Carbohydrates

Simple sugars that are polyhydroxy aldehydes (aldoses) or ketones (ketoses)

Glyceraldehyde

Dihydroxyacetone

Trioses (either an aldose or ketose)

• Trioses
• Tetroses
• Pentoses
• Hexoses

Pentoses and Hexoses

Aldohexose

• Draw on separate paper.
• Slide Lecture 23:7

• 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.

• 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)

• 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

Slide 23: 9

• 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.

Stereoisomers that are not enantiomers (mirror-image isomers).

• 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.

Sugars with 4 or more carbons primarily exist in cyclic forms by means of a reaction between a hydroxyl and the carbonyl.

• Cyclic aldehydes are hemiacetals.
• Cyclic ketones are hemiketals.

Slide 23:11

5 to 6 atoms

• 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.

“a” form or the “b” form

• 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'

• Chair configuration
• Glucopyranose is the most technical name used in naming glucose.
• Also glucose occurs mostly in the D form.

• 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.

• The carbonyl and hydroxyl groups of sugars can undergo several chemical rxns:
• Oxidation
• Reduction
• Isomerization
• Esterification
• Glycoside formation
• Glycosylation reactions

In the presence of oxidizing agents, metal ions such as Ca2+, and certain enzymes, monosaccharides readliy undergo several oxidation rnxs.

• 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

• 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

• L-asorbic acid (vit C) is derived from a lactone of D-glucuronic acid.
• An important feature of oxidation

• A type of weak oxidizing agent
• Can be used to detect a reducing sugar in urine (e.g., glucose, indicative of diabetes mellitus).

• 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.

• 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)

• 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.

• 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)

• Enediol
• Several enzyme- catalyzed rxns involving enediols occur in carbohydrate metabolism.

The free OH groups of carbohydrates can be converted to esters via reaction with acids

Phosphate and sulfate esters are the most common

• Phosphate esters are found in carbohydrate metabolism.
• Sulfate esters are found in proteoglycan components of connective tissues.

• 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

Noncarbohydrate components of glycosides

• 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

• 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).

N-glycosidic bonds involve a bond between oligosaccharides and the amino nitrogen of asparagine residues

O-glycosidic bonds involve a bond between an oligosaccharide and the hydroxyl oxygens of serine or threonine

• 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.

Dextrose- for D form

• 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

Levulose

High-fructose corn syrup is made from pure corn syrup (glucose) converted to fructose.

• Galactose is used in synthesizing numerous biomolecules used by cells
• A six atom ring

• 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.

From glucose-1-phosphate when the diet does not provide enough.

• Uronic acids
• Amino sugars
• Deoxy sugars

• 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

• A monosaccharide derivative
• Monosaccharides in which an –H has replaced an –OH are called “deoxy”
• ex. DNA uses deoxyribose

• L-Fucose is also called 6-deoxygalactose
• Such as the glycoproteins of the ABO blood group determinantes onthe surfaceof red blood cells.
• (sugar dericative)

Glucoamines

• Pyranose has one carborn outside of the ring
• Furanose has two carbons outside of the ring