Biomolecules 2024 - Carbohydrates (1) PDF

# Biomolecules ## Part 1 Carbohydrates By Dr. Thommas Musyoka This document discusses carbohydrates, one of the four major classes of biomolecules. The other three are proteins, nucleic acids and lipids. ### Carbohydrates - are aldehyde or ketone compounds with multiple hydroxyl groups. - have a b...

# Biomolecules ## Part 1 Carbohydrates By Dr. Thommas Musyoka This document discusses carbohydrates, one of the four major classes of biomolecules. The other three are proteins, nucleic acids and lipids. ### Carbohydrates - are aldehyde or ketone compounds with multiple hydroxyl groups. - have a basic molecular formula of $(C_nH_{2n}O_n)$ where n=3 or more. - This formula comes from the observation that when you heat sugars, you get carbon and water (hence "hydrate of carbon"). ### Structure and Occurrence - Carbohydrates consist of carbon, hydrogen and oxygen. - The general empirical structure for carbohydrates is $[CH_2O]_n$. - They are organic compounds organized in the form of aldehydes or ketones with multiple hydroxyl groups coming off the carbon chain. - The building blocks of all carbohydrates are simple sugars called monosaccharides. - A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose). ### Forms of Carbohydrates The carbohydrates can be structurally represented in one of these three forms: 1. **Open chain structure/Fischer projection** - It is the long straight-chain form of carbohydrates 2. **Haworth Structure** - Shows the presence of the pyranose ring structure 3. **Hemi-acetal structure** - Here the 1st carbon of the glucose condenses with the -OH group of the 5th carbon to form a ring structure ### Fischer Projection - German chemist Emil Fischer in 1891 devised this projection. - In a Fisher projection, the carbohydrate is shown in its open chain form, rather than a cyclical one. Carbon atoms in the main chain of the carbohydrate molecule are connected vertically, while hydrogen atoms and hydroxyl groups are bonded horizontally. ### Monosaccharides - They are the simplest carbohydrate units that can not be hydrolysed into simpler units. - They have the general formula: $(CH_2O)_n$, n≥3 - Some notable monosaccharides are Glucose, Fructose and Galactose. #### Glucose This monosaccharide, often called "dextrose" or "blood sugar", is important for the body because it: - Is the primary fuel for the body - Is found in all disaccharides and polysaccharides #### Fructose This monosaccharide, often called "fruit sugar", is important for the body because it: - Is found in fruit, honey and syrup - Converts to glucose in the body #### Galactose This monosaccharide is important for the body because it: - Is a component of lactose - Is found in milk - Converts to glucose ### Nomenclature 1. **According to the active group in the sugar:** - If monosaccharide contains an aldehyde group (CHO) → it's called an aldose. - If it contains a ketone group (C=O) → it's called a ketose. 2. **According to the number of carbon atoms (n): ** - If sugar contains 3 carbons → it's called a triose. - If sugar contains 4 carbons → it's called a tetrose. - If sugar contains 5 carbons → it's called a pentose. - If sugar contains 6 carbons → it's called a hexose. - If sugar contains 7 carbons → it's called a heptose. 3. **By combining the two methods, we find that:** - **3 carbons**: aldotriose or ketotriose - **4 carbons**: aldotetrose or ketotetrose - **5 carbons**: aldopentose or ketopentose - **6 carbons**: aldohexose or ketohexose ### Isomers - Are compounds which have the same molecular weight, but differ in their physical and chemical properties. #### Stereoisomers - These are compounds that have the same structural formula but differ in spatial configuration (arrangement of groups of atoms in space around the asymmetric carbon(s)). #### Isomers The more important types of isomers include: - D and L isomers - Pyranose and furanose ring structures - Alpha and beta anomers - Epimers - Aldose-ketose isomers ### D and L isomers (enantiomers) - These are a special type of isomerism found in the pairs of structures, which are mirror images of each other. - The mirror images are called enantiomers. - The two members of the pair are designated as a D and an L-sugar. - A monosaccharide is designated "D" if the hydroxyl group on the highest numbered asymmetric carbon (the prelast carbon) is drawn to the right as in D glyceraldehyde. - A monosaccharide is designated "L" if the hydroxyl group on the highest numbered asymmetric carbon is drawn to the left as in L-glyceraldehyde. -The majority of the sugars in humans are D-sugars. - There are two exceptions: L-fucose (in glycoproteins) and L-iduronic acid. ### Pyranose and furanose ring structures - The monosaccharides are either **pyran** (a six-membered ring) or **furan** (a five-membered ring). - For glucose in solution, more than 99% is in the pyranose form. ### Hemiacetals or Hemiketals - An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. ### Boat and Chair forms - These forms represent the three-dimensional configuration of sugar in nature. <start_of_image> Diagrams of the different forms of fructose are provided. ### Anomeric Carbon - It is the carbon atom attached to four different groups or atoms. - Formation of a ring results in the creation of an anomeric carbon at carbon 1 of an aldose or at carbon 2 of a ketose. ## Reducing Sugars - If the oxygen on the anomeric carbon (the carbonyl group) of a sugar is not attached to any other structure, that sugar is a reducing sugar. - Only the state of the oxygen on the anomeric carbon determines if the sugar is reducing or nonreducing, which means that other hydroxyl groups on the molecule are not involved. - A reducing sugar can react with chemical reagents (for example, Benedict's solution) and reduce the reactive component, with the anomeric carbon becoming oxidized. ### Optical Activity - The ability of the compound to rotate plane polarized light to the right or left. - If the compound rotates plane polarized light to the right, it is called **dextrorotatory**, d or (+). - If it rotates plane polarized light to the left, it is called **levorotatory**, l or (-). - The direction of rotation is independent of the stereochemistry of the sugar. - It may be designated D(-), D(+), L(-), or L(+). - For example, the naturally occurring form of fructose is the levorotatory sugar (l or -). - dextrorotatory sugar (d or +): glucose, galactose, and starch - levorotatory sugar (l or -): fructose and invert sugar. ### Stereoisomerism - Compounds that have the same structural formula but differ in spatial configuration are known as stereoisomers, and the phenomenon is called stereoisomerism. - The number of possible isomers of a compound depends on the number of asymmetric carbon atoms (n) and is equal to 2n. - Glucose, with four asymmetric carbon atoms, has 16 isomers. ## Disaccharides - A disaccharide consist of two sugars joined by an O-glycosidic bond. - The most abundant disaccharides are sucrose, lactose, and maltose. -Other disaccharides include isomaltose, cellobiose and trehalose. - Disaccharides can be classified into **homo disaccharides ** and **hetero disaccharides**. #### Homo Disaccharides - Are formed of the same monosaccharide units. - Include maltose, isomaltose, cellobiose, and trehalose. #### Hetero Disaccharides - Are formed of different monosaccharide units. - Include sucrose and lactose. ### Lactose - It is formed of β-galactose and α-glucose linked by β-1,4-glucosidic linkage. - It contains a free anomeric carbon, making it a reducing sugar. - It may appear in urine in late pregnancy and during lactation. ### Sucrose - It is formed of α-D-glucopyranose and β-D-fructofuranose by a 1-2 glycosidic bond. - It has no free aldehyde or ketone groups, making it a nonreducing sugar. - It is hydrolysed to glucose and fructose by sucrase (invertase) enzyme. - Sucrose is dextrorotatory +66.5. ### Maltose (malt sugar) - It consists of 2 α-glucose units linked by α-1,4-glucosidic linkage. - It contains a free anomeric carbon, making it a reducing sugar. ### Trehalose - It is formed from 2 α-glucose units linked by a-1,1- glucosidic linkage. - It does not contain a free anomeric carbon, making it a nonreducing sugar - It is present in a highly toxic lipid extracted from Mycobacterium tuberculosis. ### Condensation - The two glucose molecules bond together with a single O atom to form the disaccharide maltose ### Hydrolysis - When water is added to the glycosidic linkage of the disaccharide, the linkage is broken. - This results in two glucose molecules. ## Oligosaccharides - Oligosaccharides contain from 3 to 10 monosaccharide units - Raffinose is an oligosaccharide found in peas and beans. ## Polysaccharides (glycans) - Polysaccharides consist of more than 10 monosaccharide units. #### Types of Polysaccharides - **Homo Polysaccharides:** consist of only one type of monosaccharide molecule. - Examples: starch, glycogen, dextrin, cellulose, inulin, and chitin - **Hetero Polysaccharides:** contain more than one type of monosaccharide. - Examples: glycosaminoglycan, glycoprotein. ### Starch - It is a homopolymer of glucose forming an α-glucosidic chain called a glucosan or glucan. - It is the most abundant dietary carbohydrate in cereals, potatoes, legumes, and other vegetables. - The two main constituents are amylose (15-20%), which has a nonbranching helical structure and amylopectin (80-85%), which consists of branched chains composed of 24-30 glucose residues united by 1→4linkages in the chains and by 1 →6 linkages at the branch points. ### Dextrins - Are intermediates in the hydrolysis of starch. ### Glycogen - Is the storage polysaccharide in animals - It is a more highly branched structure than amylopectin, with chains of 12-14 α-D-glucopyranose residues in an α[1 → 4]-glucosidic linkage and with branching by means of α(1→6)-glucosidic bonds. ### Inulin - Is a polysaccharide of FRUCTOSE (and hence a fructosan) - It is found in plants. - It is readily soluble in water. - It is used to determine the glomerular filtration rate. ### Cellulose - Is the chief constituent of the framework of plants - It is insoluble. - It consists of β-D-glucopyranose units linked by β (1→ 4) bonds to form long, straight chains strengthened by cross-linked hydrogen bonds. - Cellulose cannot be digested by mammals because of the absence of an enzyme that hydrolyzes the β linkage. - It is an important source of bulk in the diet, so it prevents constipation. ### Chitin - Is a structural polysaccharide in the exoskeleton of crustaceans and insects, and also in mushrooms. - It consists of N-acetyl-D-glucosamine units joined by β (1→ 4)-glycosidic linkages. ### Glycosaminoglycans (GAGs) or (Mucopolysaccharides) - Are long linear (unbranched) heteropolysaccharide chains generally composed of a repeating disaccharide unit (acidic sugar-amino sugar)n. - The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, and sometimes sulphated. - There are 6 types: - heparin - heparan sulfate - dermatan sulfate - keratan sulfate - chondroitin sulfate - hyaluronic acid - All of the glycosaminoglycans except hyaluronic acid and heparin are found covalently attached to protein, forming proteoglycan monomers. - They have the property of holding large quantities of water and occupying space lubricating other structures, which is due to the large number of OH groups and negative charges on the molecules. - Repulsion keeps the carbohydrate chains apart. ### Glycoproteins - Are proteins to which oligosaccharides are covalently attached. - Oligosaccharide chains formed mainly of sialic acids and L-fucose. - Sialic acids are N- or O-derivatives of neuraminic acid/ - Neuraminic acid is a nine-carbon sugar derived from mannosamine and pyruvate. - Glycoproteins have many functions, which include: - **Soluble** as enzymes, hormones and antibodies - **In lysosomes** - **Attached** to the cell membrane (The membrane bound glycoproteins participate in: - **Cell surface recognition** (by other cells, hormones, viruses). - **Cell surface antigenicity** (as blood gp antigens). ### Comparison of Proteoglycans and Glycoproteins | Feature | Proteoglycan | Glycoprotein | |---|---|---| | Carbohydrate components | Glycosaminoglycans | Oligosaccharides | | Repeating disaccharide unit | Yes | No | | Structure | Linear (unbranched) | Branched | | Length | Long | Short | | Components | Contain uronic acids (glucuronic acid and iduronic acid) | Contain sialic acid derivatives (NANA) | | &nbsp; | N-acetyl hexosamine | N-acetyl hexasamine | | &nbsp; | Contain hexoses as galactose (in keratin sulfate) | Contain hexoses as galactose and mannose | | &nbsp; | Contain sulfate | No sulfate | | &nbsp; | No pentoses | Contain pentose | | &nbsp; | &nbsp; | Contain deoxy sugar as L-fucose | | Tissue Distribution and Function | - Structural: cartilage, bone, tendons, cell membrane, cornea | -Functional and structural: mucins, blood groups antigens, some hormones, enzymes, immunoglobulins and receptors | ### NANA=N-acetyl-Neuraminic Acid - Neuraminic Acid = sialic acid = mannosamine + pyruvic acid - Neuraminic acid is also known as amino sugar acid. - NANA is found glycoproteins. ### L-Fucose - It is known as 6-deoxy-L-galactose or methyl pentose. - It is found in glycoproteins. # The End!