Chem 1152: Ch. 17 Carbohydrates
IntroductionIntroduction
Biomolecules: Organic compounds produced by living organisms•Carbohydrates•Lipids•Proteins•Nucleic acids
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Functions of carbohydrates1.Provide energy through their oxidation2.Supply carbon for synthesis of cell components3.Serve as stored form of chemical energy4.Form structural elements of some cells and tissues
Classes of Carbohydrates (saccharides)Classes of Carbohydrates (saccharides)
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm
Hydrates of carbon Cm(H2O)n characterized by having multiple functional groups
•Hydroxyl (alcohols)•Carbonyl (aldehydes or ketones)Mono- and disaccharides are simple sugars
glucosefructose
galactoseribose
deoxyribose
sucroselactose
Fructo-oligosaccharidesgalactooligosaccharides
starchglycogencellulose
chitin
Stereochemistry of CarbohydratesStereochemistry of Carbohydrates
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://web.fccj.org/~ethall/stereo/stereo.htm
Stereoisomerism: Isomeric molecules can have the same molecular formula and sequence of bonded atoms (constitution), but that differ in the three-dimensional orientations of their atoms in space.Enantiomers: Stereoisomers that are mirror images of each other.Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image.-Defined relative to central, chiral atom (carbon)
enantiomers
Stereochemistry of CarbohydratesStereochemistry of Carbohydrates
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Chiral Carbon: Carbon in organic compound that has four different groups attached to it.Chirality: “Handedness”. Refers to compounds that cannot be superimposed on mirror image.
enantiomers
OHC
H
C OH
CH2OH
CHO
H
CHO
CH2OH
OHCC
OH
CH2OH
CHOC
HO
CH2OH
Chiral carbon
CHO indicates aldehyde
Identify Chiral CarbonsIdentify Chiral Carbons
OHC
H
C OH
CH2OH
OHCCHOH
CH2OH
CH3CCH2CH3
O
*
CH3CHCH3
OHCH3CHCH2CH3
OH
OH
O
Identify chiral carbons in glucoseIdentify chiral carbons in glucose
glucoseglucose
C
C
C
C
CH2
C
OH
OH
OH
OH
HO H
H
H
H
H O1
2
3
4
5
6
p. 528
chiral carbons in glucosechiral carbons in glucose
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Max # of possible stereoisomers = 2n where n = number of chiral carbon atoms
Fischer ProjectionsFischer Projections
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Chiral Carbons represented by intersection of two lines For fischer projections the prefixes (L-) and (D-) do not indicate Levorotatory
(L-) and Dextrorotatory (D-) related to rotation of polarized light, but represent orientation of functional groups when compared to glyceraldehyde
Fischer ProjectionsFischer Projections
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
When carbonyl is up: D-family: OH (or NH2) group of chiral C most distant from anomeric center projects to right
L-family: OH (or NH2) group of chiral C most distant from anomeric center projects to left Anomeric center: Carbonyl (aldehyde or ketone) carbon
Biological systems can only utilize the D- isomers.
*
* *
*
*
*
*
*
*
*
*
*
MonosaccharidesMonosaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Contain single polyhydroxy aldehyde or ketone unit Further classified based on number of C atoms Aldehydes contain prefix aldo-, ketones have prefix keto-
Monosaccharides: D- aldosesMonosaccharides: D- aldoses
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Chiral C’s?
1
2
3
4
stereoisomers
2
4
8
16
Monosaccharides: Chemical PropertiesMonosaccharides: Chemical Properties
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Because aldehydes and ketones react with alcohols to form hemiacetals and hemiketals, all monosaccharides with at least 5 carbon atoms exist predominantly in cyclic forms.
These are depicted using Haworth structures Ring is drawn with oxygen to the back, and anomeric carbon to the right.
Furanose ring: A 5-member ring containing an oxygen atom. Pyranose ring: A 6-member ring containing an oxygen atom.
Glucose (Blood Sugar)Glucose (Blood Sugar)
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Primary source of energy in cells Precursor for Vitamin C synthesis Modified subunits can form long polymer chains
starch, cellulose, glycogen
C
C
C
C
CH2
C
OH
OH
OH
OH
HO H
H
H
H
H O
1
2
3
4
5
6
Beta-hydroxy group: OH attached to anomeric carbon above ring. Alpha-hydroxy group: OH attached to anomeric carbon below ring.
Hemiacetal (on C1)
64%
0.02%
36%
FructoseFructose
http://upload.wikimedia.org/wikipedia/commons/1/1e/Fructose-isomers.jpg
Fruit sugar Absorbed directly into bloodstream during digestion (like glucose and galactose) Anomeric carbon is C2
70%
22%
Hemiketal (on C2)
1
23
456
Chemical Reactions of SugarsChemical Reactions of Sugars
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Open chain forms of monosaccharides (aldehydes, hydroxyketones) can be readily oxidized. Ex. Use of Benedict’s reagent to oxidize aldehydes and ketones with hydroxy
group on adjacent carbon. At the same time, the cyclic forms are converted to open-chain forms and also react. Reducing sugars: Monosaccharides that can be oxidized Oxidation of carbohydrates to CO2 and H2O very important at cellular level, serves as
source of heat and energy.
Reducing sugar + Cu(II) oxidized cmpd + Cu2ODeep blue solution
Red-orange precipitate
Other Important MonosaccharidesOther Important Monosaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011: http://www.istpace.org/Web_Final_Report/WP_4_chem_subsystems/descr_chem_subsystems/pna_intro/pna_intro.html
Ribose and Deoxyribose Used to synthesize RNA and DNA Used in protein synthesis
Phosphodiester bond
Other Important MonosaccharidesOther Important Monosaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Galactose Synthesized in mammary gland, incorporated into milk lactose Component of the antigens present on blood cells that determine blood type
Joining MonosaccharidesJoining Monosaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Monosaccharide units can be joined together by glycosidic linkages. Glycosidic linkages are same as adding alcohol to hemi- intermediates
O
R H
+ H
R
O
RO
HO
R H
H+, RO-H
RO
RO
R H + H-O-H
Hemiacetal Hemiacetal intermediateintermediate
acetalacetal
O
R H
+ H
R
O
RO
HO
R H
H+, RO-H
RO
RO
R H + H-O-H
O
OH
CH2OH
OH
O
O
CH2OH
OHH
+O
CH2OH
OH
O
CH2OH
OHO
+ OH2
Polymerization of Monosaccharide SubunitsPolymerization of Monosaccharide Subunits
•Through glycosidic linkages at the hemiacetal carbons, many monosaccharide subunits can be put together to form long, branching chains via 1,4 or 1,6 linkages.•These can be between 2 α, 2 β, or between an α and a β.
DisaccharidesDisaccharidesMaltose
Lactose Sucrose
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Properties of PolysaccharidesProperties of Polysaccharides
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
•Components of plant starch built from glucose residues.Plant starchesPlant starches
Amylose (10-20%)No branching
Amylopectin (80-90%)Branching 24-30 residues
Structure of glycogenStructure of glycogen
•Polymer built from glucose subunits.•Glucose in Glycogen are connected via α-1,4 or α-1,6 linkage.•α-1,4 linkage makes a linear chain, α-1,6 linkage makes a branch (~every 10 residues).•The end glucose residues without open 1’-OH is called nonreducing ends.•Branches provide more non-reducing ends for rapid degradation.
What is glycogen and why do we need it?What is glycogen and why do we need it?
•Long-term energy reservoir for glucose in animal and fungal cells. •Found primarily in muscle and liver cells.•Liver produces glycogen for needs of organism, while muscle takes care of only itself.•Glycogen not as energy rich as fatty acids, and is used differently. •Controlled breakdown of glycogen and glucose release maintain blood-glucose levels.•Glucose is the primary fuel used by the brain.•Unlike fatty acids, glucose from glycogen is readily mobilized and can provide energy for sudden, strenuous activity.
•Most important structural polysaccharide and single most abundant organic compound on earth.• Provides strength and rigidity to plant cell walls.•Wood is ~50% cellulose.•Contains 300-3000 glucose subunits.•Form extended straight chains that hydrogen bond with parallel chains, creating long, rigid fibers.•Undigestible by humans.
CelluloseCellulose
Intramolecular addition of alcohols to aldehydeIntramolecular addition of alcohols to aldehyde
C1 is hemiacetal carbon. Attached to it you will find: H, OH, OR and R, just like non-cyclical compounds.
Intramolecular Intramolecular hemiacetalhemiacetalglucoseglucose
C
C
C
C
CH2
C
OH
OH
OH
OH
HO H
H
H
H
H O1
2
3
4
5
6
*CH2OH
C
C
C C
O
C
6
5
4
3 2
1
OH
H
OH
OH
OH
H
H
H
H