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2010-10-17 18:48:45

Biochem block3 Lecture 23 Chapter 7
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  1. What are monosaccharides?
    Simple sugars that are polyhydroxy aldehydes (aldoses) or ketones (ketoses)
  2. What is the simplest aldose?
  3. What is the simplest ketose?
  4. What is the simplest form of sugar?
    Trioses (either an aldose or ketose)
  5. Name simple sugars containing 3,4,5, and 6 carbons.
    • Trioses
    • Tetroses
    • Pentoses
    • Hexoses
  6. What two sugars are the most abundant?
    Pentoses and Hexoses
  7. What is another name for the abundant sugar glucose?
  8. Draw the structure of an aldose, ketose, glyceraldehyde, dihydroxyacetone.
    • Draw on separate paper.
    • Slide Lecture 23:7
  9. What are sugar stereoisomers and how is the number of possible isomers determined?
    • Sugars that have one or more chiral centers.
    • van't Hoff's rule: A compound with n chiral carbon atoms has a maximum of 2^n possible stereoisomers.
  10. What is the confirmation of the most naturally occuring sugar and what are they considered to be derived from?
    • The most common confimation is 'D'
    • They are derived from either the triose D-glyceraldehyde (the aldoses) or the nonchiral (still parent) triose dihydroxyacetone (the ketose)
  11. How are the configurations of sugar stereoisomers named?
    • Sugars are called either “D” or “L” based on the chiral carbon furthest from the carbonyl carbon (carbon 1 on the glyceraldehyde structure).
    • Named 'D' if the OH is on the right
    • Named 'L' if the OH is on the left
  12. Be able to identify D and L forms
    Slide 23: 9
  13. What are enantiomers?
    • In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are "non-superposable" (not identical), much as one's left and right hands are "the same" but opposite.
    • Enantiomers (D vs. L) – mirror images.
  14. What are diastereomers?
    Stereoisomers that are not enantiomers (mirror-image isomers).
  15. What are epimers?
    • Diastereomers that differ in the configuration at a single asymmetric carbon atom.
    • Ex. D-glucose and D-galactose are epimers because their structures differ only in the configuration at a single asymmetric carbon atom.
  16. What is the primary form of sugars with 4 or more carbons and how is it formed?
    Sugars with 4 or more carbons primarily exist in cyclic forms by means of a reaction between a hydroxyl and the carbonyl.
  17. What are cyclic aldehydes and cyclic ketones called?
    • Cyclic aldehydes are hemiacetals.
    • Cyclic ketones are hemiketals.
  18. Draw a Furan and Pyran.
    Slide 23:11
  19. How many atoms are in the most stable cyclic hemiacetals and hemiketals?
    5 to 6 atoms
  20. What happens to the carbonyl carbon when a sugar cyclizes?
    • When sugars cyclize, the carbonyl carbon becomes a chiral center called the anomeric carbon
    • There are two possible disastereomers that form from cyclization, these are called anomers.
  21. What are the possible forms of the anomeric carbon?
    “a” form or the “b” form
  22. How are anomers classified?
    • The beta boat sails atop 'D' sea.
    • The alpha fish swims beneath 'D' sea.
    • *L-sugars are opposite: alpha is 'up' and beta is 'down'
  23. What is the typical configuration of D-glucose and what is the technical name?
    • Chair configuration
    • Glucopyranose is the most technical name used in naming glucose.
    • Also glucose occurs mostly in the D form.
  24. What is mutarotation?
    • The spontaneous introconversion of alpha and beta monosaccharides when dissolved in water.
    • Produces an equilibrium mixture of alpha and beta forms in both furanose and pyranose ring structures.
  25. Reactions of monosaccharides (6):
    • The carbonyl and hydroxyl groups of sugars can undergo several chemical rxns:
    • Oxidation
    • Reduction
    • Isomerization
    • Esterification
    • Glycoside formation
    • Glycosylation reactions
  26. How does the oxidation of monosaccharides occur?
    In the presence of oxidizing agents, metal ions such as Ca2+, and certain enzymes, monosaccharides readliy undergo several oxidation rnxs.
  27. Describe the 3 oxidation reactions of monosaccharides and their products (in glucose form too).
    • An aldehyde to a carboxylic acid (called an aldonic acid): D-glucose to D-gluconic acid
    • Terminal CH2OH to a carboxylic acid (called a uronic acid): D-glucose to D-glucuronic acid
    • Both of the above to a carboxylic acid (called an aldaric acid): D-glucose to D-glucaric acid
  28. What are lactones and how are they formed?
    • Cyclic esters that are formed from the carbonyl group in either an aldonic or uronic acid with an OH group in the same molecule.
    • An important feature of oxidation
  29. How is Vitamin C derived?
    • L-asorbic acid (vit C) is derived from a lactone of D-glucuronic acid.
    • An important feature of oxidation
  30. What is a Benedict reagent and what is its significance?
    • A type of weak oxidizing agent
    • Can be used to detect a reducing sugar in urine (e.g., glucose, indicative of diabetes mellitus).
  31. What is a reducing sugar?
    • Sugars that can be oxidized by weak oxidizing agents.
    • Because the reaction can only occur with sugars that can revert to the
    • open chain form, all monosaccharides are reducing sugars.
  32. Explain the reduction of a monosaccharide and what is formed.
    • Reduction of aldehyde and ketone groups of sugars results in “sugar alcohols” also called alditols
    • The C-O double bond becomes a single bond and 2 H's are added when Hydrogen gas and catalyst are added.
    • These have commercial application (processing of foods and pharmaceuticals)
  33. What is soritol, how is it formed, and what is its significance?
    • Sorbitol is an alditol (sugar alcohol) that is formed from the reduction of D-glucose.
    • D-sorbitol (D-glucitol) is commonly found in candy.
    • Sorbitol increases the shelf life of candy by helping to reduce moisture loss.
    • Sorbitol is converted to fructose in the liver.
  34. Explain the isomerization of a monosaccharide and what is formed.
    • Isomerization involves an intramolecular shift of a hydrogen atom and a relocation of a double bond (during the intermediate step).
    • Configuration at the first single chiral carbon (epimerization) occurs.
    • D-glucose (alpha-hydroxyaldehyde) can convert to D-fructose (alpha-hydroxyketone) or to D-mannose (alpha-hydroxyaldehyde)
  35. What is the intermediate of monosaccharide isomerization called and ?
    • Enediol
    • Several enzyme- catalyzed rxns involving enediols occur in carbohydrate metabolism.
  36. Exaplin the esterification of a monosaccharide and what is formed.
    The free OH groups of carbohydrates can be converted to esters via reaction with acids
  37. What are the most common esters found in nature?
    Phosphate and sulfate esters are the most common
  38. Where are phosphate esters and sulfate esters commonly found?
    • Phosphate esters are found in carbohydrate metabolism.
    • Sulfate esters are found in proteoglycan components of connective tissues.
  39. How are glycosides formed?
    • Reaction of an alcohol with a hemiacetal or hemiketal produces an acetal or ketal
    • *Carbohydrate molecules that contain only acetal groups don't test positive with Benedict's reagent (ring doesn't open).
    • The new bond is called a “glycosidic linkage”
    • The molecule is called a glycoside
  40. What are aglycones?
    Noncarbohydrate components of glycosides
  41. How is a disaccharide formed?
    • When two monosaccharides combined via a glycosidic bond, the new molecule is called a disaccharide
    • Polysaccharides are long chains of monosaccharides combined by a series of glycosidic linkages
  42. What are glycosylation reactons and what are two examples?
    • Reactions that attach sugars (monomers and polymers) to proteins or lipids.
    • N- glycosidic and O-glycosidic
    • A spontaneous (non-enzymatic) form of the reaction is even involved in some disease processes (like atherosclerosis).
  43. How are N-glycosidic bonds formed?
    N-glycosidic bonds involve a bond between oligosaccharides and the amino nitrogen of asparagine residues
  44. How are O-glycosidic bonds formed?
    O-glycosidic bonds involve a bond between an oligosaccharide and the hydroxyl oxygens of serine or threonine
  45. Glucose (important monosaccharide)
    • Primary fuel for living cells and is the preferred energy source for brain cells and also for cells that have few mitochondria (e.g., RBCs)
    • A six atom ring
    • Dietary sources: plant starch, disaccharides (lactose, maltose, sucrose)
    • Liver cells can synthesize glucose in certain circumstances (gluconeogenesis)- can be produced from different materials and cells in the body.
  46. What was the original name of glucose?
    Dextrose- for D form
  47. Fructose (Important Monosaccharide)
    • D-fructose is referred to as fruit sugar
    • A five atom ring
    • This ketose is found in fruits, some vegetables, and honey
    • Twice as sweet as sucrose (per gram) and used in food production
    • Also used in the male reproductive tract as an energy source for sperm
  48. What was fructose originally named
  49. How is high-fructose corn syrup made?
    High-fructose corn syrup is made from pure corn syrup (glucose) converted to fructose.
  50. Galactose (Important Monosaccharide)
    • Galactose is used in synthesizing numerous biomolecules used by cells
    • A six atom ring
  51. What is galactosemia?
    • A genetic disorder that occurs when an enzyme needed to metabolize galactose is missing
    • Galactose, galactose-1-phosphate, and galacitiol accumulate and cause liver damage, cataracts, and severe mental retardation.
  52. How can cells systhesize galactose?
    From glucose-1-phosphate when the diet does not provide enough.
  53. What are three monosaccharide derivatives?
    • Uronic acids
    • Amino sugars
    • Deoxy sugars
  54. What are amino sugars, how are they formed, and where are they often found?
    • A monosaccharide derivative that is formed when hydroxyl group is replaced by an amino group
    • Often found in thecomplex carbohydrates attached to proteins and lipids
    • ex. D-glucosamine and D-acetylglucosamine
  55. What are deoxy sugars, how are they formed, and where are they often found?
    • A monosaccharide derivative
    • Monosaccharides in which an –H has replaced an –OH are called “deoxy”
    • ex. DNA uses deoxyribose
  56. What is the deoxy sugar often found among the carbohydrate components of glycoproteins?
    • L-Fucose is also called 6-deoxygalactose
    • Such as the glycoproteins of the ABO blood group determinantes onthe surfaceof red blood cells.
    • (sugar dericative)
  57. What are important for joints?
  58. Be able to identify pyranose and furanose...
    • Pyranose has one carborn outside of the ring
    • Furanose has two carbons outside of the ring