Carbohydrates PDF

Carbohydrates By Dr/ Rasha Shoaib Lecturer of Biochemistry *Carbohydrates are considered the most common organic compounds found in plants and animals. *The elements (C, H, O) are the main elements in carbohydrate compounds. *Carbohydrates were generally given names that end wit...

Carbohydrates By Dr/ Rasha Shoaib Lecturer of Biochemistry *Carbohydrates are considered the most common organic compounds found in plants and animals. *The elements (C, H, O) are the main elements in carbohydrate compounds. *Carbohydrates were generally given names that end with the syllable (-ose) such as glucose which is the main sugar in the blood, and maltose, which is malt sugar, and sucrose which is regular food sugar. *Carbohydrate properties: -Carbohydrates are organic substances that have three characteristics: (1) It is composed of three elements carbon, hydrogen, and oxygen. (2) The presence of an aldehyde or a ketone group. (3) The presence of more than one hydroxy group (polyhydroxy). The benefits of carbohydrates: (1) A great source of energy, as its decomposition and oxidation results in energy used in the biochemical reactions of all living organisms. (2) The chemical energy derived from carbohydrates is stored in the form of energy-rich compounds such as adenosine triphosphate (ATP) and Guanosine triphosphate (GTP). (3) Carbohydrates are part of the structure of the cell wall. (4) Fuel for the central nervous system: for the brain and the rest of the parts of the central nervous system to carry out its functions in regulating the body, glucose must be available because it is the main energy source for this important system, and a lack of glucose in the blood leads to weak thinking and mental focus. Carbohydrates classification *Classification of carbohydrates according to the number of building units: (1) Monosaccharides: Also called simple sugars, consist of only one sugar unit aldehydes and ketones which can not be broken down into simpler sugars such as glucose, galactose, and mannose. (2) Disaccharides: sugars that produce two monosaccharide units of two similar or different types such as maltose, lactose, and sucrose. (3) Oligosaccharides: are polysaccharides consisting of 3-10 units of monosaccharide molecules. (4) Polysaccharides: consist of the association of a large number (thousands) of monosaccharide units linked together by a glycosidic bond. Monosaccharides *Monosaccharides are sugars that cannot be hydrolyzed into smaller units by hydrolysis. *All the monosaccharides have the formula as (CH2O)n. *Monosaccharides may have between three and eight carbon atoms. *Classification of monosaccharides according to the presence of the functional group: -A monosaccharide is called a polyhydroxy aldehyde (aldose) if the aldehyde group is present at the first carbon of the chain of carbon atoms, but if the ketone group is present at carbon number 2 it is called a polyhydroxy ketone (Ketose). *Classification of monosaccharides according to the number of carbon atoms: *Classification of monosaccharides according to the number of carbon atoms and functional group: *When n = 3, the carbohydrates that contain three carbon atoms are called trioses and they are either aldehydes such as glyceraldehyde or ketones such as Dihydroxyacetone (DHA). *When n = 4, the carbohydrates containing four carbon atoms are called tetrose which is either an aldehyde such erythrose or ketone as erythrulose. *When n = 5, carbohydrates containing five carbon atoms are called pentose which is either an aldehyde such as ribose or xylose or ketone as ribulose or xylulose. *When n = 6, carbohydrates containing six carbon atoms are called hexose , which is either an aldehyde such as glucose, galactose, mannose or ketone as fructose. *Physical properties of monosaccharides: 1) Optical activity of monosaccharides: *If a compound contains one or more asymmetric carbon atoms, a carbon atom contains four different groups, so the compound is optically active as in monosaccharides and amino acids. *Asymmetric carbon atom or chiral carbon atom -There are four different groups attached to the carbon atom and there is no double bond between it and any other carbon atom. *The aldehyde triose (glyceraldehyde) contains only one asymmetric carbon atom, carbon atom number 2, While glucose contains four asymmetric carbon atoms. *All sugars contain an asymmetric carbon atom except for dihydroxyacetone. 2) Isomers: *Compounds that have the same structural formula but differ in spatial configuration, i.e. they have the same number of C, H, and O atoms but they differ in the arrangement of groups and atoms in space. * The presence of asymmetric carbon atoms allows the formation of isomers. *The number of isomers is determined according to the number of chiral carbon atoms where the number of isomers is 2n , where n represents the number of chiral carbon atoms present in the sugar. *For example, a six-carbon aldehyde sugar contains four chiral carbon atoms. Thus, the hexose sugar has a number of 24 , which represents 16 isomers. *Note: the number of asymmetric carbon atoms in ketose is one less than in aldose for example fructose contains 3 asymmetric carbons. *Important types of isomers include: 1) Aldose and ketose isomerism (based on the functional group) -They are isomers that differ in the position of the active carbon of the functional group, whether it is an aldehyde group in C1 or a ketone group in C2 for example: Glyceraldehyde and Dihydroxyacetone. Erythrose, and Erythrulose. Ribose and Ribulose. Glucose and Fructose. 2) D & L isomers (enantiomers) *They are isomers that are mirror images of each other. For example D and L-sugar. D and L indicate the composition of the preterminal carbon. *Most sugars in humans are in the D form except for L-fucose, L-arabinose, and L-xylulose. *When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory(+), or to the left, levorotatory (-). *When equal amounts of D and L isomers are present, the resulting mixture has no optical activity, since the activities of each isomer cancel one another. Such mixture is said to be a racemic or DL-mixture. 3) α and β anomers: They are isomers that differ in the position of the OH group attached to the carbon of the functional group (the anomeric carbon). If the –OH group is below, the form is α-anomer. If the –OH group is above, the form is β-anomer. *This equilibration is accompanied by optical rotation (mutarotation) as the hemiacetal ring opens and reforms with the change of position of the –H and – OH groups on carbon 1. 4) Epimers *They are homologs that differ in the position or configuration of a single hydroxyl group at one carbon (asymmetric carbon). *Glucose and galactose at carbon number 4. *Glucose and mannose at carbon number 2. *Stereocyclic representation of sugars 1) Fischer’s representation An open chain (glucose, for example, is a polyhydric aldehyde) where the D, L monosaccharides are assigned based on the orientation of the –OH group on the last chiral carbon away from the carbonyl and are the right D and L the left. It clearly shows the relative position of the atoms in the molecule. However, it is not accurate in representing the angles of the bonds and the geometry of the molecule. 2) Haworth representation *The second situation is Haworth's proposal and it has the cyclic form of the sugars represented as a hexagonal form of pyranose derived from pyran or pentagonal furanose derived from furan as shown in the following figure for both glucose and fructose. We find Haworth's suggestion useful, but somewhat imperfect. As it shows that the hexagonal ring has its atoms in one plane. 3) Chair and Boat forms *Although the Haworth formula is usually used to clarify the cyclic image of monosaccharides, it indicates that the molecule is flat, but the fact is that the cyclic molecule is bent in some way to reduce the angular stress. Therefore, the molecule takes a form in which the atoms on adjacent carbon atoms are in alternating positions. Most of the sugars in the hexagonal cyclic form are found in the chair form, but some may be found in the boat form. The cyclic structure of monosaccharides *Where the aldehyde group, which is carbon atom No. (1), interacts with the (OH) alcoholic group located on carbon atom No. (5). The symbols alpha (α) and beta (β) indicate the location of the OH group attached to carbon atom number (1) where the letter (α) indicates that the OH group is located below the plane of the ring. The letter (β) indicates that the (OH) group is located above the level of the ring. *The α and β anomers i.e. they turn into each other through the formation of the open chain. This mutual transformation is called the phenomenon of mutarotation. It arises from the formation of an equilibrium mixture that contains the α anomer by approximately one-third and the β anomer by two-thirds. Also, the composition of the open chain of glucose is very low, less than 1%. 66% 1% 33%  *The hexagonal sugar ring resulting from the implicit hemiacetal reaction is called pyranose because the resulting shape resembles the ring of an organic compound called by the same name (pyran). *Therefore, the name of the glucose molecule is D-Glucopyranose and the two anomeric cyclic structures are: α-D-Glucopyranose and β-D-Glucopyranose. *An example of a ketonic monosaccharide is fructose. Where the ketone group, which is carbon atom No. (2) interacts with the alcoholic (OH) group located on carbon atom No. (5), so a pentagonal ring is formed. The designation alpha (α) and beta (β) indicates the location of the OH group attached to carbon atom number (2). letter (α) indicates that the OH group is located below the level of the ring. The letter (β) indicates that the (OH) group is located above the plane of the ring. *The carbon atom No. (2) in the cyclic structure, which used to represent the carbonyl group and is a symmetrical atom in the structure of the open chain, has become an asymmetric atom because of this reaction, and the carbon atom No. (2) in fructose is called an anomeric atom. Thus, the cyclic structures α and β formed around this atom are called anomers. The biological importance of monosaccharides 1) Glucose is blood sugar and is the main source of energy in humans and animals. 2) Fructose is found in semen (seminal vesicles). 3) Galactose is present in the mammary glands. It is included in the formation of lactose disaccharides 4) Mannose is important in glycoproteins formation. 5) Ribose and deoxyribose are involved in the synthesis of nucleic acids. 6) Inositol is the main sugar in muscles. Sugar derivatives 1) Sugar acids: *Most of the usual compounds of glycolic acids are formed by the oxidation of aldoses (aldehyde sugars) to carboxylic acids by oxidizing the aldehyde's 1st carbon, the 6th hydroxymethylated carbon, or the 1st and 6th carbons together. These sugar acids may be aldonic acids, uronic acids, or aldaric acids, respectively. Their structural formulas are as follows: And if we take glucose as an example, it is given by oxidation to the following: 2) Sugar alcohols: *Important sugar alcohols (alditols), formed by the reduction of (i.e., addition of hydrogen to) a monosaccharide, include sorbitol (glucitol) from glucose and mannitol from mannose; both are used as sweetening agents. 3) Amino sugars: *An amino sugar is a sugar molecule in which a hydroxyl group has been replaced with an amine group. 4) Deoxy sugars: These sugars include compounds in which a hydrogen atom replaces a hydroxyl group with one of the pyranose or furanose sugar rings. The sugar 2-D-oxyribose is the repeating unit in the polymers of nucleic acids (DNA).