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Carbohydrate Nomenclature I. Monosaccharides

Carbohydrate Nomenclature I. Monosaccharides.

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Carbohydrate Nomenclature I. Monosaccharides

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  1. Carbohydrate Nomenclature I. Monosaccharides We may think of carbohydrates as sugar and spice and everything nice, but, to first year biochemistry students, carbohydrates are a terror. One reason is the nomenclature for the monosaccharides. A comprehensive nomenclature is needed to obtain precision in a classification scheme, distinguishing one sugar from another when only subtle differences exist between the two. Organic chemistry gives us terms such as D and L isomers, chiral centers, enantiomers, diastereoisomers, etc. Biochemical terminology builds on the organic using terms such as anomers, alpha and beta sugars, glycosides, oligosaccharides, all attempting to signify uniqueness in the complexity of the structures. Sprinkled in among the vocabulary are specific names of sugars, e.g., glucose, maltose, amylopectin, that can be as daunting to learn as the structures. This tutorial is designed to point out features that will help you remember the structures and names of this all-important class of biomolecules. Taming the Mayhem

  2. C C C C C O C C C C C C C H C C=O C C C C C C C C Tri- C C C C C Tetra- C C C Penta- C C C C Hexa- C C Aldopentose Hepta- Ketohexose Numbers, Groups and Names “OSE” is the suffix denoting a sugar and “ULOSE” denotes a keto sugar. The monosaccharides you will encounter in biochemistry have 3, 4, 5, 6, and 7 carbons. Prefixes such as tri, tetra, penta, hexa, and hepta alert you to the number. Know these prefixes (click 1). You will also see “aldo” and “keto” to denote the type of functional group (click 1). The term aldopentose denotes two structural features, chain length and functional group. Aldo refers to aldehyde and keto to ketone. The figures show an aldopentose and a ketohexose. An aldo sugar always has the “carbonyl” group on C-1, a keto sugar has it on C-2. This is good to remember. –OH groups are not indicated by this terminology. More terms are need, therefore, to describe a specific sugar. Click to go on.

  3. D-glucose D-ribose L-glucose D-fructose D-mannose L-mannose Balls and Sticks: All sugars have one carbonyl group and at least two –OH groups. All have at least one –CH2OH group. What distinguishes one from another is the right-left orientations of internal –OH groups. Stereochemistry is best learned by using balls and sticks. For example, D-glucose, an aldohexose, is show as (click 1). The ball represents the –CHO group, the sticks –OH groups. Because a –CH2OH group is common to all sugars, it is not necessary to draw this group each time. The red line shows the –OH group whose left/right orientation determines if its a D- or L-sugar. L-glucose is the mirror image of D-glucose (click 1). Focus on chiral center orientations to learn the sugars. Keto sugars can only be represented by sticks, with a = to show the keto group (click 1). Ball and stick representations are a quick way to help you see differences between sugars and imprint these in you memory (click 1)

  4. gluco gulo galacto manno allo ido altro talo ribo arabino xylo lyxo erythro threo fructo Rules for Configurations What you saw on the previous slide was the importance of –OH group orientation to pinpoint a specific sugar’s name. Now you will see that internal configurations have their own prefixes, such as “gluco, manno, galacto, fructo, etc. Configuration prefixes help you compare sugars. Here are examples of D sugar configurations (click 1). Note that glucose and galactose differ by orientation around C-4 (click 1) and glucose and mannose differ at C-2 (click 1). The other aldohexoses are allose with all –OH groups on the same side (click 1) to idose (the idiot sugar) that can decide which side to put its –OH groups (click 1). In the pentoses, one sees that ribose has all –OH groups on the same side (click 1). The tetroses differ by orientation around C-2. Fructose is set apart because of the keto group on C-2 (click 1). But, note that C-3 to C-6 of fructose have the same configuration as glucose (click 1). Click to go on.

  5. D-erythrose D-threose D-glucose L-glucose D-mannose D-galactose D-ribose D-xylose D-fructose D vs L Classification Simplifies Names Recall, aldohexoses have 4 chiral centers, or 16 (24) stereoisomers possible. Does that mean 16 individual sugar names? No. When we denote aldohexoses as D or L, only 8 (23) D-steroisomers are possible. This is because the D, L designation fixes one of the centers. Therefore, of the 16, 8 D and 8 L will have the same name. Common aldohexoses you will encounter in your studies are D-glucose, D-mannose, and D-galactose. Know these and D-fructose (click 1). X Applying the same rule, there are 4, D-aldopentoses and 2, D-aldotetroses. The common D-aldopentoses are D-ribose and D-xylose (pronounced zy-lose) (click 1). If you use your imagination you should see an X in the structure of xylose (click 1). Perhaps calling xylose the “idiot sugar of pentoses” will help you remember the structure. The aldotetroses are represented by D-erythrose and D threose (click 1). D-erythrose, like D-ribose has all –OH groups on the right. D-threose has one right and one left (another idiot sugar?). Click to go on.

  6. OH H HO H C C C-OH C-OH HO- C C HO- C-OH C-OH C C O O CH2OH CH2OH alpha D-glucopyranose beta D-glucopyranose Rings Only 5-, 6-, and 7-carbon sugars form rings. The ring can either have 6 atoms (pyranose) or 5 atoms (furanose). The –OH group on a hexose will attack C-1 to form a ring (click 1). When the ring forms, a new asymmetric carbon is introduced into the molecule (click 1). The –OH on the new asymmetric carbon can be drawn so as to appear on the same side of the ring-forming oxygen (alpha sugar) (click one), or it can be drawn to be on the side away from the ring-forming oxygen (beta sugar) (click 1). O C * H -OH C HO- C C-OH C-OH CH2OH You should now be able to see how all the nomenclature discussed thus far is needed to pin down a specific monosaccharide. To help you see this, consider the alternatives to alpha D-glucose. Its alpha (not beta), D (not L), gluco (not galacto, manno, fructo, etc.) pyranose (not furanose). The nomenclature is precise for just one sugar. Click one to go on.

  7. Terminology We end the lesson by considering the terminology that describes the properties of sugars. Understand that terminology is needed to draw comparisons between structures. So, the question you must ask is “how does this term tell me how two sugars differ? Remember isomers must have something in common as well as different. 1. Glucose and galactose are epimers (click 1). The word epimer is used when comparing sugars with multiple chiral centers. It literally says only one center is different. 2. L-glucose is the enantiomer of D-glucose (click 1) This means that one is the mirror image of the other. 3. Alpha D-glucose is the anomer of beta D-glucose (click 1) Anomers differ in the stereochemistry around the ring-forming carbon. Since alpha and beta differ in only one chiral center, anomers can also be considered epimers 4. Glucose and galactose are diastereoisomers (click 1) Diastereoisomers have different physical properties. Generally optical isomers with one chiral center differ only in the direction they rotate plane-polarized light. Diastereoisomers differ both in rotation and physical properties. D-galactose and D-glucose, for example have the same chemical formulas (C6H12O6), the same straight carbon chain and the same number of –OH groups. But, besides rotation, they also differ in melting point, solubility, heat of vaporization, etc. That is why they are considered “dia” (lit., opposed to being simple) “stereoisomers”.

  8. Test and Extend Your Understanding Q: Don’t make the mistake of thinking that left/right orientation of the critical –OH group changes a D into an L sugar. To show this, what sugar would you form if C-5 on D-glucose was oriented to the left instead of the right? A: L-idose Q: Are D-glucose and D-ribose isomers? If so, what term describes the relationship? A: D-glucose and D-ribose are not isomers of one another because they have different chemical formulas. To be considered a structural or stereoisomer, the two molecules must have the same empirical formula but differ only in the positioning of the atoms. Q: What is the relationship between D-glucose and D-fructose? A: This is a tough call. Both have the same formula and both have a carbonyl functional group. D-glucose has an aldehyde as its functional group and D-fructose has a ketone. The two must, therefore, be considered “structural isomers” and not stereoisomers. Q: How many epimers are there of D-glucose? Of -D-glucose? A: Two, D-mannose and D-galactose. -D-glucose has 3: -D-glucose, -D-mannose, -D-galactose. Q: How many stereoisomers of a heptulose are possible? How many are D and how many are L sugars? How many names will be needed for all the isomers? (hint: the name tells you the structure of this sugar). A: A heptulose is a 7 carbon keto sugar. Therefore, it has 4 chiral centers, which means the straight chain form has 16 isomers; 8 are D and 8 are L, just like glucose. There will be 8 names needed.

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