Carbohydrates

most abundant molecules on earth.
Most are produced by photosynthesis.
Uses: Yield energy (ATP) to drive metabolic processes.
Energy-storage molecules (i.e. glycogen, starch).
Structural - cell walls and exoskeletons of some organisms.
Carbohydrate derivatives found in coenzymes (FAD) and nucleic acids.
Can be described by the number of monomers they contain:
1) monosaccharides - (CH2O)n where n = 3-6; one sugar molecule
2) oligosaccharides - polymers from 2-20
3) polysaccharides - polymers of greater than 20 sugar residues
4) glycoconjugates - derivatives; attached to proteins, lipids, peptide chains
Monosaccharides
Also known as polyhydroxy aldehydes or ketones.
Classified based upon type of carbonyl group (-C-) and number of carbon atoms
aldose - sugar with aldehyde group
ketose - sugar with ketone group
There are two important trioses:
glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose)
Can have stereoisomers, but the D-form predominates.
Naming of sugars: xylose aldose
xyulose ketose add “ul” before “ose”.
Both aldoses and ketoses engage in intramolecular cyclization
Alcohol + aldehyde = hemiacetal
Alcohol + ketone = hemiketal
Both 5 and 6 carbon monosaccharides can form hemiacetals.
5-membered = furanose
6-membered = pyranose
Most oxidized carbon (C-1; attached to 2 oxygen atoms) is known as an anomeric carbon.
Can adopt either of two configurations.
- OH group down
- OH group up
Aldoses and ketoses equilibrate between cyclic and open forms.
Ring structures can adopt different conformations
5-member - twist (2 carbons approximately coplanar)
envelope (4 carbons approximately coplanar)
6-member - chair (more stable due to lack of steric hindrance from 6’ carbon)
boat
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Derivatives of monosaccharides:
1) Sugar phosphates
Metabolized as phosphate esters
2) Deoxy sugars
Hydrogen atoms replaces -OH group on C-2.
Important to structure of nucleic acids.
3) Amino sugars
Amino group (NH-) substituted for -OH group in monosaccharide.
4) Sugar alcohols
Replace carbonyl oxygen to form polyhydroxy alcohols
e.g. glycerol --> glyceraldehyde
Replace “-ose” with “-itol”.
Ribose --> ribitol
5) Sugar acids
Oxidation of carbonyl carbon or highest carbon.
glucose --> gluconate or glucuronate
Important in many polysaccharides.
6) Ascorbic acid
Derived from D-glucuronate.
Primates cannot do the conversion, so must be supplied in the diet.
Disaccharides
Two monosaccharides joined by covalent bond called a glycosidic linkage via a condensation reaction.
Bond is created between the C-1 of one sugar and the -OH of another carbon
Examples:
1) maltose - 2 glucose molecules joined by -glycosidic bond
C-1 of one residue and C-4 of second residue
also known as -D-glucopyranosyl-(1--> 4)--D-glucopyranose
2) cellobiose - 2 glucoses joined by -glycosidic bond; plant polysaccharide
3) lactose - galactose and glucose in -glycosidic bond; major carbohydrate in milk
4) sucrose - glucose and fructose in 1--> 2 linkage; table sugar
Reducing and nonreducing sugars
Some monosaccharides and most disaccharides have a reactive carbonyl group or anomeric carbon
that can be oxidized.
Examples: glucose, maltose, cellobiose, lactose
Detected by the ability to reduce Cu
2+
--> Cu
+
with Benedict’s reagent (blue --> red-orange).
Nonreducing sugars have both anomeric carbons in a glycosidic bond (e.g. sucrose).
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Polysaccharides
Divided in two two classes:
1) homoglycans - homopolysaccharides composed on one monosaccharide
2) heteroglycans- heteropolysaccharides made of more than one type of monosaccharide
Often classified according to their biological role:
1) starch and glycogen - storage polysaccharides
Both are homoglycans.
Starch is storage form in plants and fungi.
Glycogen is storage form in animals.
Bacteria contain both.
Starch - mixture of amylose and amylopectin
amylose is an unbranched polymer of 100-1000 D-glucose in an -(1 --> 4) glycosidic
linkage.
amylopectin is a branched polymer -(1--> 6) branches of residues in an -(1 --> 4)
linkage; overall between 300-6000 glucose residues, with branches once every
25 residues; side chains are 15-25 residues long
-amylase is an endoglycosidase found in human saliva but also plants
that randomly hydrolyzes the (1--> 4) bond of amylose and amylopectin.
-amylase is an exoglycosidase found in higher plants that hydrolyzes
maltose residues from non-reducing ends of amylopectin.
Glycogen - branched polymer of glucose residues with branches every 8-12 residues with
branches containing as many as 50,000 glucose residues
2) cellulose and chitin - structural polysaccharides
cellulose - straight chain homoglycan of glucose with -(1--> 4) linkages with alternating
glucose molecules; ranges in size from 300-15,000 glucose residues
Extensive H-bonding within and between cellulose chains.
Makes bundles or fibrils ---> rigid.
chitin - linear polymer of N-acetylglucosamine residues
Alternating 180o with - (1 --> 4) linkage.
Lots of H-bonding between adjacent strands.
Glycoconjugates
Heteroglycans of three types:
1) Proteoglycans
Complexes of polysaccharides called glycosaminoglycans and core proteins.
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Found in extracellular matrix of connective tissues.
Glycosaminoglycans are unbranched heteroglycans of disaccharide units (amino
sugar, D-galactosamine or D-glucosamine, and alduronic acid). e.g. hyaluronic acid
Found in cartilage and synovial fluid.
Proteoglycan cartilage
2) Peptidoglycans
Found in cell wall of bacteria.
Composed of alternating residues of N-acetylglucosamine and N-acetylmuramic acid
joined by - (1--> 4) linkages.
3) Glycoproteins
Proteins with oligosaccharides attached.
Carbohydrate chains are from 1-30 residues in length.
Examples: enzymes, hormones, structural proteins, transport proteins.
Found in eucaryotic cells.
Can be attached to proteins with one of two configurations:
1) O-linked - carbohydrate bonded to -OH of serine or threonine
2) N-linked - carbohydrate (usually N-acetylglucosamine) linked to asparagines