Chapter 9: carbohydrates

Functions of carbohydrates

metabolism, storage and generation of energy, molecule recognition, cellular protection, cell adhesion, biological lubrication, and maintenance of biological structure

Carbohydrate formula

(CH2O)n

When is a carbohydrate not a saccharide?

when n= 2 or 1

Saccharides

carbohydrates of n>3 and their derivatives (contain amino, sulfate or phosphate groups)

Sugar

saccharides that do not have modifications

Monosaccharide

simple sugars and derivatives with 3 to 9 C atoms

Oligosaccharide

compound from by linking several monosaccharides together (eg disaccharides that are two monosaccharides linked together)

Polysaccharide

polymer from multiple saccharide units. May be homopolysaccharide or heteropolysaccharide.

Glycan

generic term for oligosaccharides and polysaccharides

Formaldehyde

carbohydrate of n=1. Not a sugar, poisonous

Acetaldehyde

carbohydrate of n=2. Not a sugar, toxic

Carbohydrates of n= ? gives compounds with properties of sugars

3-9

Aldose vs ketose

aldoses contain aldehyde functional groups while ketose contain ketones

Enantiomers

optical isomers that are are mirror images

How to identify L vs D isoform

Use the direction of the −OH group on the chiral carbon furthest from the carbonyl group to determine which isomer it is. On a Fischer projection, it is the L isomer if the −OH group points left and the D isomer if it points right.

What is the 1 carbon of an aldose?

the C in the CHO group (aka where the aldehyde group is attached)

How does a fischer projection show stereochemistry

horizontal bonds point towards viewer and vertical bonds point away

How to identify a chiral carbon?

carbon must have 4 unique functional groups attached to it

Diastereomers

optical isomers that are NOT mirror images of one another

All life forms only use —- - amino acids and — - sugars

L -amino acids and D-sugars

What may be an explanation for D-sugars being more common in nature than L-sugars?

higher ring stability

What may be an explanation for L-amino acids being more common in nature than D-amino acids?

preference in terms of enzyme evolution

Ketoses always have —- (more or less) chiral carbons than aldoses

1 less chiral carbon

what is the stereochemical relationship between D-threose and D-Erythorse? What is the stereochemical relationship between D-threose and L-Threose? Between L-Thresose and D-erythrose?

diastereomers; enantiomers; diastereomers

What is the 1 carbon of ketoses

the CH2OH group on the end of the sugar closest to the ketone group

The more chiral carbons, the more —- a sugar has

the more isomers

Which one has more isomers, a five carbon aldose or a five carbon ketose and why?

the aldose because aldoses usually have more chiral centers than ketoses of the same carbon number

Show the reaction for hemiacetal formation. What is notable about the product of this reaction?

The carbon now has four different substituents so it becomes a chiral center

5 or 6 carbon sugars may undergo —-- in order to form a ring

hemiacetal formation

Furanose

5 membered ring formation of a sugar

Pyranose

6 membered ring formation of a sugar

Show how this sugar may cyclize into rings

Alpha vs beta form of sugar rings

cyclization creates a new asymmetric center called the anomeric center. Beta form has an anomeric center with the -OH group pointing up while alpha has the -OH group pointing down

Anomeric center

new asymmetric center created by ring formation of sugar. May be designated as alpha or beta

Mutarotation

the process where cyclic sugars, like glucose, change their optical rotation as their anomeric forms alpha and beta interconvert in solution until they reach an equilibrium mixture. This occurs because the cyclic form can open into a straight-chain form and then re-close into either the alpha or beta cyclic structure

Haworth projection

a way to draw cyclic sugar molecules (carbohydrates) as flat, ring-like structures, showing their 3D stereochemistry with groups pointing up or down relative to the ring

Epimer

stereoisomers differing in configuration about one carbon (not the anomeric carbon)

Anomer

stereoisomer that differs in configuration at at the anomeric carbon

Alpha vs beta cyclic structure of sugars

the alpha structure occurs when the -OH attached to the anomeric carbon is pointing down while the beta conformation has it pointing up

For pyranoses, when O is away from the viewer and the hemiacetal group is at the right side, how can you identify if the sugar is L or D and alpha or beta

CH2OH group above the plane=D or CH2OH group below the plane=L; C1 –OH above the plane=β or C1 –OH below the plane=α

Chair vs boat form of pyranoses (which is more stable)

chair form is lower energy and therefore more stable

t/f all stereoisomers have the same chemical formula

true

Sugar phosphates

important intermediates in metabolism. Functions as activated compounds in syntheses

Alditol

result of reduction of the sugar carbonyl. Sometimes creates inner molecular symmetry. These sugar alcohols are used widely in the food industry as thickeners and sweeteners. Does not cause tooth decay and arent absorbed well to small intestine.

Overconsumption of sugar alcohols can lead to what?

bloating, diarrhea and flatulence

Aldonic acid

result of oxidation of C1 of a sugar.

Uronic acid

result of oxidation of C6 of a sugar

How to experimentally test for reducing sugars?

The solution of Cu(II) (Fehling’s/Benedict’s/Tollen’s) may be used as an analytical test for reducing sugars. If the sugar’s reducing end is oxidized by this solution, the color will change from blue to red indicating a positive test.

O-Glycosides

formed by elimination of water between the hydroxyl group of the anomeric carbon of a cyclic saccharide and the hydroxyl group of another compound. Newly formed bond is called a glycosidic bond

Glycosidic bond formation

bond formed by elimination of water between hydroxyl group of anomeric carbon of a cyclic sugar and hydroxyl group of another compound

Reducing end in glycosidic bond formation

here in the example, the reducing end of Galactose interacts with C4 of Glucose to form lactose and eliminate one water

Monsaccharides can form polysaccharide via —- bonds

glycosidic

Maltose (structure and abbreviation)

𝛼-D-Glcp-(1->4)𝛼-D-Glcp

Sucrose(structure and abbreviation)

𝛼-D-Glcp(1->2)𝛽-D-Fruf

Cellobiose(structure and abbreviation)

𝛽-D-Glcp-(1->4)-𝛽-D-Glcp

Lactose(structure and abbreviation)

𝛽-D-Galp-(1->4)-𝛽-D-Glcp

alpha-D-glucose(structure and abbreviation)

Glc

alpha-D-galactose(structure and abbreviation)

Gal

alpha-D-fructose(structure and abbreviation)

Fru

When drawing disaccharides, how should it be oriented?

reducing end (if there is one) must be on the right

What polysaccharides may not be oxidized by Fehling’s solution?

polysaccharides which do not contain a reducing end (example; sucrose)

How to tell if a polysaccharide has a reducing end or not based on chemical structure

check the anomeric carbon of the monosaccharide subunit not involved in a glycosidic bond. If there is an -OH attached, that sugar is reducing. Otherwise it is not. Other method is to check if the sugar has a hemiacetal group (Carbon attached to a OH, OR, R, and H), then they are a reducing sugar

5 major features of disaccharides

sugar monomers involved and their stereochemistry, carbons involved in the linkage, order of the sugars (note free anomeric carbon can undergo oxidation), configuration of the anomeric carbon (alpha or beta), and ring configuration (p or f) (p means 6 membered ring and f means 5 membered)

If there is a glycosidic bond between carbon – and carbon — of two monosaccharides, that disaccharide will not have a reducing end. Why?

1 and 1; there will be no anomeric carbon with an -OH group

Starch

storage polysaccharide of plants. Contains both amylopectin and amylose

Amylopectin

alpha (1->4) glucose polymer with a (1->6) branches. Part of starch.

Amylose

alpha (1->4) unbranched polymer. Part of starch. Forms into a helix with a large interior core and stabilized by H-bonds

Glycogen

storage polysaccharide of animals and microbes. Like amylopectin but higher MW with shorter and more frequent branch points

Sticky rice has more —- in its starch, making it sweeter

more branches in the amylopectin

Cellulose

major structural polysaccharide in plants. Linear glucose polymer with beta(1->4) linkages.

Xylans and glucomannans

modified polysaccharides that are found in fibrous parts of plants

How is the cell wall of plant cells organized

cellulose chains form microfibrils which form the plant cell wall

Chitin

homopolymer of N-acetyl-beta-D-glucosamine. Like a derivatized form of cellulose. Major structural component of the exoskeleton of arthropods and mollusks. Comparable to collagen in providing a matrix for mineralization

Glycosaminoglycans

serve structural and nonstructural roles in vertebrates.

Gram positive vs gram negative bacteria cell wall

gram negative bacteria has double layer of lipid membrane while gram positive bacteria has a thick peptidoglycan cell wall

Peptidoglycan cell wall

cell wall of gram positive bacteria.Cross-linked, multilayered polysaccharide-peptide complex. Site of action of the earliest antibiotics

Penicillin

strong antibiotic as it inhibits formation of the peptidoglycan cell wall

glycoproteins

proteins with attached carbohydrate chains that play crucial roles in cellular functions like cell recognition, immune response, and structural support

N-linked glycoprotein

saccharide bound to a protein sidechain via a Nitrogen found in the peptide side chain ( for example asparagine)

O-linked glycoprotein

saccharide bound to a protein sidechain via a Oxygen found in the peptide side chain ( for example threonine)

How do glycoproteins determine blood type

Glycoproteins on red blood cell surfaces act as antigens (A, B, or neither) that define your ABO blood type

Which of these polymers has a lower energy compared to its hydrolyzed (monomeric) state- DNA, RNA, polypeptides, and poly saccharides

none of these polymers are in a lower energy state than their respective monomer