Carbohydrates.docx

Carbohydrates have range of different uses such as source of energy e.g., glycolysis and amino acid synthesis (transamination) Also used structurally e.g., cellulose and is attached to structures like proteins (glycosylation). Carbohydrates contains ONLY carbons, hydrogens, and oxygens. Molar ratio...

Carbohydrates have range of different uses such as source of energy e.g., glycolysis and amino acid synthesis (transamination) Also used structurally e.g., cellulose and is attached to structures like proteins (glycosylation). Carbohydrates contains ONLY carbons, hydrogens, and oxygens. Molar ratio of carbohydrates= C1H2O1 = hydrates of carbons Types of carbohydrates Monosaccharides E.g., glucose, galactose, and fructose are hexose sugars (6 carbons). Simple building blocks of carbohydrates Can be triose (3C), tetrose (4C) and pentose (5C) Polymerisation- overall loss of water therefore a condensation reaction Disaccharides is 2 monosaccharides. Oligosaccharides is between 3-20 monosaccharides. Polysaccharides is between 21 and 1000’s monosaccharides e.g., starch and amylopectin. Different combinations of monosaccharides can form different disaccharides. E.g. Galactose + glucose= lactose Glucose + glucose= maltose (reducing sugar) Glucose+ fructose= sucrose. Why are the bond types between disaccharides drawn differently? Carbohydrate numbering All of hexoses has same nomenclature. The labels for the carbons are always the same. Carbon number 1 is always the anomeric carbon which dictates how the 2 sugars interact when forming the bond e.g., orientation and shape of the bond between 2 disaccharides. Different ways to represent same molecules. Carbohydrate equilibrium Any carbohydrates can exist in 2 forms- cyclic and linear. Cyclic sugar dominates the equilibrium so its most likely seen compared to the linear version of the sugar. For the linear form of glucose to form the cyclic form of glucose, the alcohol group attacks the aldehyde group to form an intramolecular hemiacetal group. A hemiacetal group is a functional group that is formed when aldehyde reacts with alcohol. Intramolecular means the reaction occurs within the same molecule. Depending on which direction the alcohol group attack the aldehyde group, either from the back or from the top; that then has implications of the stereochemistry of the anomeric carbon. So, if the alcohol group attacks the aldehyde from the back, this results in the OH group pointing down = Axial position If the alcohol group attacks the aldehyde from the top, this results in the OH group pointing across= Equatorial position Therefore they are anomers The bond between maltose is an alpha (1-4) bond due to the anomeric carbon being in alpha position. The bond between lactose is a beta (1-4) bond due to the anomeric carbon being in beta position. Order matters in which alpha and beta glucoses are combined e.g., alpha glucose + beta glucose isn’t the same as beta glucose + alpha glucose. Alpha glucose + any other glucose= alpha glycolytic bond Beta glucose + any other glucose= beta glycolytic bond Chirality When an atom is bound to different groups therefore has a ‘chiral centre’ Enantiomers – same chemical formula but atoms are arranged as mirror image. Cannot be superimposed no matter how we rotate or translate. D glucose and L glucose are mirror images of each other therefore sugar enantiomers We look at these carbons called penultimate carbon to identify the D and L glucoses. If hydroxyl is pointing right, its D glucose. If hydroxyl group pointing left, its L glucose. Sugar epimers- When sugars are different to one another in configuration with regards to single carbon atom other than the penultimate carbon. Diastereoisomers- same chemical composition but multiple chiral centres are not enantiomers or epimers as they are not mirror images of each other. E.g. Aldohexoses and ketohexoses Glucose contains CHO group associated with aldehydes= aldohexose Glucose can also contain a C=O group associated with ketones= ketohexose Any sugar with a free aldehyde group or a free ketone group is a reducing sugar- can reduce other molecules but gets oxidised itself. Anomers and Mutarotation Anomer is ////////// Mutarotation is disproportion and biased of alpha and beta positions of glucose. Different chemical tests Benedict’s test Used to test for glucose in urine (glucosuria) Contains sodium carbonate, copper sulphate and sodium citrate. When reducing sugars are heated in presence of an alkali, they are converted to enediols Enediols reduce Cu2+ to Cu+ ions which precipitate as insoluble red copper oxide. Glucose oxidase test- GOD-PAD/PAP GOD converts glucose sample into gluconate- hydrogen peroxide produced in the reaction is degraded by peroxidase (POD) Gives a coloured product which is measurable at 505nm. Increase in absorbance correlates with glucose concentration of the sample. Test for starch Iodine is used to test for starch. Forms a blue-black colour and the colour disappears when heated and appears when cooled Starch is non reducing sugar 2 types of polysaccharides. Homopolysaccharides- same type of sugar joined together e.g., glycogen, amylose, and amylopectin. Heteropolysaccharides- different sugars joined together e.g., agar. Starch Composed of both amylose and amylopectin Amylose has alpha 1-4 glycosidic bond and so water soluble whereas amylopectin has alpha 1-6 glycosidic bonds and is water insoluble. Glycogen Stored in liver and muscles. Glycogen is more branched and more compact than amylopectin. Composed of glucose units joined by alpha 1-4 links in straight chains and has alpha 1-6 glycosidic linkages at branching points. Cellulose Glucose units linked together with beta 1-4 linkages into long linear strands with no branching. Linkages hydrolysed by enzyme cellobiose. Agar is made up of agarose (linear polysaccharide) + agaropectin (branched polysaccharide) Glycosylation Sugars can be used for recognition when added onto something such as proteins. Viruses such as corona virus are heavily glycosylated. 2 types of glycosylation: N-linked (amino acid asparagine) O-linked (amino acids serine and threonine)