Carbohydrate Structure And Function

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Monosaccharide

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The simplest form of carbohydrates. They consist of one sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose, galactose, and ribose.

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Trioses

Back 2

Contain 3 Carbons

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Tetroses

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Contain 4 Carbons

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Pentoses

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Contain 5 Carbons

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Hexoses

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Contain 6 Carbons

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Aldoses

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Contain an Aldehyde functional group as the most oxidized functional group H-C=O

Ex. Glucose

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Ketoses

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Contain a Ketone functional group as the most oxidized functional group R-C(=O)-R

Ex. Fructose

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Glyceraldehyde

(3 carbon monosaccharide with an aldehyde)

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Aldehyde = Aldo

3 Carbon = Triose

Aldo+Triose = Aldotriose

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Dihydroxyacetone

(3 carbon monosaccharide with a ketone)

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Ketone = Keto

3 Carbon = Triose

Keto+Hexose = Ketotriose

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D vs L Orientation

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For sugars with more than one chiral center, D or L designates the asymmetric carbon farthest from the aldehyde or keto group.

Hydroxyl (OH) group on the last chiral carbon = right side, D enantiomer. Hydroxyl on the last chiral carbon = left side, L enantiomer.

Most naturally occurring sugars are D isomers.

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D-Fructose

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D-Glucose

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D-Galactose

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D-Mannose

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Enantiomers

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Same sugar (molecule), different optical family

Ex. D-Glucose vs. L-Glucose

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Diastereomers

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Two sugars in the same family (both are either Ketoses or Aldoses and have the same number of carbons), but are not identical and are not mirror images.

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Epimers

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A special subtype of diastereomers. Molecules that differ in configuration at exactly one chiral center.

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Hemiacetal

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The product of adding an Alcohol (R-O-H) to a Carbonyl (Aldehyde, R-C(=O)-H) is known as a __________.

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Hemiketal

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The product of adding an Alcohol (R-O-H) to a Carbonyl (Ketone, R-C(=O)-R) is known as a __________.

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Hemiacetals

(Cyclic)

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Monosaccharides contain both a hydroxyl (electrophile) and a carbonyl (nucleophile). Aldoses can undergo intermolecular reactions to form Cyclic Hemiacetals.

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Hemiketals

(Cyclic)

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Monosaccharides contain both a hydroxyl (electrophile) and a carbonyl (nucleophile). Ketoses can undergo intermolecular reactions to form Cyclic Hemiketals.

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Pyranose Rings

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6-membered monosaccharide rings.

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Furanose Rings

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5-membered monosaccharide rings.

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Anomeric Carbon

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The Hydroxyl group acts as a nucleophile during ring formation, causing the Oxygen to become a member of the ring structure.

During this process, the Carbonyl becomes chiral, and is known as the Anomeric Carbon (C-1).

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α-Anomer

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The Hydroxyl (-OH) group on C-1 is Trans to the -CH2OH substituent.

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β-Anomer

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The Hydroxyl (-OH) group on C-1 is Cis to the -CH2OH substituent.

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Mutarotation

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Exposing Hemiacetal rings to water causes them to spontaneously cycle between the open/closed forms. This in turn causes the ability for both α & β anomers to form due to rotation on the single bond between C-1 and C-2.

-The spontaneous change in configuration is known as Mutarotation.

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Aldonic Acids

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Monosaccharide (aldehyde) in open chain form becomes oxidized, creating __________ _________.

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Reducing Sugar

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Any monosaccharide with a hemiacetal ring.

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Lactone

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An Aldose in ring form (Hemiacetal) undergoes oxidation to yield a ________.

This is a cyclic ester with a carbonyl group on the anomeric carbon.

Carbonyl group: R-C(=O)-R

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Tollen's Reagent

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Utilizes Ag(NH3)2, Silver Nitrate, as an oxidizing agent.

-Positive: Aldehydes reduce Ag+ to metallic silver.

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Benedict's Reagent

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Oxidizes the Aldose group of an Aldehyde, and is indicated by the precipitate of Cu2O, which is Red.

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Keto-Enol Shift

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The rearrangement of a compound, usually by moving a hydrogen ion and forming a double bond (Tautomerization).

In this case, the ketone group picks up a hydrogen and the double bond is moved between two adjacent carbons, resulting in an enol (compound with a double bond and an alcohol group).

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Alditol

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When the Aldehyde group of a sugar is reduced to an alcohol, the compound is considered an __________.

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Deoxy Sugar

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Contains a Hydrogen that replaces a Hydroxyl group (H insteand of OH) on the sugar.

The most well known of these sugars is D-2-Deoxyribose, the carbohydrate found in DNA.

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Phosphorylation

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A type of esterification reaction that includes a Nucleophillic attack by a Hydroxyl (OH) group on a Phosphate (PO4) group from ATP, reducing it to ADP and transferring the phosphate group.

-Nucleophile: OH

-Electrophile: PO4

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Acetals

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Hemiacetals react with alcohols to form _______. The anomeric hydroxyl group is transformed into an alkoxy group (RO), yeilding a mixture of α & β acetals, with water as a leaving group.

The resulting C-O bonds are Glycosidic bonds, and the acetals formed are Glycosides.

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Ketals

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Hemiketals react with alcohols to form _______. The anomeric hydroxyl group is transformed into an alkoxy group (RO), yeilding a mixture of α & β ketals, with water as a leaving group.

The resulting C-O bonds are Glycosidic bonds, and the ketals formed are Glycosides.

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Glycoside

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A molecule that contains a bond between a functional group and a carbon of the sugar.

Glycoside formation is a dehydration reaction, so breaking a Glycosidic bond requires hydrolysis.

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Glycosidic Bond

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The bond that forms between a functional group and a carbon of the sugar.

This is the same type of bond that attaches monosaccharides together to create disaccharides.

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Furanosides

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Glycosides derived from Furanose Rings are referred to as ___________.

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Pyranosides

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Glycosides derived from Pyranose Rings are referred to as ____________.

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Disaccharides

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A Glycosidic bond forms between hydroxyl groups of two monosaccharides, resulting the the expulsion of H2O and the creation of a Disaccharide.

The Hydroxyl on the anomeric carbon reacts with the hydroxyl on another sugar to form an acetal/ketal with a 1,2; 1,4; or 1,6 glycosidic link.

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α & β Glycosidic Linkage

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α or β Anomeric carbon forms a bond with a hydroxyl group on another sugar (of either α or β).

-Ex. α-1,6 between two D-Glucose molecules. The bond is between the OH on the Anomeric carbon on G1, and the 6th carbon on G6.

-Ex. β,β-1,1 bond is between the OH on both Anomeric carbons.

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Sucrose

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glucose-α-1,2-fructose

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Lactose

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galactose-β-1,4-glucose

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Maltose

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glucose-α-1,4-glucose

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Polysaccharides

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Long chains of monosaccharides linked together by glycosidic bonds.

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Homopolysaccharides

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A Polysaccharide comprised of one type of monosaccharide (ex. all glucose).

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Heteropolysaccharides

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A Polysaccharide comprised of more than one type of monosaccharide (ex. glucose and fructose).

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Cellulose

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Main structural component of plants. Comprised of β-D-glucose molecules linked byβ-1,4 glycosidic bonds.

-Homopolysaccharide.

-Requires cellulase to digest.

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Starches

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Polysaccharides that are more digestible by humans due to linked α-D-glucose monomers.

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Amylopectin

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Starch comprised of α-D-glucose molecules linked by α-1,4 glycosidic bonds, with branches via α-1,6 glycosidic bonds (approx. 1/25 glucose molecules)

Debranching enzymes help to degrade the polysaccharide.

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Amylose

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Plants mainly store starch as Amylose, comprised of α-D-glucose molecules linked by α-1,4 glycosidic bonds.

-Broken down by α or β amylase.

Iodine tests for the presence of starch by fitting inside the helix conformation made by amylose, creating a starch-iodine complex.

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β Amylase

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Cleaves Amylose at non-reducing end (the end with the acetal), yielding Maltose.

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α Amylase

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Cleaves Amylose randomly along the chain to create shorter polysaccharide chains, maltose, and glucose.

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Glycogen

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Main carbohydrate storage in animals. Similar structure to starch, but includes α-1,6 glycosidic bonds (approx. 1/10 glucose molecules) which yields a highly branched product.

Branching optimizes energy efficiency, increases solubility in solution, and allows for enzymes to work on multiple sites simultaneously.

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Glycogen Phosphorylase

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Functions by cleaving glucose from the nonreducing end of a glycogen branch and phosphorylating it, producing glucose 1-phosphate.

Glucose 1-phosphate plays an important role in metabolism.