1 Carbohydrates 2: Oligosaccharides

2 Are meteorites “sugar free”?? Read OR OR Do a search on meteorites and sugars I find the second address sometimes doesn’t work.

3 Glycosidic Bonds Glycosidic bonds are formed by sugars in ring form and occur between the anomeric hydroxyl of one sugar and any other available hydroxyl. These bonds DO NOT form spontaneously and always require the action of an appropriate enzyme. Glycosidic bonds are broken by enzymes, by the addition of water (hydrolysis)

4 Disaccharides made only from glucose - cellobiose Two  D glucopyranose cellobiose Condensation or hydrolysis rx HOH  1  4 glycosidic bond

5 Disaccharides made only from glucose - maltose + HOH  1  4 glycosidic bond Two  D glucopyranose Maltose + water

6 Disaccharides made only from glucose - Trehalose

7 How many different ways are there to link two glucose molecules ?( involves ring forms only)

8 Other important disaccharides - lactose

9 Other important disaccharides - sucrose

10 Sideline – reducing sugars Aldehydes and ketones can act as reducing agents i.e. they can give up electrons and reduce another compound Can detect the presence of reducing sugars with the Benedicts test Reducing sugars donate an electron and convert: Cu 2+ (cupric ion)  Cu + (cuprous ion ) Then Cu + reactes with oxygen to produce 2Cu + + O  2CuO (red precipitate) Important in detecting ??

11 Reducing Sugars and Ring Formation 1,5 bridge Recall that both pyranose and furanose rings are free to open and close. Therfore ring forms are still reducing

12 What happens when the anomeric hydroxyl gets used up in a glycosidic bond? – the ring gets locked shut! Recall that the oxygen in the anomeric hydroxyl originally came from the oxygen of the aldehyde or ketone. There is no “free” aldehyde or ketone in the ring form, when the anomeric OH is tied up in a glycosidic bond. “Ring locked” (not reducing) Not locked, Free to open Still reducing

13 Other Oligosaccharides Oligosaccharide chains coat the outside of the some animal cells and form a “sugar coat” or glycocalyx. The oligosaccharides are attached to both proteins and lipids embedded in the plasma membrane. These sugar chains are important in “cell recognition processes”. Eg blood group antigens are short oligosaccharide chains attached to lipids in the plasma membrane of blood cells Most of the proteins in the PM have attached oligosaccharide chains eg MHC proteins

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15 Blood group antigens