Biochem Carbohydrates

Geromil J. Lara, RMT, MSMTCARBOHYDRATES

CARBOHYDRATES

• Most abundant organic compounds in the plant word

• Act as storehouses of chemical energy

• Components of supportive structures in plants

• Crustacean shells

• Connective tissues in animals

• Essential components of nucleic acids

CARBOHYDRATES

• means “hydrate of carbon”

• derives from the formula Cn(H2O)m

• Glucose: C6H12O6 or C6(H2O)6

• most are polyhydroxyaldehydes, polyhydroxyketones, or compounds that yield them after hydrolysis

• saccharides – simpler members of the CHO family (Latin: saccharum – sugar)• Monosaccharide, Disaccharide, Oligosaccharide, and Polysaccharide

A. MONOSACCHARIDES

• C6H12O6

• with one of the carbons being the carbonyl group of either an aldehyde or a ketone

• have 3 to 9 carbon atoms

• -ose indicates that a molecule is a carbohydrate

• tri-, tetr-, pent-, so and so forth - indicate the number of carbon atoms in the chain

A. MONOSACCHARIDES

•Aldoses

• monosaccharides containing an aldehyde group

A. MONOSACCHARIDES

•Ketoses

• monosaccharides containing a ketone group

A. MONOSACCHARIDES

•2 Trioses• aldotriose glyceraldehyde

• ketotriose dihydroxyacetone

A. MONOSACCHARIDES

• Fischer Projection Formulas

• Glyceraldehyde – contains a stereocenter and therefore exists as a pair of enantiomers

• Enantiomer - is one of two stereoisomers that are non-superposable complete mirror images of each other, much as one's left and right hands are "the same" but opposite

A. MONOSACCHARIDES

• D- and L- Monosaccharides

•D-Monosaccharide• has the –OH group on its penultimate carbon to the right

•L-Monosaccharide• has the –OH group on its penultimate carbon to the left

CONFIGURATIONAL RELATIONSHIPS AMONG

THE ISOMERIC D-ALDOTETROSES, D-

ALDOPENTOSES, AND D-ALDOHEXOSES

CONFIGURATIONAL RELATIONSHIPS AMONG D-2-KETOPENTOSES AND

D-2-KETOHEXOSES

A. MONOSACCHARIDES

•D-• specifies the configuration at the stereocenter farthest from the carbonyl group

•Prefixes (rib-, arabin-, and gluc-)• specify the configuration of all other stereocenters in the monosaccharide relative to one another

•-ose

• indicates that the compound is a carbohydrate

A. MONOSACCHARIDES

•Most Abundant Hexoses

•D-Glucose

•D-Galactose• D-aldohexoses

•D-Fructose

• D-ketohexose

A. MONOSACCHARIDES

•Amino Sugars• a monosaccharide in which an –OH group is replaced by an –NH2 group

•D-Glucosamine

•D-Mannosamine

•D-Galactosamine

A. MONOSACCHARIDES

•Physical Properties

• are colorless, crystalline solids

•Are very soluble in water

• because hydrogen bonding is possible between their polar –OH groups and water

•Slightly soluble in ethanol

•Insoluble in nonpolar solvents such as diethyl ether, dicloromethane, and benzene

A. MONOSACCHARIDES

•Cyclic Structure

• Haworth Projection

•A way to view furanose and pyranose forms of monosaccharides

•The ring is drawn flat and viewed through its edge with the anomeric carbon (new C stereocenter created in forming the cyclic structure) on the right and the oxygen atom to the rear

•A 5- or 6-membered cyclic hemiacetal is represented as a planar pentagon or hexagon, respectively, lying roughly perpendicular to the plane of the paper

A. MONOSACCHARIDES

•Cyclic Structure


•β means that the –OH on the anomeric carbon lies on the same side of the ring as the terminal –CH2OH

•α means that the –OH on the anomeric carbon lies on the side of the ring opposite from the terminal –CH2OH

A. MONOSACCHARIDES

•Cyclic Structure


•-pyran- a 6-membered hemiacetal ring

•-furan- a 5-membered hemiacetal ring

•Pryanose and Furanose

•Are used because monosacharide 5- and 6-membered rings correspond to the heterocyclic compunds pyran and furan

REACTION OF MONOSACCHARIDES

•Formation of Glycosides (Acetals)

•Acetal – yields when hemiacetal is treated with one molecule of alcohol

•Glycoside

•A carbohydrate in which the –OH group on its anomeric carbon is replaced by an –OR group

•Glycosidic Bond

•the bond from the anomeric carbon of a glycoside to an –OR group


•Reduction to Alditols

•Carbonyl group is reduced to an hydroxyl group by a variety of reducing agents, including hydrogen in the presence of a transition metal catalyst and sodium borohydride – alditols

• drop –ose and change to -itol

NaBH4

D-Glucitol(D-Sorbitol)

Xylitol – used as a sweetening agent in “sugarless” products


•Oxidation to Aldonic Acids (Reducing Sugars)

•Aldehyde group of an aldose can be oxidized, under basic conditions, to a carboxylate group


•Oxidation to Uronic Acids

•Enzyme-catalyzed oxidation of the primary alcohol at carbon 6 of a hexose yields a uronic acid


•Phosphoric Esters

•Important in the metabolism of monosaccharides

• eg. Conversion of glucose to glucose-6-phosphate

B. DISACCHARIDES and OLIGOSACCHARIDES•Disaccharides

• contain 2 monosaccharide units joined by a glycosidic bond

•Oligosaccharides

• to describe any of the carbohydrates that contain from 6 to 10 monosaccharide units

•Polysaccharides

• a carbohydrate containing a large number of monosaccharide units, each joined to the next by one or more glycosidic bonds


• Sucrose (table sugar)

• most abundant

• obtained principally from the juice of sugar cane and sugar beets

• nonreducing sugar


• Lactose

• principal sugar present in milk

• consists of D-galactopyranose bonded by a beta-1,4-glycosidic bond to carbon 4 of D-glucopyranose

• reducing sugar


• Maltose

• derives its name from its presence in malt, the juice from sprouted barley and other cereal grains

• consists of 2 units of D-glucopyranose joined by a glycosidic acid between carbon 1 of one unit and carbon 4 of the other unit

B. DISACCHARIDES and OLIGOSACCHARIDES•Relative Sweetness

•Fructose

• sweetest – even sweeter than sucrose

• Honey

• D-fructose and D-glucose

•Lactose

•almost no sweetness and is sometimes added to food as a filler

C. POLYSACCHARIDES

•Polysaccharides

•Starch: Amylose and Amylopectin

• used for energy storage in plants

• complete hydrolysis yields only D-Glucose

• Amylose – composed of continuous, unbranched chains of as many as 4000 D-glucose units joined by alpha-1,4-glycosidic bonds

• Amylopectin – contains chains of as many as 10,000 D-glucose units joined by alpha-1,4-glycosidic bonds• new chains of 24-30 units are started by alpha-1,6-glycosidic bonds

C. POLYSACCHARIDES

•Polysaccharides

•Glycogen

• acts as the energy-reserve carbohydrate for animals

• it is branched polysaccharide containing approximately 106 glucose units joined by alpha-1,4 and alpha-1,6-glycosidic bonds

C. POLYSACCHARIDES

•Polysaccharides

•Cellulose

• most widely distributed plant skeletal polysaccharide

• is a linear polysaccharide of D-glucose units joined by beta-1,4-glycosidic bonds

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Biochem Carbohydrates