7: Carbohydrates and the Glycoconjugates of Cell Surfaces

carbohydrates

- hydrates of carbon

- general formula of (CH2O)n with n >= 3

- major form of stored energy

- important structural roles

Monosaccharides

- simple sugars

- cannot be broken down into simple sugars under mild conditions

Oligosaccharides

- 2-10 simple sugar residues linked by glycosidic bonds

Polysaccharides

polymers of the simple sugars

aldoses

- monosaccharides that contain an aldehyde function

- simplest one is glyceraldehyde

- numbering of carbons is starts one at the aldehyde carbon

ketoses

- monosaccharides that contain a ketone group

- simplest one is dihydroxy-acetone

- numbering of carbons start at the end closest to the carbonyl carbon

most naturally occurring stereochem of carbs is _______ isomers

D

- D/L is based on the highest numbered asymmetric carbon

- isomer is D if hydroxyl group on the highest numbered carbon is drawn on the right in a flat projection

enantiomers

- stereoisomers that are non-superimposable mirror images of each toehr

diastereomers

- pairs of isomers that have opposite configuration at one or more chiral centers but NOT mirror images

epimers

- two sugars that differ in configuration at only ONE chiral center

hemiacetal and hemiketal

- rxn of an alcohol group of a carbohydrate with the internal carbonyl aldehyde or chetone form

- rxn introduce an additional asymmetric center

cyclization of linear sugars

- in solution, sugars will become cyclic and form pyranose or furanose

pyranose

- six membered ring sugar

- usually from hemiacetal

furanose

- 5 membered ring sugar

- usually from hemiketal

anomers

- cyclization of D-glucose generates of mixture of α and β -> called anomers

- mutarotation in solution

how do we tell if a sugar is alpha or beta ?

- D sugar -> OH up -> alpha ; Oh down -> beta

- L sugar -> reverse of D

phospho sugars

- aka sugar esters

- phosphorylated through an ester linkable

free anomeric carbon

- HOH group, usually on the left side

- common disaccharides have an unsubstituted anomeric carbon, except for sucrose

- the "reducing end"

is sucrose and reducing sugar?

no bc it does have a free anomeric carbon

glycosidic bonds are formed bu

- elimination of water between two hydroxyl groups

alpha 1,4 glycosidic bond

- both OH groups are facing down

|_o_|

beta 1,4 glycosidic bond

- one OH is up (usually left one) and other OH is down (usually right one)

alpha 1,6 glycosidic bond

- Type of bond for glycogen branching

- connects the saccharides up and down, instead of left and right

glucose + glucose (alpha 1,4 connection) =

maltose

glucose + glucose (beta 1,4 connection) =

cellobiose

glucose + glucose (alpha 1,6 connection) =

isomaltose

galactose + glucose (beta 1,4 connection) =

lactose

- bond is hydrolyzed by lactase

- lactose intolerance is caused by lactase deficiency

glucose + fructose (alpha 1,2) =

sucrose

glycan

- aka starch

- polysaccharide

- mix of amylose and amylopectin

glucan

- long homopolysaccharide -> only contains glucose

amylose

- linear chain of glucose units in alpha 1,4 linkage

amylopectin

- branched chain

- branches are created by alpha 1,6 links and occur every 12,30 resides for about 24-30 residues

starch in plants

- starch phosphorylase release glucose phosphate -> catabolize by glycolysis

- cleavage at branch points -> needs alpha 1,6 glucosidase

- 1 ATP is "saved" since glucose is already phosphorylated in comparison to glycolysis

digestion of starch in animals start ...

in the mouth via salivary amylase

- salivary alpha amylase to break the alpha 1,4 linkage

- alpha 1,6 glucoside to break the alpha 1,6 linkage

glycogen

- glucose units joined by alpha 1,4 linkages and alpha 1,6 branches

- major form of storage polysaccharides in animals

how are starch and glycogen different?

by the number/degree of branching

- note: starch is stored in plants, glycogen is stored in animals

how are amylose and cellulose differ?

- in the configuration of the glycosidic linkages

- amylose is alpha 1,4

- cellulose is beta 1,4

- leads to different properties of cellulose

cellulose

- structural polysaccharide

- found in the cell walls of nearly all plants

- ex wood and bark of trees -> highly organized structure of cellulose and lignin -> insoluble

- alternating 180 degree flips to generate an extended ribbon conformation -> Hbonding bt chains

- resistant to hydrolysis by acid and amylases to can degrade starch and glycogen -> animals that digest cellulose need specific bacteria that can hydrolyze cellulose

in water, amylose structure becomes

helical conformation

chitin

- found in the cell walls of fungi and the exoskeletons of crustacea and insects

- identical to cellulose, except for the addition of NHCOCH3 at the C2 position -> repeating units is N-acetyl-D-glucosamine

agarose

- component of agar that is obtained from marine red algae -> chain of alternating D-galatase and 3,6-anhydro-L-galactose

- agarose gels used in labs to separate biomolecules via size

glycosaminoglycans

- linear chains of repeating disaccharides in which one unit is an amino sugar and one or both is negatively charged

- usually have to do with connective tissues

- ex heparin -> anticoagulant

- ex hyaluronates -> synovial fluid for joints and liquid humor in eyes

- ex chrondroitins and keratan sulfate -> tendons, cartilage and other connective tissue

- ex dermatan sulfate -> for the skin