Introduction to Biochemistry Lecture 2 PDF

040817231 Introductory biochemistry Introduction to Biochemistry NASHWA W. YASSA, PHD Carbohydrates  Carbohydrates ▪Carbohydrates are organic molecules found in nature. ▪Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. ▪The basic molecular formula (CH...

040817231 Introductory biochemistry Introduction to Biochemistry NASHWA W. YASSA, PHD Carbohydrates  Carbohydrates ▪Carbohydrates are organic molecules found in nature. ▪Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. ▪The basic molecular formula (CH2O)n where n = 3 or more. Also, carbohydrates are polyhydroxylated compounds having at least 3 carbon atoms and a potentially active carbonyl group which may be aldehydic or ketonic group. So, carbohydrates are Also, defined as aldoses (contain aldehyde group) or ketoses (contain ketone group) polyhydroxy (contain more than one hydroxyl group) compounds The carbohydrates are widely distributed both in animal and in plant tissues Where do carbohydrates come from? Carbon cycle in nature photosynthesis. Light energy Carbon Water Dioxide Glucose Oxygen  In plants, they are produced by photosynthesis. HOW……. ? Plants and photosynthesis Chlorophyll captures light energy from the sun which is transformed into chemical energy (ATP) Chemical energy is used to combine carbon dioxide (CO2) and water (H2O) to form glucose (C6H12O6)  The by-product is oxygen (O2) Extra glucose is stored in plants as starch or used for synthesis of cellulose (a main component of cell wall)  Importance of Carbohydrates ▪ The most widely abundant organic molecules in nature ▪ Serve many functions in living organisms: ▪ Energy storage→ Form the majority of dietary calorie intake ▪ Sugars are components of nucleic acids ▪ Cell membrane component and cell-cell communication ▪ Form structural tissues in plants (cellulose, lignin) and murein in microorganisms ▪ Disorders of carbohydrate metabolism could lead to Diabetes Classification They are classified according to the number of structural units into: 1‐Monosaccharides= They are the simplest carbohydrates that can not be hydrolysed into simpler units. ◦ The most abundant monosaccharide in nature is the six- carbon sugar D-glucose. 2‐ Disaccharides= produce 2 molecules of monosaccharide on hydrolysis. 3‐ Oligosaccharides= produce three to ten monosaccharide units on hydrolysis 4‐ Polysaccharides= produce more than 10 monosaccharide units on hydrolysis Nomenclature 1- According to active group in the sugar: – If monosaccharide contains aldehyde group (CHO) → it's called aldose. And if contain ketone group (c=o) → it's called ketose. 2-According to the number of carbon atoms (n):‐ If sugar contains 3 carbons → it's called triose, 4c→ tetrose 5c→ pentose 6c→ hexose 7c→ heptose  Monosaccharides Aldoses (e.g., glucose) have an aldehyde group at one end. Ketoses (e.g., fructose) have a keto group, usually at C2. D-Glucose 2) Disaccharides : Two monosaccharide units covalently linked by glycosidic linkage and classified according the type of units to : a)Homo - disaccharides (two similar units) or b)Hetero - disaccharides (two different units). And according the reducing power to : a)Reducing-disaccharides (have free active gp.). Maltose (2 glu.) Lactose (glucose and glactose) b)Non-reducing-disaccharides (have no free active gp. where they used in the formation of glycosidic linkage). Sucrose (glucose and fructose) Maltose 3) Oligosaccharides: A few monosaccharides (3 - 9 or 10 units) covalently linked by glycosidic linkages. 4) Polysaccharides : Polymers consisting of chains of monosaccharide units (tens, hundreds or more than thousands). All are non-reducing (has low number of free active group or reducing ends and large molecular weight). They classified to: a) Homo-polysaccharides and b) Heteropolysaccharides. Structural representation of sugars straight chain representation Fisher projection Haworth projection The structure of glucose can be represented in one of the following ways: 1.The straight – chain (open ‐ chain) structural formula : Aldohexose can account for some of the properties of glucose, but can not explain some reaction D-glucose The cyclic structure accounts for the remainder of the chemical properties of glucose. This cyclic structure can be represented in two forms: a. Fischer projection formula :where the aldehyde group reacts with an alcohol group on the same sugar to form a hemiacetal ring. B.Haworth formula: Where the cyclic structure is represented in pyranose (six membered) and furanose (five – membered) rings resembling pyran and furan rings. Here oH and H written above and below instead of Right and left so what is on Right → below left →above except last carbon (which close the ring) in which left →below Right →above C-Boat and chair forms represents the three dimensional configuration of sugar in nature. Monosaccharides Are Aldoses and Ketoses Monosaccharides are colorless, crystalline solids Freely soluble in water but insoluble in nonpolar solvents Most have a sweet taste. carbon atoms at backbone are linked by single bonds. Monosaccharides Have Asymmetric Centers Asymetric or chiral carbon atom provides optically active isomeric forms of sugar called enantiomers expect dihydroxyacetone. The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers. By convention, one of these two forms is designated the D-isomer, the other the L-isomer. Monosaccharides Are Reducing Agents Oxidation of the anomeric carbon of glucose and other sugars makes them reducing sugar. This is the basis for Fehling’s reaction. The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. The carbonyl carbon is oxidized to a carboxyl group. For many years, this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of diabetes. Properties of Monosaccharides 1. Optical isomerism 2. Epimerism 3. Hemiacetal and hemiketal formation 4. L and D forms 5. Anomers 6. Formation of glycosidic bonds 7. Reducing properties Isomers or isomerisms are the compounds which have the same molecular formula but differ in its structural formula (configuration in the space ) 1-Optical isomerism Have Asymmetric Centers Asymetric or chiral carbon atom provides optically active isomeric forms of sugar called enantiomers expect dihydroxyacetone. The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers. By convention, one of these two forms is designated the D-isomer, the other the L-isomer. 1-Optical isomerism Enantiomers : They are mirror images. 2-Epimers Epimers are isomers that have different configurations at only one carbon atom. This kind of isomerism was formed due to the internal distribution of hydroxyl group and hydrogen atom around carbon atom no. 2,3,4 for hexoses or 2,3 for pentoses. FOR EXAMPLE: Glucose and mannose are epimers at carbon no. 2 while, glucose and galactose are epimers at carbon no. 4 Isomers & Epimers Monosaccharides having the same chemical formula are called isomers All hexoses (glucose, fructose, galactose, mannose) have the formula C6H12O6 Epimers are monosaccharides differing in the orientation around one carbon atom other than the carbonyl group α and β anomers: An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal. These representations of the cyclic sugars are called Haworth projections. Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH. Glucose forms an intra- molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6- member pyranose ring, named after pyran.  ANOMERIC CARBON ATOM The carbon atom which is part of the carbonyl group Alpha(α) and Beta(β) anomers differ from each other only in respect to configuration around anomeric carbon atom. Biologically Important Sugar (Glucose) Derivatives 1. Sugar Acids 2. Sugar Alcohols 3. Deoxy Sugars 4. Amino Sugars Sugar Acids Oxidation of aldo group (c1)of sugars produces aldonic acids. Ketoses are not easily oxidized. Oxidation of terminal alcohol group (–OH sixth carbon atom of glucose produces Glucuronic acid or uronic acid : Sugar acids Sugar alcohols  RIBOSE DEOXYRIBOSE CH2OH CH2OH O OH O OH C C C C H H H H H H H H C C C C OH OH OH H © 2016 Paul Billiet ODWS Amino sugar Glycosidic Bond 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 Thanks