Carbohydrates Monosaccharides and Disaccharides

Question 1

Define the term carbohydrate and list at least four different biological roles of carbohydrates.

Answer 1

Carbohydrates or saccharides
* most abundant biological molecules
* chemically simpler than nucleotides or amino acids,
* containing only carbon, hydrogen, and oxygen
* formula (C ∙ H2O)n, where n ≥ 3.

Functions:
1. Structural compounds (Cellulose/Chitin/ECM)
1. Fuel source/Storage (Starch/Glycogen)
1. Structural component in nucleic acids (Ribose/Deoxyribose)
1. Recognition/Regulation of extracellular proteins

Question 2

Define monosaccharide

Answer 2

A carbohydrate consisting of a single saccharide (sugar)
Monosaccharides are aldehyde (Aldose) or ketone (Ketose) derivatives of straight-chain polyhydroxy alcohols containing at least three carbon atoms
- Generally Soluble in Water

Classified according to:
- Chemical Nature of their carbonyl group
- Number of C atoms

Triose, Tetrose, Pentose, Hexose etc

D-sugars: have the same absolute configuration at the asymmetric center farthest from their carbonyl group as does D-Glyceraldehyde (-OH at C5 of D-glucose is on the Rt side of Fischer projection)

Question 3

Define disaccharide

Answer 3

A carbohydrate consisting of two monosaccharides linked
by a glycosidic bond.
- Simplest polysaccharides (more common)
- Generally soluble in water
- Many occur as hydrolysis products of larger molucules
- Occur through glycosidic linkages between monosaccharides (ie Galactose + Glucose -> Lactose)
- Sucrose most common (Glucose + Gructose -> Sucrose)

Systemic Names:
Lactose: O-β-D-galactopyranosyl-(1→4)-D-Glucopyranose
- C1 of the β anomer of galactose to O4 of glucose

Sucrose: O-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside
- Anomeric carbon on each sugar (C1 in glucose and C2 in fructose) participates in the glycosidic bond
- Not a Reducing Sugar

β Anomer: A carbohydrate in which the group bonded to the anomeric carbon is cis (both up) to the CH2O group on the other side of the pyranose or furanose ring ether oxygen atom

Question 4

Define oligosaccharide

Answer 4

A polymeric carbohydrate containing a few
monosaccharide residues
- Generally Soluble in water

Question 5

Define polysaccharide (homo vs hetero?)

Answer 5

Polysaccharide: A polymer of one or more carbohydrate unit (consisting of multiple monosaccharide residues). AKA glycan

Homopolysaccharide: A polysaccharide consisting of one type of monosaccharide unit.

Heteropolysaccharide: A polysaccharide consisting of more than one type
of monosaccharide

Generally Insoluble in Water

eg Cellulose and Chitin (Structural Polysaccharides)
- Starch & Glycogen (Storage)

Cellulose: Linear (Homo) polymer of up to 15,000 D-Glucose residues linked by β(1→4) glycoside bonds

Chitin: homopolymer of β(1→4) linked N-acetyl-D-glucosamine Residues

Starch: Composed of α-Amylose and amylopectin
- α-Amylose: linear polymer of several thousand glucose residues linked by α(1→4) bonds

Question 6

Define Glycoconjugate

Answer 6

Glycoconjugate: Carbohydrate covalently linked to other molecular classes
- Proteins/peptides: Proteoglycans, glycoproteins, peptidoglycans
- Lipid: Glycolipids

Lectins: Proteins that bind carbohydrations
Selectins: Mediate leukocyte attachment to endothelial cells

Question 7

Define the terms aldose and ketose

Answer 7

Aldose: A sugar whose carbonyl group is an aldehyde.
Ketose: A sugar whose carbonyl group is a ketone.

Question 8

Describe the structural features that are common to all monosaccharides in the open chain form.

Answer 8

Monosaccharides: Aldehydes (carbonyl @ C1) or ketones (Carbonyl at C2) with 2 or more hydroxyl (OH) groups.
- Unbranched carbon chain connected by single bonds

Multiple chiral centres (Except for dihydroxyacetone)
- For N chiral centers, there are 2^N possible Stereoisomers

One C attached to O via Carbonyl:
- Aldehyde (C1): Aldose monosaccharide
- Ketone (C2): Ketose monosaccharide

D or L
- based on configuration of the chiral carbon most distant from the Carbonyl group (highest number chiral centre)
- D: Hydroxyl on Right of fischer projection
- L: Hydroxyl on Left of Fischer Projection

Named by number of Carbon Atoms
- 3C = triose // 6C = hexose
- Hexoses most common
- Numbering begins at end closest to carbonyl
- Type + Length = Sugar type (Glucose = Aldohexose)

Epimers: differ in configuartion at only ONE chiral centre (invert a chiral centre)

Enantiomers: Mirror reflection stereoisomers (Invert every chiral center)

Question 9

SLIDE 3 Name any monosaccharide as a D or L isomer from inspection of its Fischer projection

Answer 9

D or L
- based on configuration of the chiral carbon most distant from the Carbonyl group (highest number chiral centre)
- D: Hydroxyl on Right of fischer projection
- L: Hydroxyl on Left of Fischer Projection

Question 10

Describe the numbering system for the carbons in monosaccharides

Answer 10

numbered from 1 to n, starting from the end closest to the carbonyl.
- If the carbonyl is at the very beginning of the chain (c1), the monosaccharide is said to be an aldose
- Carbonyl at C2: ketose.
These names can be combined with the chain length prefix, as in aldohexose or ketopentose

Question 11

Define epimer and enantiomer

Answer 11

Epimers: differ in configuration at only ONE chiral centre (invert a chiral centre)
- D-mannose is the C-2 epimer of D-glucose
- D-galactose is the C-4 epimer of D-glucose

Enantiomers: Mirror reflection stereoisomers (Invert every chiral center)
- L-glucose is enantiomer of D-glucose
- D-Galactose and D-Mannose are enantiomers (NOT epimers)

Question 12

SLIDE 5!!! Draw the reaction that converts a straight-chain monosaccharide to a hemiacetal or hemiketal

Answer 12

Alcohol + Carbonyl -> hemiacetal/hemiketal
Further addition of another alcohol can yield a full acetal or ketal
Yields a new chiral center (previous Carbonyl Carbon)

• (R-OH (alcohol) - nucleophile) H from incoming alcohol joins to the O on the carbonyl to produce C-OH
• Carbonyl Carbon becomes new chiral centre called Anomeric Carbon

Question 13

Formation of the hemiacetal in glucose

Answer 13

C5 -OH reacts with C1 aldehyde (HC=O)
6-membered ring forms pyranose
C1 becomes new chiral centre after reaction
- C1 = Anomeric Carbon
- May be alpha/beta configuration

Alpha = OH on opposite side of ring relative to C-6 (Anomeric C -OH pointing down for D-Sugars)
Beta = OH on same side of ring as C-6 (Anomeric C -OH pointing up for D-sugars)

alpha and beta forms may be interconverted by breaking and reforming the hemiacetal (Mutarotation)

Hawthorne Projection:
- Hydroxyls on Right of Fischer projection will be pointing down on Haworth
- Hydroxyls on Left side of Fischer projection will be pointing up on Haworth

Question 14

SLIDE 6 Draw the structure of glucose in linear and pyranose forms.

Answer 14

In sol’n monosaccharides will adopt a cyclic conformation through an intramolecular reaction
- Hydroxyl group acts as nucleophile
- Pyranose (6 Member ring) and Furanose (5 Member Ring)
- Carbonyl is converted to either a hemiacetal (Aldoses) or Hemiketal (ketoses)
- Anomeric Carbon is chiral in cyclic form (can be alpha or beta)

Question 15

PENDING Given either the linear, pyranose or furanose structure of a monosaccharide, redraw in one of the other forms.

Answer 15

pending

Question 16

Define the terms anomer, anomeric carbon, mutarotation, pyranose, furanose.

Answer 16

• Anomer: Sugars that differ only in the cofiguration around the anomeric carbon
• Alpha anomer: OH on anomeric carbon is on the opposite side of the ring from the CH2OH group at the chiral centre that designates the D or L configuration (Beta anomer = same side)
• Anomeric Carbon: Carbonyl Carbon of a monosaccharide unit which becomes a chiral center when the sugar cyclizes to a hemiacetal or hemiketal
• Mutarotation: Ability to convert back and forth between alpha and beta anomer
• Pyranose: monosaccharide with 6 Membered Ring
• Furanose: Sugar with 5 Membered Ring

Aldoses with 5 or more carbons may form either pyranoses or furanoses, although the larger pyranose ring is generally more stable

Aldose or ketose with 6C can form either Pyranose or furanose

Question 17

SLIDE 7/8: Describe the advantages of Haworth projections and conformational formulas in representing cyclic saccharide structures.

Answer 17

Haworth Projections:
- show stererochemistry more explicitly
- (-OH) below the ring = Right side on Fischer Proj’n
- (-OH)s on Left side of Fischer projection will be pointing up on Haworth

Chair Conformation:
- Shows both conformation and stereochemistry
- Substituents either axial (str8 up/down) or equatorial
- Favours “bulkier” substituents in equatorial positions

Fischer projection drawing
- horizontal bonds are directed toward the viewer (Wedge - coming out) (forward of the stereogenic carbon).
- vertical bonds are directed behind the central carbon DASH (away from the viewer).

Question 18

PENDING Describe and identify common substitutions and modifications to monosaccharides.

Answer 18

(1) Oxidized Sugars (Aldehydes)
- 1st and last carbons can be oxidized
- Aldonic Acids: Oxidation at C1 // “-onic acid”
- Uronic Acids: Oxidation of primary alcohol // “-uronic acid”

(2) Reduced Sugars (Aldehydes & Ketones)
- Alditols: Reduction of carbonyl to alcohol
- “-itol”
- Ribitol, Xylitol, Glycerol

(3) Deoxy Sugars
- Removal of -OH from monosaccharides yield deoxy-sugars

(4) Amino Sugars
- Replacement of OH (usually at C2) in monosaccharides with NH2/NH3+
- “-amine” (glucosamine)
- Likely to carry POS charge unless acetyl group added
- Often acetylated on the amino group (eg N-acetylglucosamine)

(5) Sialic Acid
- The sialic acid family includes many derivatives of the nine-carbon sugar N-Acetylneuraminic acid (derived from N-acetylmannosamine and pyruvate)
- Found as part of numerous glycoproteins and glycolipids (eg gangliosides)
- Neg charge due to carboxylic acid and Uronic
- Oxidized structures

Question 19

PENDING Recognize and name substituted forms of glucose, including D-glucosamine and N-acetyl-D-glucosamine.

Answer 19

Pending

Question 20

Define the terms

Glycosidic bond:
N-Glycosidic Bonds:
O-Glycosidic Bonds:

Answer 20

Glycosidic Bond: the covalent linkage (acetal or ketal) between the anomeric carbon of a saccharide and an alcohol (O-glycosidic bond) or an amine (N-glycosidic Bond)
- Link the monosaccharide residues of a polysaccharide

N-Glycosidic Bonds: connect the bases in nucleic acids to the associated sugars
O-glycosidic Bonds: connect anomeric carbons to alcohols

Once formed, anomeric carbons in glycosidic bonds will not MUTAROTATE (fixed in cyclic form)

Reducing Sugar: A saccharide bearing an anomeric carbon that has NOT formed a glycosidic bond and can therefore reduce mild oxidizing agents
- Sugars with a Hemiacetal/hemiketal anomeric carbon

Non-Reducing Sugar: Has all Anomeric Carbons in Glycosidic Bonds

Question 21

Define

Non-reducing and Reducing sugar.

Answer 21

Reducing Sugar: A saccharide bearing an anomeric carbon that has NOT formed a glycosidic bond and can therefore reduce mild oxidizing agents
- Sugars with a Hemiacetal/hemiketal anomeric carbon

Non-Reducing Sugar: Has all Anomeric Carbons in Glycosidic Bonds

Glycosidic Bond: the covalent linkage (acetal or ketal) between the anomeric carbon of a saccharide and an alcohol (O-glycosidic bond) or an amine (N-glycosidic Bond)
- Link the monosaccharide residues of a polysaccharide

Once formed, anomeric carbons in glycosidic bonds will not MUTAROTATE (fixed in cyclic form)

Question 22

Guide to Naming Disaccharides

Identify and explain the meaning of the different parts in the formal name of a disaccharide or polysaccharide, and in the abbreviated name.

(1) Lactose: β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose // Gal(β1→4)Glc // Lactose (β form)

(2) Sucrose: α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru

(3) Maltose: α-D-glucopyranosyl-(1→4)-D-Glucopyranose

Answer 22

Naming:
- Specifies monosaccharide Order // Configuration of anomeric carbon, and which Carbons are involved in the glycosidic bond
- Begins at non-reducing end (left by convention)

1. Name the configuration (α or β) at the anomeric carbon of the linkage from monosacc 1 to monosacc 2
2. Name the monosaccharide 1, including the pyranose or furanose designation; end with “-osyl” suffix
3. Name the two C’s joined, with an arrow showing the direction of linkage (anomeric → hydroxyl) // If both anomeric C are involved use (↔)
4. Name monosaccharide 2, including the pyranose or furanose designation (and α or β unless hemiketal/hemiacetal)
5. Suffix “-ose” for Reducing Sugars // “-oside” for non-reducing sugars

(1) β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose /? Gal(β1→4)Glc // Lactose (β form)
Interpretation:
- β : -OH on Anomeric Carbon is on same side as C6
- (1→4): C1 of galactose is joined to C4 of Glucose
- Reducing sugar (glucose end)
- Both sugars are pyranoses

(2) Sucrose: α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru
- Disaccharide made from glucose and fructose)
- Glucose = pyranose
- Fructose = Furanose
- Glycosidic linkage is between C1 of Glucose and C2 of Fructose (non-reducing) = Connects TWO ANOMERIC C’s
- 2nd sugar indicated as -oside (not -ose) indicating the glycoside joining anomeric carbons
- Non-Reducing Sugars are indicated with a double-headed arrow in the abbreviated form

Question 23

Guide to Naming Disaccharides

Interpret the formal Name for lactose
Lactose: β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose // Gal(β1→4)Glc // Lactose (β form)

(2) Sucrose: α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru

(3) Maltose: α-D-glucopyranosyl-(1→4)-D-Glucopyranose

Answer 23

(1) β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose /? Gal(β1→4)Glc // Lactose (β form)
Interpretation:
- β : -OH on Anomeric Carbon is on same side as C6
- (1→4): C1 of galactose is joined to C4 of Glucose
- Reducing sugar (C1 of glucose unit) - hemiacetal = mutarotation possible
- Both sugars are pyranoses

Galactose is C4 epimer of glucose (OH @ C4 pts up in Haworth)

(2) Sucrose: α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru
- Disaccharide made from glucose and fructose)
- Glucose = pyranose
- Fructose = Furanose
- Glycosidic linkage is between C1 of Glucose and C2 of Fructose (non-reducing) = Connects TWO ANOMERIC C’s
- 2nd sugar indicated as -oside (not -ose) indicating the glycoside joining anomeric carbons
- Non-Reducing Sugars are indicated with a double-headed arrow in the abbreviated form

Question 24

Guide to Naming Disaccharides

Interpret the Formal Name for Sucrose

Sucrose: α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru

(1) Lactose: β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose // Gal(β1→4)Glc // Lactose (β form)

(3) Maltose: α-D-glucopyranosyl-(1→4)-D-Glucopyranose

Answer 24

α-D-glucopyranosyl β-D-fructofuranoside // Glc(α1 ↔2β)Fru
- Disaccharide made from glucose and fructose
- Glucose = pyranose
- Fructose = Furanose
- Glycosidic linkage is between C1 of Glucose and C2 of Fructose (non-reducing) = Connects TWO ANOMERIC C’s
- 2nd sugar indicated as -oside (not -ose) indicating the glycoside joining anomeric carbons
- Non-Reducing Sugars are indicated with a double-headed arrow in the abbreviated form

(1) β-D-galactopyranosyl-(1→4)-β-D-Glucopyranose /? Gal(β1→4)Glc // Lactose (β form)
Interpretation:
- β : -OH on Anomeric Carbon is on same side as C6
- (1→4): C1 of galactose is joined to C4 of Glucose
- Reducing sugar (glucose end)
- Both sugars are pyranoses

Maltose: α-D-glucopyranosyl-(1→4)-D-Glucopyranose
- α (1→4) Linkage between two D-glucose units
- C1 of rt side glucose is hemiacetal = reducing end

Question 25

Pending Identify reducing and non-reducing sugars in structural diagrams and in names of disaccharides or polysaccharides.

Answer 25

Pending