Simple sugars that are polyhydroxy aldehydes (aldoses) or ketones (ketoses)
What is the simplest aldose?
What is the simplest ketose?
What is the simplest form of sugar?
Trioses (either an aldose or ketose)
Name simple sugars containing 3,4,5, and 6 carbons.
What two sugars are the most abundant?
Pentoses and Hexoses
What is another name for the abundant sugar glucose?
Draw the structure of an aldose, ketose, glyceraldehyde, dihydroxyacetone.
Draw on separate paper.
Slide Lecture 23:7
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.
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)
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
Be able to identify D and L forms
Slide 23: 9
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.
What are diastereomers?
Stereoisomers that are not enantiomers (mirror-image isomers).
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.
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.
What are cyclic aldehydes and cyclic ketones called?
Cyclic aldehydes are hemiacetals.
Cyclic ketones are hemiketals.
Draw a Furan and Pyran.
How many atoms are in the most stable cyclic hemiacetals and hemiketals?
5 to 6 atoms
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.
What are the possible forms of the anomeric carbon?
“a” form or the “b” form
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'
What is the typical configuration of D-glucose and what is the technical name?
Glucopyranose is the most technical name used in naming glucose.
Also glucose occurs mostly in the D form.
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.
Reactions of monosaccharides (6):
The carbonyl and hydroxyl groups of sugars can undergo several chemical rxns:
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.
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
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
How is Vitamin C derived?
L-asorbic acid (vit C) is derived from a lactone of D-glucuronic acid.
An important feature of oxidation
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).
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.
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)
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.
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)
What is the intermediate of monosaccharide isomerization called and ?
Several enzyme- catalyzed rxns involving enediols occur in carbohydrate metabolism.
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
What are the most common esters found in nature?
Phosphate and sulfate esters are the most common
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.
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
What are aglycones?
Noncarbohydrate components of glycosides
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
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).
How are N-glycosidic bonds formed?
N-glycosidic bonds involve a bond between oligosaccharides and the amino nitrogen of asparagine residues
How are O-glycosidic bonds formed?
O-glycosidic bonds involve a bond between an oligosaccharide and the hydroxyl oxygens of serine or threonine
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)