Carbohydrates PDF

# Carbohydrates ## Session Two: Introduction to Carbohydrates ### Aims: - The session aims are to provide a basic understanding of the core principles of carbohydrates. - Recognize carbohydrates and classify them as mono-, di-, or polysaccharides. - Classify monosaccharides as aldoses or ketoses...

# Carbohydrates ## Session Two: Introduction to Carbohydrates ### Aims: - The session aims are to provide a basic understanding of the core principles of carbohydrates. - Recognize carbohydrates and classify them as mono-, di-, or polysaccharides. - Classify monosaccharides as aldoses or ketoses and as Trioses, Tetroses, Pentoses, or Hexoses. - Distinguish between a D sugar and an L sugar and identify the structures of D-glucose, D-galactose, and D-fructose and describe how they differ from each other. - Identify the structures of sucrose, lactose, and maltose. - Identify the monosaccharides that are needed to form sucrose, lactose, and maltose. - Compare and contrast the structures and uses of starch, glycogen, and cellulose. ## Mass Composition Data for the Human Body in Terms of Major Types of Biochemical Substances | Biochemical Substances | Bioinorganic Substances | Bioorganic Substances | |---|---|---| | | Substances that do not contain carbon: | Substances that contain carbon: | | | Water(about 70%) | Proteins(about 15%) | | | Inorganic salts(about 5%) | Lipids(about 8%) | | | | Carbohydrates(about 2%) | | | | Nucleic acids(about 2%) | ## General Description of Carbohydrates - **Carbohydrates** are compounds containing C, H, and O. The general formula for a carbohydrate is $C_x(H_2O)_y$. All carbohydrates contain C=O and -OH functional groups. - In green (chlorophyll-containing) plants carbohydrates are produced via photosynthesis. - **Plants:** 6 $CO_2$ + 6 $H_2O$ chlorophyll, sunlight $C_6H_{12}O_6$ + 6 $O_2$ (+)-glucose - **Cellular Respiration:** $C_6H_{12}O_6$ + 6 $O_2$ 6 $CO_2$ + 6 $H_2O$ + energy ## Importance of Carbohydrates In humans carbohydrates have the following functions: - Oxidation of carbohydrates provides energy. - Storage of carbohydrates as glycogen provides a short-term energy reserve. - Carbohydrates provide carbon atoms for the synthesis of proteins, lipids, and nucleic acids. - Central to materials of industrial products: paper, lumber, fibers - Key component of food sources: sugars, flour, vegetable fiber - Important part of nucleic acids and free nucleotides and coenzymes. - Biological role as a part of hormones and their receptors and enzymes. ## Definition of Carbohydrates ### First Definition (Old definition) - Carbohydrates are substances containing carbon, hydrogen, and oxygen having the general formula $C_nH_{2n}O_n$. - Hydrogen and oxygen are present in 1:2 ratio the same ratio as water, so the French called them "Hydrates de Carbon", i.e., carbo-hydrates $C_n(H_2O)_n$. - The old definition is inaccurate because: - There are substances which are not carbohydrates but have the formula $C_nH_{2n}O_n$, e.g., acetic acid $CH_3COOH$ ($C_2H_4O_2$) and lactic acid. - There are some carbohydrates, which do not have this general formula, e.g., amino sugars and deoxy sugars. ### Second Definition (new definition): - Carbohydrates are aldehyde (CHO) or ketone (C=O) derivatives of polyhydric alcohols (have more than one OH group) or compounds which yield these derivatives on hydrolysis. ## Classification of Carbohydrates - The classification of carbohydrates is based on four different properties: (1) the size of the base carbon chain, (2) the location of the CO function group, (3) the number of sugar units, and (4) the stereochemistry of the compound. - Carbohydrates can be grouped into generic classifications based on the number of carbons in the molecule. For example, trioses contain three carbons, tetroses contain four, pentoses contain five, and hexoses contain six. - In actual practice, the smallest carbohydrate is glyceraldehyde, a three-carbon compound. - Carbohydrates are hydrates of aldehyde or ketone derivatives based on the location of the CO functional group. - The two forms of carbohydrates are aldose and ketose. The aldose form has a terminal carbonyl group (O=CH) called an aldehyde group, whereas the ketose form has a carbonyl group (O=C) in the middle linked to two other carbon atoms (called a ketone group). ## Examples of Carbohydrates ### Aldotrioses: - Glyceraldehyde - CHO - H-C-OH - CH2OH - Dihydroxyacetone - CH2OH - C=O - CH2OH ### Tetroses: Aldotetroses - Four carbons - CHO - H-C-OH - H-C-OH - CH2OH - D-Erythrose - Four carbons - CHO - HO-C-H - H-C-OH - CH2OH - D-Threose ### Aldopentoses - CHO - H-C-OH - H-C-OH - H-C-OH - CH2OH - D-(-)-ribose - CHO - HO-C-H - H-C-OH - H-C-OH - CH2OH - D-(-)-arabinose ### Hexoses: Adohexoses - CHO - H-C-OH - HO-C-H - H-C-OH - HO-C-H - H-C-OH - CH2OH - D-(+)-glucose - CHO - HO-C-H - HO-C-H - H-C-OH - HO-C-H - H-C-OH - CH2OH - D-(+)-mannose - CHO - H-C-OH - HO-C-H - HO-C-H - H-C-OH - H-C-OH - CH2OH - D-(+)-galactose ### Hexoses: Ketohexoses - CH2OH - C=O - HO-C-H - H-C-OH - H-C-OH - CH2OH - D-Fructose ## Models are Used to Represent Carbohydrates - The Fisher projection of a carbohydrate has the aldehyde or ketone at the top of the drawing. The carbons are numbered starting at the aldehyde or ketone end. - The compound can be represented as a straight chain or might be linked to show a representation of the cyclic, hemiacetal form. ## The Haworth Projection - The Haworth projection represents the compound in the cyclic form that is more representative of the actual structure. - This structure is formed when the functional (carbonyl) group (ketone or aldehyde) reacts with an alcohol group on the same sugar to form a ring called either a hemaketal or hemiacetal ring. ## Linear and Ring Forms of Glucose - H - C=O - H-C-OH - OH-C-H - H-C-OH - H-C-OH - CH2OH - a-D-Glucose (linear form) - CH2OH - H-C-OH - OH-C-H - H-C-OH - HO-C-H - H-C-OH - CH2OH - a-D-Glucose (ring form) - CH2OH - H-C-OH - OH-C-H - HO-C-H - H-C-OH - H-C-OH - CH2OH - B-D-Glucose (ring form) ## Linear and Ring Forms of Fructose - CH2OH - C=O - HO-C-H - H-C-OH - H-C-OH - CH2OH - Fructose - CH2OH - HO-C-H - OH-C-H - H-C-OH - HO-C-H - H-C-OH - CH2OH - B- D-Fructose (ring form) - CH2OH - H-C-OH - OH-C-H - HO-C-H - H-C-OH - H-C-OH - CH2OH - a- D-Fructose (ring form) ## Stereoisomers - The central carbons of a carbohydrate are asymmetric (chiral): four different groups are attached to the carbon atoms. This allows for various spatial arrangements around each asymmetric carbon also called stereogenic centers forming molecules called stereoisomers. - Stereoisomers have the same order and types of bonds but different spatial arrangements and different properties. - For each asymmetric carbon, there are $2^n$ possible isomers; therefore, there are two forms of glyceraldehyde. - A monosaccharide is assigned to the D or the L series according to the configuration at the highest-numbered asymmetric carbon. This asymmetrically substituted carbon atom is called the “configurational atom” or chiral center. - Thus, if the hydroxy group (or the oxygen bridge of the ring form) projects to the right in the Fisher projection, the sugar belongs to the D series and receives the prefix D-, and if it projects to the left, then it belongs to the L series and receives the prefix L-. These stereoisomers, called enantiomers, are images that cannot be overlapped. - If there is more than one chiral C-atom: absolute configuration of chiral C furthest away from carbonyl group determines whether D- or L- - D-glucose is represented in the Fisher projection with the hydroxy group on carbon number 5 positioned on the right. L-Glucose has the hydroxy group of carbon number 5 positioned on the left. Most sugars in humans are in the D-form. ## Epimers - Sugars that differ only in their stereochemistry at a single carbon. D-glucose and D-mannose, which differ only in the stereochemistry at C-2, are epimers, as are D-glucose and D-galactose (which differ at C-4). - Ribose is an epimer to each of arabinose and xylose. ## Monosaccharides, Disaccharides, and Polysaccharides - Another classification of carbohydrates is based on the number of sugar units in the chain: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. - This chaining of sugars relies on the formation of glycoside bonds that are bridges of oxygen atoms. When two carbohydrate molecules join, a water molecule is produced. When they split, one molecule of water is used to form the individual compounds. This reaction is called *hydrolysis*. - The glycoside linkages of carbohydrate can involve any number of carbons; however, certain carbons are favored, depending on the carbohydrate. ### Monosaccharides - are simple sugars that cannot be hydrolyzed to a simpler form. - These sugars can contain three, four, five, and six or more carbon atoms (known as trioses, tetroses, pentoses, and hexoses, respectively). - The most common include glucose, fructose, and galactose. ### Disaccharides - are formed when two monosaccharide units are joined by a glycosidic linkage. On hydrolysis, disaccharides will be split into two monosaccharides by disaccharide enzymes (e.g., lactase) located on the microvilli of the intestine. - These monosaccharides are then actively absorbed. - The most common disaccharides are maltose (comprising 2-D-glucose molecules in a 1→ 4 linkage), lactose, and sucrose. ### Oligosaccharides - are the chaining of 2 to 10 sugar units, whereas polysaccharides are formed by the linkage of many monosaccharide units. - On hydrolysis, polysaccharides will yield more than 10 monosaccharides. - The most common polysaccharides are starch (glucose molecules) and glycogen ## Chemical Properties of Carbohydrates - Some carbohydrates are reducing substances; these carbohydrates can reduce other compounds. - To be a reducing substance, the carbohydrate must contain a ketone or an aldehyde group. This property was used in many laboratory methods in the past in the determination of carbohydrates. - Carbohydrates can form glycosidic bonds with other carbohydrates and with noncarbohydrates. - Two sugar molecules can be joined forming a glycosidic bond between the hemiacetal group of one molecule and the hydroxyl group on the other molecule. - In forming the glycosidic bond, an acetal is generated on one sugar (at carbon 1) in place of the hemiacetal. If the bond forms with one of the other carbons on the carbohydrate other than the anomeric (reducing) carbon, the anomeric carbon is unaltered and the resulting compound remains a reducing substance. - Examples of reducing substances include glucose, maltose, fructose, lactose, and galactose. - If the bond is formed with the anomeric carbon on the other carbohydrate, the resulting compound is no longer a reducing substance. - Nonreducing carbohydrates do not have an active ketone or aldehyde group. They will not reduce other compounds. - The most common nonreducing sugar is sucrose-table sugar. - All monosaccharides and many disaccharides are reducing agents. This is because a free aldehyde or ketone (the open chain form) can be oxidized under the proper conditions. - As disaccharide remains a reducing agent when the hemiacetal or ketal hydroxyl group is not linked to another molecule. Both maltose and lactose are reducing agents, whereas sucrose is not. ## Glycosidic Bonds - The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: - R-OH + HO-R’ → R-O-R’ + H2O - E.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose). ## Disaccharides: ### Reducing Disaccharides - **a-Maltose (malt sugar):** - It consists of 2 a-glucose units linked by a -1,4-glucosidic linkage. - It has a free aldehyde group (anomeric carbon) therefore, it is reducing sugar. ### Non-reducing Disaccharides - **Sucrose:** - It is table sugar and is formed of a-glucose linked to b-fructose by b-1,2-linkage. - It is a fermentable sugar. - The 2 anomeric carbons (C1 of glucose and C2 of fructose) are involved in the linkage(no free groups) so it is: Non-reducing sugar. ## Polysaccharides - Polysaccharides are classified into: - **Homopolysaccharides:** They yield only one type of monosaccharides on hydrolysis, e.g., Hexosans & Pentosans - **Heteropolysaccharides:** They are polysaccharides that on hydrolysis produce several types of sugars. - **Starch:** - Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. - It is in the form of starch granules. - The core of the granule is amylose (20%) and the shell is amylopectin (80%). - **Amylose:** - is a glucose polymer with a(1→4) linkages of a helix formed of a large number of a-glucose. - It forms the inner part of starch granules. - **Amylopectin:** - It forms the outer coat of starch granule and is insoluble in water. - It is branched chains formed of a large number of a-glucose units linked by a-1,4-glucosidic linkage along the branch and by a-1,6-glucosidic linkage at the branching point that occur every 25-30 glucose units. - Starch can be hydrolyzed by HCl or amylase. ### Dextran - A compound formed of a-glucose units linked by a-1,4, a-1,3- and a-1,6-linkage present in the form of a network that is synthesized by certain bacteria having sucrose in its media. - It has a great biochemical importance: - It is used as plasma substitute to restore blood pressure in cases of shock. - It is used for treatment of iron deficiency anemia as dextran ferrous sulfate intramuscular injection. - Sodium dextran sulfate is an anticoagulant. ### Glycogen - It is the stored form of carbohydrate in animal, particularly in muscles and liver. - Its structure is similar to amylopectin a branched tree with a-1,4-glucosidic linkage along the branch and a-1,6-glucosidic linkage at the branching point. - The glycogen tree is shorter and more branched (a branch point every 8-10 glucose units) than amylopectin. - It is digestible because human amylases hydrolyze a-glucosidic linkage. ### Cellulose - It is a structural polysaccharide and forms the skeleton of plant cells and does not enter in animals cell structures. - It is a straight chain molecule formed of a large number of b-glucose units linked by b-1,4-glucosidic linkage. - It is water insoluble and enters in the structure of cotton and paper. - It gives cellobiose on hydrolysis with HCl. ## Heteropolysaccharides - They are polysaccharides that on hydrolysis produce several types of sugars. - There are two types: - Non-nitrogenous heteropolysaccharides: a. Plant gums b. Pectin - Nitrogenous heteropolysaccharides: - They contain sugar amines and are of two types: - Neutral nitrogenous heteropolysaccharides: e.g. Glycoproteins or mucoproteins - Acidic nitrogenous heteropolysaccharides: - Sulfur-free mucopolysaccharides: e.g. hyaluronic acid - Sulfur-containing mucopolysaccharides: e.g. Heparin ### Glycoproteins or mucoproteins - They are formed of a large protein core to which are attached smaller branched or unbranched chains of carbohydrate. - Distribution: They include: 1. Cell membranes where they play an important part in cell-cell attachment. 2. Blood group substances A and B antigens. 3. Mineral and vitamin transporting protein, e.g., trasferrin. 4. Immunoglobulins (antibodies) 5. Some hormones, e.g., anterior pituitary hormones. 5. Some enzymes such as peptidases and alkaline phosphatase. 6. Intrinsic factor that is responsible for vitamin B12 absorption. 7. Structural function as a part of collagen of connective tissue. ### Heparin - Structure: It is formed of a long repeat of sulfated a-glucosamine and sulfated b-L-iduronic acid linked by alternating a- and b-1,4-glycosidic linkages, synthesized on a core protein. - Source: It is produced by mast cells (kidney, lung, liver skin). - Function: - It is an anticoagulant and prevents intravascular clotting. - It binds activates lipoprotein lipase enzyme (the plasma clearing factor) to clear the turbid plasma from the absorbed lipids after meals. - It has a structural role in extracellular matrix ## Fibers - Found in food derived from plants - Includes polysaccharides such as cellulose, hemicellulose, pectins, gums. - Also includes non-polysaccharides such as lignin, cutins, and tannins. - Fibers are not a source of energy because Human digestive enzymes cannot break down fibers - The bacteria in human Gl tract can breakdown some fibers.