Carbohydrates: Structure, Properties, and Biological Importance, Lecture notes of Biochemistry

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Chapter 1

CARBOHYDRATES

Topic Outline

• Monosaccharides
  • Aldoses
  • Ketoses
• Representation of Monosaccharides
  • Fischer Projection
  • Haworth Projection
  • Chair Conformation
• Characteristic Reactions of Monosaccharides
  • Formation of Glycosides
  • Reduction to Alditols
  • Oxidation to Aldonic Acid

Carbohydrates

• “hydrates of carbon”, derived from the formula C n (H 2 O) m
• the most abundant organic compounds in the plant world
• act as storehouses of chemical energy (glucose, starch, glycogen)
• components of supportive structures in plants (cellulose), crustacean shells (chitin), and connective tissues in animals (acidic polysaccharides)
• essential components of nucleic acids (D-ribose and 2-deoxy-D- ribose)

Carbohydrates

• account for approximately three-fourths of the dry weight of plants
• less than 1% of the body weight of animals

Monosaccharides

• have the general formula C n

H

2 n

O

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

• The most common monosaccharides have three to nine carbon atoms.
• The suffix - ose indicates that a molecule is a carbohydrate, and the prefixes tri- , tetr- , pent- , and so forth, indicate the number of carbon atoms in the chain.

Monosaccharides

• Monosaccharides containing an aldehyde group are classified as aldoses ; those containing a ketone group are classified as ketoses.
• There are only two trioses:

The Enantiomers of Glyceraldehyde

D- and L- Monosaccharides

• the configuration of carbohydrates is commonly designated using the D,L system proposed by Emil Fischer in 1891 ➢ D-Monosaccharide – a monosaccharide that, when written as a Fischer projection, has the – OH group on its penultimate carbon * to the right. ➢ L-Monosaccharide – a monosaccharide that, when written as a Fischer projection, has the – OH group on its penultimate carbon * to the left. *Penultimate carbon – 2 nd^ to the last

Enantiomers (D- and L-)

Configurational Relationships among the Isomeric D- Aldotetroses, D-Aldopentoses, and D-Aldohexoses.

D- and L- Monosaccharides

• The three most abundant hexoses in the biological world are D-glucose, D-galactose, and D-fructose.
• Glucose, by far the most abundant of the three, is also known as dextrose because it is dextrorotatory. Other names for this monosaccharide include grape sugar and blood sugar. Human blood normally contains 65–110 mg of glucose/ mL of blood.

Amino Sugars

• Monosaccharides in which an – OH group is replaced with an
  • NH 2 group.
• Only three amino sugars are common in nature: D- glucosamine, D-mannosamine, and D-galactosamine.

Physical Properties of Monosaccharides

• Monosaccharides are colorless, crystalline solids.
• All monosaccharides are very soluble in water.
• They are only slightly soluble in ethanol and are insoluble in nonpolar solvents such as diethyl ether, dichloromethane, and benzene.

Cyclic Structures of Monosaccharides

• Aldehydes and ketones react with alcohols to form hemiacetals. Cyclic hemiacetals form very readily when hydroxyl and carbonyl groups are part of the same molecule and that their interaction produces a ring.