených peptidovou vazbou. Ta spojuje jednoduchou kovalentní vazbou aminoskupinu jedné aminokyseliny a karboxylovou skupinu druhé aminokyseliny. Polykondenzací vzniká různě dlouhý řetězec aminokyselin ukončený na jedné straně volnou aminoskupinou, N-konec, a na straně opačné volnou karboxylovou skupinou, C-konec.

Proteins are macromolecular organic substances formed by a chain of amino acids connected to each other by peptide bonds. It connects the amino group of one amino acid and the carboxyl group of the other amino acid by a simple covalent bond. Polycondensation results in a chain of amino acids of varying lengths terminated on one side by a free amino group, the N-end, and on the opposite side by a free carboxyl group, the C-end.

Peptides are shorter chains of amino acids than proteins (less than 100) and have a molecular weight of less than 10,000.

Protein classification

Proteins can be divided into two basic groups according to their composition.

Simple proteins

Simple proteins contain chains made up of only amino acids. We distinguish:

• Fibrillar proteins - scleroproteins

Fibrillar proteins, insoluble in water, mainly perform structural functions. Individual peptide chains are interconnected by crosslinks so that they form parallel fibers. Examples of fibrillar proteins are collagen or keratin, found in hair, skin or nails.

Kolagenová vlákna I. typu (elektronový mikroskop)

• Globular proteins - spheroproteins

The chain of globular proteins has a spherical shape allowing the hydrophobic parts of the molecule to be wrapped inside. They are therefore soluble in water, e.g. albumin.

Complex proteins

Složené proteiny obsahují kromě proteinové části i jinou neproteinovou strukturu. Rozlišujeme:

• Glycoproteins – containing a glycosidically linked carbohydrate;

• Metalloproteins – obsahující iont kovu (Fe, Cu), například ferritin nebo transferrin;

• Chromoproteiny – containing a pigment as a prosthetic group, for example hemoglobin, cytochromes or myoglobin;

• Nucleoproteins – containing linked nucleic acids;

• Lipoproteins – containing lipids.

Protein structure

We can distinguish several structures in the protein molecule:

Primary structure

The primary structure is determined by the sequence of amino acids in the protein chain. We read their order from N- to C-end. The amino acids of the chain are connected to each other by peptide bonds.

Secondary structure

The secondary structure is conditioned by the formation of hydrogen bonds between the NH– and C=O groups of the peptide bond. The most common secondary structures include the alpha-helix and the beta-folded sheet.

• Alpha-helix

The protein chain is coiled into a right-rotating or left-rotating helix with a single loop length of 3.6 amino acid residues. The side chains protrude outward from the helix.

Alfa-helix

• Beta sheet

There are two chains, arranged in parallel and antiparallel. They stabilize each other by H-bridges.

Beta sheet

In addition to these two secondary structures, there are many others, such as Zn-finger or Leu-zipper.

Tertiary structure

Tertiary structure describes the spatial arrangement of a molecule conditioned by interactions between side groups of the chain – electrostatic forces, H-bridges, SH-bonds, non-polar interactions, etc.

Quaternary structure

Quaternary structure describes the spatial arrangement of protein subunits composed of more than one chain. The subunits are not linked to each other by peptide bonds.

Denaturation is a process in which all higher protein structures except the primary one are disrupted. The protein loses its functionality, but its energy value remains. Causes may include high temperature, pH change, or the presence of heavy metal salts.

Amino acids

Aminokyseliny jsou základní stavební složkou proteinů a peptidů. Obsahují nejméně jednu amino- (–NH₂) a jednu karboxylovou (–COOH) skupinu.

Amino acids are the basic building blocks of proteins and peptides. They contain at least one amino- (–NH2) and one carboxyl (–COOH) group.

According to biosynthesis, we can divide them into:

1. Non-essential that the body can synthesize: Gly, Ala, Pro, Ser, Tyr, Cys, Asp, Asn, Glu, Gln
2. Essential, which we must ingest in food

• Branched: Val, Leu, Ile

• 

  Valine

• 

  Leucine

• 

  Isoleucine

• aromatic: Phe, Trp

• 

  Phenylalanine

• 

  Tryptophane

• basic: His and Arg (both essential only in childhood or in critical conditions), Lys

• 

  Histidine

• 

  Arginine

• 

  Lysine

• with sulfur content: Met

• 

  Methionine

• with the OH− group: Thr

• 

  Threonine

 For more information see Amino acids.

The importance of proteins for the body

The importance of proteins for the human body is enormous. Furthermore, they represent the only source of nitrogen for the body.

 For more information see Proteins (1. LF UK, NT).

Functions:

• structural – collagen, elastin;
• motor - actin, myosin;
• informational - protein hormones;
• defensive – immunoglobulins, complement, antigens;
• transport – albumin, enable catalysis (enzymes) and others.