Carbohydrates Lecture Notes PDF

**BCH 201- LECTURE NOTE** **CARBOHYDRATES: NOMENCLATURE, CLASSIFICATION, FUNCTION & DERIVATIVES** **INTRODUCTION** - The name *carbohydrate*, "hydrate of carbon," are the aldehydic or ketonic derivatives of polyhydroxy alcohols and their polymers having hemiacetal glycosidic linkages....

**BCH 201- LECTURE NOTE** **CARBOHYDRATES: NOMENCLATURE, CLASSIFICATION, FUNCTION & DERIVATIVES** **INTRODUCTION** - The name *carbohydrate*, "hydrate of carbon," are the aldehydic or ketonic derivatives of polyhydroxy alcohols and their polymers having hemiacetal glycosidic linkages. - - Carbohydrates are the main source of energy in the body. Brain cells, red blood cells, kidney medulla, lens, and cornea of the eye, testes, and exercising muscle exclusively depend on carbohydrates (glucose) as the energy source. - They make up most of the organic matter on Earth because of their extensive roles in all forms of life. - First, carbohydrates serve as energy stores, fuels, and metabolic intermediates. - Second, ribose and deoxyribose sugars form part of the structural framework of RNA and DNA. - Third, polysaccharides are structural elements in the cell walls of bacteria and plants. Cellulose, the main constituent of plant cell walls, is one of the most abundant organic compounds in the biosphere. - Fourth, carbohydrates are linked to many proteins and lipids, which play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment. - A few common types of carbohydrates are milk, bread, popcorn, potatoes, maize, etc. - The carbohydrates are largely distributed in both plant and animal tissues. Carbohydrates occur mainly in the form of glycogen and glucose in animal cells and as cellulose and starch in plant cells. Naming the Carbohydrate Length https://www.organicchemistrytutor.com/wp-content/uploads/2019/05/glucose-is-a-hexose.png The simplest carbohydrate has 3 carbons. We use the greek numerals to call the number, aka tri-, tetra-, penta-, hexa-, and add the ending **-ose** to denote that it's a carbohydrate. For instance, a triose is a carbohydrate with 3 carbons, while hexose is a carbohydrate with 6 carbons in the molecule. For instance, glucose is an example of a hexose because it has six carbons in the molecule. This type of name, however, doesn't tell us the exact nature of the molecule. For instance, there are 24 different hexoses (12 of which exist in nature). This list includes glucose, galactose, fructose, mannose, etc. So, when I say that we're dealing with a hexose, that doesn't mean much except for the fact that the molecule contains 6 carbons. Naming the Major Functional Group in a Carbohydrate Sugars, or carbohydrates, have two major functional groups: an aldehyde or a ketone (both are collectively called carbonyls), and an alcohol functional group. Carbohydrates generally have multiple alcohol functional groups, so we never focus on those. However, sugars will only have one aldehyde OR one ketone functional group. We specify this in the name by adding **aldo**-- or **keto**-- prefix to the carbohydrate name. As you can guess, aldo- goes together with an aldehyde, and keto- with the ketone-containing carbohydrates.  As you can see from these examples, we start the name by saying that the molecule is an aldehyde using the aldo- prefix. We then say how many carbons are there in the molecule. And we finish by adding the -ose ending to specify that it's a carbohydrate we're dealing with. We can do the same for ketoses. As ketoses contain a ketone functional group, we obviously cannot have it at the beginning of the chain. Most common ketoses have a ketone functional group on the second carbon in the chain. Those with their ketone function at C2 are the most common form. Note that some of these ketoses are named by the insertion of **-*ul-*** before the suffix -*ose* in the name of the corresponding aldose; thus **D-xylulose** is the ketose corresponding to the aldose **D-xylose. Dihydroxyacetone, D-fructose,** D-ribulose, and D-xylulose are the biologically most prominent ketoses. Examples of ketoses Examples of ketoses The nomenclature of ketoses follows the same principles as for aldoses: you start by saying keto- to point out the functional group. Then, say how many carbons you have in the molecule and add -ose to signify the carbohydrate.  Some molecules are not superimposable on their mirror images and that these mirror images are optical isomers (stereoisomers) of each other. A chiral (asymmetric) carbon atom is the usual source of optical isomerism, as was the case with amino acids. The simplest carbohydrate that contains a chiral carbon is glyceraldehyde, which can exist in two isomeric forms that are mirror images of each other. Glyceraldehyde has a single asymmetric carbon; thus, this sugar has two stereoisomers. D-Glyceraldehyde and L-glyceraldehyde (or D and L- glucose; for example) are ***enantiomers,* or mirror images** of each other. Molecules that have multiple asymmetric carbons exist as **diastereoisomers**, isomers that are not mirror images of each other. D-glucose and D-mannose differ in configuration only at C-2. Sugars differing in configuration at a single asymmetric center are called **epimers**. Thus, D-glucose and D-mannose are epimeric at C-2; D-glucose and D-galactose are epimeric at C-4. However, D-mannose and D-galactose are not epimers of each other because they differ in configuration about two of their C atoms.  **The stereochemical relationships among the D-ketoses with three to six carbon atoms. The configuration about c3 (red) distinguishes the members of each pair. The biologically most common ketoses are boxed.** The **configuration** is the three-dimensional arrangement of groups around a chiral carbon atom, and stereoisomers differ from each other in configuration. The *D,L* system to denote Stereochemistry is widely used by biochemists. *D-Glucose is the only aldose that commonly occurs in nature as a monosaccharide.* However, it and several other monosaccharides including D-glyceraldehyde, D-ribose, D-mannose, and D-galactose are important components of larger biological molecules. L Sugars are biologically much less abundant than D sugars. Alcohols react with the carbonyl groups of aldehydes and ketones to form **hemiacetals** and **hemiketals,** respectively. **The reactions of alcohols with (a) aldehydes to form hemiacetals and (b) ketones to form hemiketals.** The hydroxyl and either the aldehyde or the ketone functions of monosaccharides can likewise react intramolecularly to form cyclic hemiacetals and hemiketals (Fig. 11-4). The configurations of the substituents to each carbon atom of these sugar rings are conveniently represented by their **Haworth projection formulas.** A sugar with a six-membered ring is known as a **pyranose** in analogy with **pyran,** the simplest compound containing such a ring. Similarly, sugars with five-membered rings are designated **furanoses** in analogy with **furan.** **\ ** The cyclic forms of glucose and fructose with six- and five-membered rings are therefore known as **glucopyranose** and **fructofuranose,** respectively. The cyclization of a monosaccharide renders the former carbonyl carbon asymmetric. When hemiacetals and hemiketals are formed, the carbon atom that carried the\ carbonyl function becomes an asymmetric carbon atom. Isomers of monosaccharides\ that differ only in their configuration about that carbon atom are called **anomers,** designated as α or β, and the carbonyl carbon is thus called the **anomeric carbon.** In the anomer, the OH substituent to the anomeric carbon is on the opposite side of the sugar ring from the CH~2~OH group at the chiral center that designates the D or L configuration (C5 in hexoses). The other anomer is known as the β form. **Cyclization reactions for hexoses. (*a*) D-Glucose in its linear form reacting to yield the cyclic hemiacetal -Dglucopyranose and (*b*) D-fructose in its linear form reacting to yield the hemiketal -D-fructofuranose. The cyclic sugars are shown as Haworth projections**.   **The anomeric monosaccharides -Dglucopyranose and -D-glucopyranose, drawn as both Haworth projections and ball-and-stick models.** These pyranose sugars interconvert through the linear form of D-glucose and differ only by the configurations about their anomeric carbon atoms, C1. The two anomers of D-glucose, as any pair of diastereomers, have different physical and chemical properties. For example, the values of the specific optical rotation, \[α\]^20^~D~, for α-D-glucose and β-D-glucose are, respectively, +112.2° and +18.7°. When either of these pure substances is dissolved in water, however, the specific optical rotation of the solution slowly changes until it reaches an equilibrium value of \[α\]^20^~D~ = +52.7°. This phenomenon is known as **mutarotation;** in glucose, it results from the formation of an equilibrium mixture consisting of 63.6% of the anomer and 36.4% of the anomer (the optical rotations of separate molecules in solution are independent of each other so that the optical rotation of a solution is the weighted average of the optical rotations of its components). The interconversion between these anomers occurs via the linear form of glucose. Although Haworth projections are convenient for displaying monosaccharide structures, they do not accurately portray the conformations of pyranose and furanose rings. Given C-C-C tetrahedral bond angles of 109° and C-O-C angles of 111°, neither pyranose nor furanose rings can adopt true planar structures. Instead, they take on puckered conformations, and in the case of pyranose rings, the two favored structures are the **chair conformation** and the **boat conformation**. Note that the ring substituents in these structures can be **equatorial,** which means approximately coplanar with the ring, or **axial,** that is, parallel to an axis drawn through the ring as shown. Two general rules dictate the conformation to be adopted by a given saccharide unit. First, bulky substituent groups on such rings are more stable when they occupy equatorial positions rather than axial positions, and second, chair conformations are slightly more stable than boat conformations. For a typical pyranose, such as -D-glucose, there are two possible chair conformations. Of all the D-aldohexoses, -D-glucose is the only one that can adopt a conformation with all its bulky groups in an equatorial position. With this advantage of stability, it may come as no surprise that -D-glucose is the most widely occurring organic group in nature and the central hexose in carbohydrate metabolism. **(a) Chair and boat conformations of a pyranose sugar. (b) Two possible chair conformations of\ β-D-glucose.** **FUNCTIONS** Carbohydrates are for the following functions: - - - - - - - - **CLASSIFICATION & NOMENCLATURE** Generally, carbohydrates are classified into three major groups. They are as follows:  - - - **Monosaccharides** - - - - **Aldoses (Aldo sugars)** - **Ketoses (Keto sugars)** - - Glyceraldehyde is the aldose with three carbons (an aldotriose), and dihydroxyacetone is the ketose with three carbon atoms (a ketotriose) **Glucose:** Glucose can be seen generally in the fruit juices and formed in the body by hydrolysis of cane sugar, starch, lactose, and maltose. Glucose is said to be the sugar of the body. Glucose structure can be depicted in the form of a ring or chain. It is found in blood, fruits, honey, and under abnormal conditions, in urine. Structure of Glucose:  **Fructose:** Fructose can be seen naturally in honey, tomatoes, and apples. Hydrolysis of cane sugar in the body can also give up fructose. C~6~H~12~O~6~ is the molecular formula for fructose. Generally, fructose is the sweetest monosaccharide and is prepared by sucrose hydrolysis. Structure of Fructose **Galactose:** An element of glycoproteins and glycolipids is galactose. It is produced in the mammary glands and hydrolyzed to make the lactose of milk. Structure of Galactose  **Mannose:** On the hydrolysis of plant gums and mannosans, mannose is obtained. A constituent of the prosthetic polysaccharide of albumins, mucoproteins, and globulins is mannose. Hexoses and pentoses exist in both ring and open-chain forms. According to the number of carbon atoms they possess, simple sugars might be further divided into tetroses, trioses, hexoses or heptoses, pentoses, and ketoses or aldoses based on whether the ketone or aldehyde groups are present. For example:   **Aldoses** **Ketoses** -------------------- ---------------- ------------------ (C₃H₆O₃) Trioses Glyceraldehyde Dihydroxyacetone (C₄H₈O₄) Tetroses Erythrose Erythrulose (C₅H₁₀O₅) Pentoses Ribose Ribulose (C₆H₁₂O₆) Hexoses Glucose Fructose **a. Trioses:** Trioses are formed throughout the metabolic breakdown of the hexoses in the body. Example: dihydroxyacetone and glyceraldehydes. https://lh4.googleusercontent.com/c6zXW40Oj2Y1UN4fC4qf2D7ZVQqow9MuDmUN20Pu1Y7DjJ\_bXdG4HwNhRTrcACeD9n9l-\_-jups9zJubXB8\_XSrdbG7On6fJh6u2dS4tpAV2ugbg4vm-k0qvSbse\_uNj7hmJFvCQ **b. Pentoses:** Pentoses are vital constituents of many coenzymes and nucleic acids. They are also formed as transitional throughout certain metabolic processes. Example: nucleic acids and coenzymes NAD, Ribose that is a structural element of ATP, and flavor proteins: Arabinose, ribulose, and xylose.  **ii. Disaccharides:** Disaccharides comprise two monosaccharides connected by a glycosidic linkage (C-O-C). Cn (H₂O)n-1 is the general formula for disaccharides. The most common disaccharides forms are lactose, sucrose, and maltose. **Maltose:** Maltose is formed as a transitional product of the action of amylases on starch and it contains two glucose residues in alpha 1, 4 linkages. It can be seen in a detectable amount in many germinating tissues and seeds where starch is being broken down. Structure of Maltose Maltose Structure **Lactose:** Lactose can be found in milk. On hydrolysis, it produces D-galactose and D-glucose. it is a reducing disaccharide, as it has a free anomeric carbon on the glucose residue.  **Sucrose:** Cane sugar or sucrose is a disaccharide of fructose and glucose. The hydrolysis of sucrose to D-glucose and D-fructose is often known as inversion as it is accompanied by a net change in [optical rotation] from dextro to levo as the equimolar mixture of fructose and glucose is formed and this mixture is known as invert sugar. Certain enzymes like invertases catalyze this reaction. Sucrose is tremendously abundant in plants and is commonly known as table sugar. Structure of sucrose C:\\Users\\oluse\\AppData\\Local\\Microsoft\\Windows\\INetCache\\Content.MSO\\3A037D0B.tmp **Trehalose:** Trehalose possesses two D-glucose residues and it is a non-reducing disaccharide like that of sucrose. It is the main sugar that can be seen in many of the insects\' haernolymph. T**rehalose** consists of two units of glucose linked by α-(1→1) glycosidic bond  **Oligosaccharides** - - https://lh5.googleusercontent.com/6SRb7zujstDMOrzATcHUUmhbKfQa8hBRJmABeXgH0k8GryVL59Kxk53973ZQKreH7e3WW7prIERFsfjkaIIugc62Um\_I8JGqxCOttWHqmjuqsmfTWYGcwvPTpDPeRCnSVfknNzsl  **3. Polysaccharides** The majority of the carbohydrates that can be found in nature take place as polysaccharides of high molecular weight. Polysaccharides are complex carbohydrates that are formed by the method of polymerization of a huge number of monosaccharide monomers. The other name for polysaccharides is also known as glycans. They are lengthy polymers of monosaccharide units joined by glycosidic linkages which might be unbranched or branched. After the completion of hydrolysis with specific enzymes and acids, the polysaccharides give up simple monosaccharide derivatives and/ or monosaccharides. Depending upon the composition, polysaccharides can be classified into two types: Homopolysaccharides and heteropolysaccharides. - - Example: agar, chitin, arabanogalactans, peptidoglycan, arabanoxylans, etc.