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Carbohydrates (Monosaccharides)

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Carbohydrates (Monosaccharides) Carbohydrates (Monosaccharides) 3rd stage Anbar University-College of Pharmacy-Clinical Laboratory Sciences Department 2019-2020 Dr. Yousif H. Khalaf References Harper’s Illustrated biochemistry, 26th edition Biochemistry – Berg, Tymoczko and Stryer, 6th edition Lippincott’s Illustrated Reviews: Biochemistry 5th edition Learning outcomes By the end of this lecture you will be able: o To know the formation of carbohydrates o To understand the nature of glycosidic bonds o To understand the structural organisation of carbohydrates o To appreciate the various functions of carbohydrates Carbohydrates . Carbohydrates, one of the four major classes of biomolecules along with proteins, lipids, and nucleic acids. Carbohydrates are built from monosaccharides (carbon atoms bound to hydroxyl groups, Empirical formula Cn(H2O)n . Monosaccharides are linked together by glycosidic bonds to form di, oligo- and a huge variety of polysaccharides . Carbohydrates are aldehyde (CHO) or ketone (C=O) compounds with multiple hydroxyl groups (Carbon-oxygen double bonds make the sugars reactive) Importance of carbohydrates • Carbohydrates serve as energy stores and metabolic intermediates. • Ribose and deoxyribose sugars form part of the structural framework of RNA and DNA. • Carbohydrates are important for tissue formation • Carbohydrates form the basis of human blood groups • Polysaccharides are structural elements in the cell walls of bacteria and plants. • Carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment. Several classifications of carbohydrates are outlined in the table Carbohydrate can be classified to: 1-Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons. 2-Disaccharides: two monosaccharides covalently linked by glycosidic bond. 3-Oligosaccharides: a few monosaccharides (3-9) covalently linked by glycosidic bonds. 4-Polysaccharides: polymers consisting of chains of monosaccharide or disaccharide units. Monosaccharides - Aldehydes or ketones - Colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. - They are important fuel molecules as well as building blocks for nucleic acids. - The smallest monosaccharides are : Monosaccharides are classified according to: (1)Aldehyde (e.g., glucose) or ketone group (e.g., fructose) (2)The number of carbon atoms present in the molecule Three-carbon monosaccharides triose (tri), e.g. glyceraldehyde Four-carbon monosaccharides tetroses (tetra), e.g. erythrose Five-carbon monosaccharides pentoses (penta), e.g. ribose Six-carbon monosaccharides hexoses (hexa), e.g. glucose Seven-carbon monosaccharides heptoses (hepta), e.g. sedoheptulose The names of all sugars end in –ose. Hexoses are the nutritionally important sugars. If the carbonyl group is an aldehyde, the sugar is an aldose. if the carbonyl group is a ketone, the sugar is a ketose D and L isomerism Isomers are molecules with the same kinds and numbers of atoms joined up in different ways. A carbon atom that contains four different chemical groups forms an asymmetric (or chiral) center. The prefixes D and L designate the absolute configuration of the asymmetric carbon farthest from the aldehyde or keto group. When the OH group on this carbon is on the right , the sugar is the D-isomer; when it is on the left, it is the L-isomer. Glyceraldehyde has a single asymmetric carbon and, thus, there are two stereoisomers of this sugar. D-Glyceraldehyde and L-glyceraldehyde are mirror images of each other (enantiomers). D-glyceraldehyde L-glyceraldehyde Stereoisomers and Epimers Stereoisomers have the same chemical formula but differ in the position of the hydroxyl group on one or more of their asymmetric carbons. Epimers are stereoisomers that differ in the position of the hydroxyl group at only one of their asymmetric carbons. D-glucose and D-galactose are epimers of each other, differing only at C4, and can be interconverted in human cells by enzymes called epimerases. D-mannose and D-glucose are also epimers of each other, differing only at C2. Fischer/Haworth projection Glucose 1 (dextrose; grape sugar) HC O 2 HC OH open-chain form • Glucose in solution exists mostly in the ring form at HO 3CH (Fischer projection) equilibrium, with less than 0.1% of the molecules in the 4 open-chain form. HC OH 5 HC OH • Fischer projections are useful for depicting carbohydrate structures because they provide clear and simple views 6 CH2OH of the stereochemistry at each carbon center. 6 CH2OH 5 H O H 4 HOH H 1 ring-form 2 OH (Haworth projection) OH 3 H OH glucose Cyclization of Monosaccharide • The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains. Rather, the open-chain forms of these sugars cyclize into rings. • For an aldohexose such as glucose, Formation of a hemiacetal by reaction of the C-1 aldehyde group with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called pyranose. because of its similarity to pyran. 1/3 Mutarotation 2/3 (Ring-form is energetically more stable) . Similarly, the C-2 keto group in the open-chain form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal. The five- membered ring is called a furanose because of its similarity to furan. For d sugars drawn as Haworth projections, the designation α means that the hydroxyl group attached to C-1 is below the plane of the ring, β means that it is above the plane of the ring. The C-1 carbon atom is called the anomeric carbon atom, and the α and β forms are called anomers. An equilibrium mixture of glucose contains approximately one-third α anomer, two-thirds β anomer, and <0.1% of the open chain form. In solution, the hydroxyl group on the anomeric carbon spontaneously (non enzymatically) changes from the α to the β position through a process called mutarotation. When the ring opens, the straight chain aldehyde or ketone is formed .When the ring closes, the hydroxyl group may be in either the α or the β position. This process occurs more rapidly in the presence of cellular enzymes called mutarotases. Pentoses and Hexoses Cyclize to Form Pyranose and furanose Rings aldehyde group HC O CH2OH ketone group HC O HC OH C O HC OH HO CH HO CH HO CH HO CH HC OH HC OH HC OH HC OH l HC OH CH2OH CH2OH CH OH hemiketa 2 CH OH hemiacetal 2 6 1 6 HO O HOCH2 O CH2OH CH OH 2 OH 5 5 HO 2 O 4 4 OH 1 OH OH 3 2 OH OH OH OH 3 a-D- galactose fructose OH pyranose pyranose furanose glucose Pentose sugars Pentoses such as D-ribose and 2-Deoxy-D-ribose form furanose rings, as we have seen in the structure of these units in RNA and DNA. HC O HC O HC OH HCH HC OH HC OH HC OH HC OH CH2OH CH2OH OH HOCH2 O OH HOCH2 O OH OH OH ribose deoxyribose Components of RNA and DNA Monosaccharides joined to alcohols and amines Through Glycosidic Bonds . Facilitate their metabolism . Linked to alcohols, amines and phosphates . The hydroxyl group on the anomeric carbon of a cyclic sugar can react with an –OH or an –NH group of another compound to form: o O-glycosidic bond between a monosaccharide and an alcohol or two monosaccharides or between a monosaccharide and a protein o N-glycosidic bond between a monosaccharide and a nitrogenous base or the amino acid lysine of a protein Adenosine triphosphate Phosphorylated sugars • Addition of a phosphoryl group to the monosaccharide • Makes sugars anionic • Trap sugar within the cell • Creates a reactive intermediate of sugar metabolism Adenosine monophosphate Sugar alcohols o Formed by the reduction of the aldehyde group of glucose to a H2OH hydroxyl group. o Used in foods suitable for diabetics as it tastes sweet. Also in cough syrup and sugar-free mints. o Energy yield roughly half that of glucose. Sorbitol.
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