CH 11: carbohydrates

carbohydrates overview

-Carbohydrates are the most abundant biomolecule in

nature

-Have a wide variety of cellular functions: energy, structure,

communication, and precursors for other biomolecules

-They are a direct link between solar energy and chemical

bond energy

carbohydrates

Monosaccharides

• Compounds that contain a single carbonyl group and two or

more hydroxyl groups

• Cannot be hydrolyzed (broken down) to simpler carbohydrates

• Have the general formula Cn (H 2 O) n

Oligosaccharides

• Several (8-20) sugars linked by glycosidic bonds

Polysaccharides

• Formed when many monosaccharides are bonded together

through glycosidic bonds

monosaccharides

-They are typically constructed of C,

H, and O atoms and adhere to the

molecular formula (CH 2 O) n , where

n ≥ 3.

- Carbohydrate derivatives—many of

which include groups containing N,

P, and other elements—are easy to

recognize by their large number of

hydroxyl (–OH) groups.

-monosaccharides from aldehydes→ aldoses

-monosaccharides from ketones→ketoses

classification of carbohydrates

Carbohydrates are also classified by the number of carbon

atoms they contain

-Trioses, tetroses, pentoses, and hexoses

-Most abundant in living cells are hexoses and pentoses

stereoisomerism in monosaccharides

Stereoisomers:

Molecules that differ from each other only in their

configuration

Configuration:

Three-dimensional arrangement of groups around a chiral

carbon atom

Possibility of stereoisomerism increases as the number of

carbon atoms increases

chirality of carbohydrates

Chiral Carbon: carbon atoms that bear four different substituents

§ Nearly all monosaccharides (the symmetric dihydroxyacetone is one exception) have a

number of stereoisomers which are easy to see when monosaccharides are written in a Fischer projection

monosaccharide stereoisomers

An increase in the number of chiral

carbons increases the number of

possible isomers

-2^n where n is the number of chiral

carbons

enantiomers

• non superimposable mirror image

• if OH on left → l enantiomer

• if OH on right → D enantiomer

enantiomers D vs. L configuration

Most highly oxidized carbon is written at the top and is designated C-1

• Other carbon atoms are numbered in sequence from the top

D configuration

• —OH is on the right of the highest-numbered chiral carbon

L configuration

• —OH is on the left of the highest-numbered chiral carbon

diastereomers and epimers

• differ at least one position, not mirror image

• epimers differ at only position

• all epimers are diastereomers, but not all diastereomers are epimers

cyclic structure of monosaccharides

Sugars with four or more carbons exist primarily in cyclic

forms

- Five-membered rings are called furanoses and six-membered

rings are pyranoses

- Cyclic form of fructose is fructofuranose, while glucose in the

pyranose form is glucopyranose

-cyclization is spontaneous

anomeric C: original carbon with C=O bond

OH in highest number carbon form bond with anomeric C

if the last C has the OH on right ( d enantiomer) the final C faces up

-OH on right → face down haworth

OH left→ face up haworth

-if D enantiomer, last C face up, if both the last C and anomeric are in the same direcrtion→ B anomer. if last C and anomeric C in opposite directions→ A anomer

mutarotation

-The a- and b-forms of

monosaccharides are readily

interconverted in aqueous

environments

-This spontaneous process,

mutarotation, produces an

equilibrium mixture of a- and b-

forms in both furanose and

pyranose ring structures

-can go from linear to ring structure

-can switch between 5 and 6 membered rings

stability of glucose

-Hexoses and pentoses form planar structures where the sugar ring

puckers so that each C atom can retain its tetrahedral bonding

geometry (“chair” conformation).

-The substituents of each carbon may point either above the ring

(axial positions) or outward (equatorial positions).

-most stable chair and abundant in nature

-the OH’s groups are all equatorial, most stable due to less steric hindrance

Glucose can adopt a chair conformation in which all its bulky

ring substituents (the –OH and –CH 2 OH groups) occupy

equatorial positions which minimizes repulsions between

neighboring groups.

- In all other hexoses, some of these groups must occupy the more

crowded—and therefore less stable—axial positions. The greater

stability of glucose may be one reason for its abundance among

monosaccharides.

derivatives of monosaccharides

Unlike enantiomers and epimers, which are not interchangeable, anomers in an aqueous

solution freely interconvert between the α and β forms, unless the hydroxyl group

attached to the anomeric carbon is linked to another molecule.

-any modification to anomeric C→ no longer have -OH on anomeric C

-reducing sugar→still has anomeric C in place,free anomeric C

-nonreducing sugar→modified anomeric C

-nonreducing sugars can not go back to linear form, reducing sugars can

types of modifications: phosphorylated monosaccharides can add phosphate. the linear form of the reducing sugar →still has anomeric C

-notes: anomeric C between two oxygens

-can add nucleic acid modifcations through a glycosidic bond, can add amine ( glucosamine), Coo- (glucuronate) , xylitol

disaccharides

-2 monomers linked by glycosidic bonds

Carbohydrate chains are

called glycans.(polysaccharides)

- Each monosaccharide has

several free –OH groups that

can participate in a

condensation reaction, which

permits different bonding

arrangements and allows for

branching.

lactose → galactose + glucose

Lactose

-Lactose (milk sugar) is the disaccharide found

in milk

-One molecule of galactose linked to one

molecule of glucose (b(1,4) linkage)-covalent, makes lactose

§It is common to have a deficiency in the

enzyme that breaks down lactose (lactase)

-Lactose is a reducing sugar

-galactose - B-1,4 linkage to glucose

beta: last C and anomeric C at same position

-one anomeric C (on glucose) is not modified, anomeric → rducing sugar

-lactose has 1 free anomeric C

sucrose

Sucrose is common table

sugar

• One molecule of glucose

linked to one molecule of

fructose, linked by an

a,b(1,2) glycosidic bond

• Glycosidic bond occurs

between both anomeric

carbons

• Sucrose is a nonreducing

sugar

-no free anomeric C

-glucose alpha 1, B -2 fructose

-both anomeric C involved in glycosidic bond

starch and glycogen are polymers of glucose

-Starch-plants, storage of glucose

• Amylose-linear form, helical structure/coils up,linked through alpha 1,4 linkage and glycosidic bond

• Amylopectin-branched formed-alpha 1,4 linkage, branched points around every 24-30 glucose molecules

-both amylose and amylopectin are made of only glucose

- Glycogen

-animals, highly branched

-alpha 1,4 linkage-linear and alpha 1,6-form branched linkage(every 8-12 glucose molecules)

-has more branched points than starch. animals need to move and release more glucose

-reducing end: has free anomeric C

-nonreducing end:anomeric C involved in glycosidic bond

cellulose is composed of glucose in B 1-4 linkages

Whereas starch molecules form

compact granules inside the cell,

cellulose forms extended fibers that

lend rigidity and strength to plant

cell walls.

- Individual cellulose polymers form

bundles with extensive hydrogen

bonding within and between

adjacent chains

-linear, no branches

-extended structure, stick up, not tightly coiled

-structural molecule for plant cell walls

Chitin

The exoskeletons of insects and crustaceans and the cell walls of

many fungi contain a polymer called chitin, in which the β(1 →

4)-linked residues are the glucose derivative N-acetylglucosamine

(glucosamine with an acetyl group linked to its amino group).

-can modify wit acetyl and amino group

-link via B 1-4 linkage, modified glucose linkages

glycoproteins are N-linked via Asn

Most of the proteins that are secreted from eukaryotic cells or

remain on their surface are glycoproteins.

- Glycoproteins are N-linked via Asn or O-linked via Ser or Thr

- The amino acid sequence or local structure of the protein, as well as the

set of processing enzymes present in the cell, roughly determine which

sugars are added to and deleted from Glycoprotein.

-linked via glycosidic bonds

-glycosylation occurs during protein synthesis(adding sugar to protein)

The ABO blood group system

The best known and one of

the clinically important

carbohydrate classification

schemes is the ABO blood

group system which involves

the oligosaccharides attached

to sphingolipids and proteins

on red blood cells and other

cells.

-oligosaccharides, proteins or lipids

-protein add oligosaccharide→ RBC identity

proteoglycans

Proteoglycans are glycoproteins in which the protein chain

serves mainly as an attachment site for enormous linear O-linked

polysaccharides called glycosaminoglycans.-negatively charged repeating disaccharides

§ Proteoglycans may be transmembrane proteins or lipid-linked,

but the glycosaminoglycan chains are invariably on the

extracellular side of the plasma membrane.

-found on membrane proteins, protection

-chondroitin sulfate-glycosaminoglycans

proteoglycan structure: have a protein backbone

-glycosaminoglycan has a strong negative charge

-O linked glycosaminoglycans

-the negative charge act as shock absorbers, when put pressure on joints they squeeze out H2O and decrease diameter of fluid surrounding joints

-to relieve pressure, negative charge repels and joint go back to normal width

proteoglycan’s function

Structural role in connective tissues by attracting various amount

of water based on mechanical pressure. This spongelike action of

glycosaminoglycans in the spaces of the joints provides shock

absorption.

- Proteoglycans, known as mucins, form the protective mucus

lining the respiratory, gastrointestinal, and reproductive tracts.

-protein backbone with glycosamino added through O linked glycosilation