Kaplan Biochemistry Chapter 4 : Carbohydrate Structure & Function

Trioses

Simplest of the monosaccharides composed of three carbon atoms

4 carbons - Tetrose

5 carbons - Pentose

6 carbons - Hexose

Aldose

Carbohydrates that contain an aldehyde as their most oxidized functional group

Can participate in glycosidic linkages

Ex - Aldohexose, Glyceraldehyde

Numbering of carbons on sugars follows IUPAC rules

Ketoses

Carbohydrates that contain a ketone group as their most oxidized functional group

Ex - ketopentose

Glycosidic Linkages

Dihydroxyacetone

d-fructose

d-glucose

d-galactose

d-mannose

optical isomers (stereoisomers)

compounds that have the same chemical formula, differ only in spatial arrangement of atoms

# stereoisomers with common backbone

2^n (n= # of chiral carbons)

Enantiomers

special optical isomer...

stereoisomers that are onidentical, nonsuperimposable mirror images

any molecule with chiral carbons and no internal planes of symmetry has an enantiomer

same sugar, different optical family (ex. D-glucose versus L-glucose)

more chiral C = more stereoisomers

b/c one compound might havee many diastereomers

# stereoisomeers with common backbone = 2^ n (n = # chiral C)

Diastereomers

not identical, not mirrors; both either ketose or aldose with same # of carbons

Epimers

subtype of diasteromers, differ iin configuration at exactly 1 chiral center

draw all possible D-stereoisomers of glucose

Hemiacetals vs. Hemiketals

hemiacetal = from aldoses

hemiketals = from ketoses

pyranose ring v furanose ring

pyranose = 6-membered ring; furanose = 5-membered ring

b/c of ring strain, only 2 that are stable

Anomeric Carbon

alpha = down; beta = up

in ring formation, hydroxyl group = nucleophile and oxygen becomes member of the ring

carbonyl carbon becomes chiral and is the anomeric carbon

a-anomer has -OH group of C-1 trans to CH2OH (AXIAL AND DOWN)

b-anomer has -OH group of C-1 cis to CH2OH (EQUATORIAL and UP)

mutarotation

spontaneous change between alpha and beta configuration; beta is more stable

Mutarotation

Monosaccharide Oxidation and Reduction

aldonic acid?

lactone?

monosaccharides are oxidized to yield energy (oxidized aldoses = aldonic acid if not in ring form) so they're reducing agents

If aldose is in ring form, oxidation = lactone

Tollen's rgnt and Benedict's rgnt

test for presence of reducing sugars, so they are oxidizing agents

ketose sugars are also reducing sugars and give positive Tollen's and Benedict's tests

ketones can tautomerize to form aldoses under basic conditions via keto-enol shift (cannot be oxidized directly to carboxylic acids)

polysaccharides: cellulose, starch, glycogen

all from D-glucose:

cellulose - structural component in plants (b-d-glucose, b-1,4)

starch (a-d-glucose, a-1,4, ex. amylose = plant starch and amylopectin = similar to amylose but branched)

glycogen - like starch but more a-1,6 branches: branches make more energy, more soluble, glycogen phosphorylase cleaves, leaving G-1-P

amylose

Acetals

hemiacetals react with alcohols to form acetals

resulting C-O bond is called a glycosidic bond, acetals formed are glycosides

*glycosides from furanose rings = furanosides

*drived from pyranose rings = pyranosides

monosaccharide + alchohol

forms an acetal

Maltose vs cellobiose

a-1,4: glucose-glucose vs B-1,4

sucrose, lactose, maltose

lactose

b-1,4: galactose, glucose

sucrose

a-1,2: glucose + fructose

Esterification

hydroxyl + carboxylic acid --> ester

glycosilation

alcohol + hemiacetal/ hemiketal --> alkoxy group

3 polysaccharides - cellulose

Cellulose (structure in plants, homopolysaccharide, B-D glucose molecules w/ B-1,4 glycosidic bonds)

*we lack cellulase

3 polysaccharides - starches

a-D-glucose monomers

plants store starch as AMYLOSE (linear glucose w/ a-1,4)

Amylopectin (a-1,4 and a-1,6) -- requires debranching enzyme

*iodine tests for starch

3 polysaccharides - glycogen

carb storage in animals

similar to starch but more a-1,6, very branched

(glycogen phosphorylase which cleaves can work in many sites at once)

(glycogen phosphorylase creates glucose 1-phosphate)

Epimers vs esteriomers

2 Notes:

1. Epimers usually have slightly different chemical + physical properties

2. Esteriomers have equal but opposite optical activity

Cellulose

chain of B-D-glucose, B-1,4 -- need cellulase

Amylose and amylopectin degradation

Amylose -- a-amylase and b-amylase

**a-amylase = cleaves randomly along chain for shorter chains, maltose, and glucose

**b-amylase = cleaves at nonreducing end to yield maltose

Amylopectin needs debranching enzyme

straight-chain fischer to ring conformation

anything on the right is "up", anything on the left is "down"

Cyclic forms of monosaccharides

hemiacetals and hemiketals