Lecture 17: Disaccharides and Polysaccharides

examples of disaccharides

sucrose and lactose

formation of disaccharides and polysaccharides

condensation reaction via glycosidic linkage aka ether bond

lactose formed by

galactose and glucose where -OH group form one sugar and the -OH group from other sugar contribute to bond formation and water is released

Sugars are either

aldoses or ketoses, they contain carbonyl group

reducing end

in aldoses the aldehyde can be oxidized to an acid, ketose is also a reducing sugar

reducing ends in disaccharide

can only have 0 or 1 reducing end

ex: lactose has one reducing end

reducing end depends on

which -OH groups are involved in the condensation reactions

Whenever condensation occurs:

one of the sugars always donates the hydroxyl group bound to the anomeric carbon

Sucrose

a non reducing sugar bc it can’t polymerize further

non reducing sugar

when the condensation between two sugars involves hydroxyl groups bound the anomeric carbons of both sugars, then there is no more reactive anomeric carbons

Hydrolysis occurs thru

enzymes and requires water

Opposite of polymerization/condensation

hydrolysis

Water is added across the glycosidic bond

One side gets H and the other side gets OH → each sugar gets back its hydroxyl group

hydrolysis restores

two glucose molecules each with an anomeric carbon again

sugar on the left

is always the one that donated the hydroxyl group of the anomeric carbon

glucose

If a sugar has its anomeric carbon locked in the glycosidic linkage

you must specify whether it is alpha or beta

alpha and beta

tell you the direction of the OH on the anomeric carbon in a cyclic sugar

Beta

the anomeric OH and the reference CH2OH are on the same side

Alpha

the anomeric OH and the reference CH2OH are on opposite sides

Figuring out the right sugar carbon number

find the anomeric carbon, “cut” the ring so each carbon gets its oxygen back, then number, and find which carbon the glycosidic linkage is

Right sugar conformation rule

If the sugar on the right has a free anomeric carbon, no need to include its alpha/beta conformation in the linkage name

naming: sugar on the left will always be

1-something or 2-something because by convention it donated its anomeric hydroxyl group

naming: sugar on the right

students must use ring-linearization and numbering to determine which carbon donated its hydroxy

Systematic Nomenclature

Polysaccharides

Indefinite length, strings of many identical monomers or dimer repeats (homopolymers or heteropolymers)

oligosaccharides

shorter discrete-length polymers

Examples of polysaccharides

cellulose, starch, glycogen, and chitin

Starch

Used to store glucose in plants

Glycogen

Used to store glucose in animals

Glycogen and starch linkages

Both have alpha(1→4) linkage

left handed helix

Digestibility of glycogen and starch

can be digested by animals

Cellulose

occurs as sheets that has beta(1→4) linkage and provides structural support to plants but cannot be digested by animals

animals cant digest cellulose bc

animals have enzymes that break alpha 1→ 4 not enzymes that break beta 1→ 4

Branching in starch and glycogen

alpha(1→6) linkage which is important bc more branching means more non reducing ends and that helps with rapid mobilization

cellulose has no branching

it has intra chain hydrogen bonds and inter chain hydrogen bonds → very sturdy

homopolymers of glucose

starch, glycogen, and cellulose

chitin

homopolymer derivative of glucose and is used for the exoskeleton of bugs

chitin linkage

beta 1→ 4 glycosidic linkage

Peptidoglycan

consists of small protein segments that are bound to sugars and is present in bacteria

gram positive

have a cell wall made of peptidoglycan

gram negative

have a very thin peptidoglycan layer and an outer membrane