BIOCHEMISTRY LEC - Carbohydrates

Carbohydrates

They are polyhydroxy aldehydes or ketones, or compounds that yield them upon hydrolysis.

Glucose

• Polyhydroxy aldehyde

• in carbon 1

Fructose

• Polyhydroxy ketone

• in carbon 2

Functions of Carbohydrates

1. Provides energy

2. Short-term energy storage (immediate)

3. Supply carbon atoms for synthesis of other biochemical substances

4. Part of the structural framework of DNA and RNA molecules

5. Linked to lipids: Structural components of cell membranes

6. Linked to proteins: Function in cell-cell and cell-molecule recognition processes

Classification of Carbohydrates (based on how many building blocks you have)

• Monosaccharides

• Disaccharides

• Oligosaccharides

• Polysaccharides

Saccharide

Part of a word that is a giveaway that it is dealing with sugar.

Monosaccharides

• Single sugar unit

• Building blocks

  • glucose

  • fructose

Disaccharide

• Two monosaccharides linked together

  • maltose

  • lactose

  • sucrose

Oligosaccharides

2-10 monosaccharides linked together

Polysaccharides

• Many monosaccharides linked together

  • starch

  • glycogen

  • cellulose

Classification of Carbohydrates by Functional Group

• Aldose (ex. glucose)

• Ketose (ex. fructose)

Classification of Carbohydrates by Number of Carbon Atoms

• Triose (3C) - dihydroxyacetone, glyceraldehyde

• Tetrose (4C) - erythrose, threose

• Pentose (5C) - ribose, arabinose

• Hexose (6C) - glucose, galactose, fructose, mannose

Isomerism

Same molecular formula but differ in the way atoms are arranged.

Sterioisomers

Same molecular and structural formula, but differ in the orientation of atoms in space.

Enantionmers

• Non-superimposable mirror images

• X - superimposable

• / - mirror image

Diastereomers

• Stereoisomers that are not mirror images

• X - superimposable

• X - mirror image

Epimers

• Differ in configuration around a single specific carbon

• Point reference is glucose

• X - superimposable

• X - mirror image (only 1 center mirrored)

Dextro Isomers

OH group is on the right side

Levo Isomers

OH group is on the left side

Classification of Carbohydrates by the Haworth Projection Formula

Cyclic Configuration

• Pyranose: 6 member ring (ex. glucopyranose)

• Furanose: 5 member ring (ex. ribofuranose)

Anomers

Differ at the anomeric carbon in cyclic structures

• Alpha = OH down

• Beta = OH up (betaas)

Alpha

anomeric OH down

Beta

anomeric OH up

Biochemically Important Monosaccharides

1. D-Glyceraldehyde and Dihydroxyacetone

2. D-Glucose

3. D-Galactose

4. Fructose

5. Ribose

D-Glyceraldehyde and Dihydroxyacetone (Biological Importance)

• The simplest of the monosaccharides

• Trioses

Glucose (Biological Importance)

• Grape Sugar

  • since ripe grapes (20%-30% glucose by mass)

• Blood Sugar

  • since blood contains dissolved glucose (70-100mg/dL)

• Dextrose

• Main energy source

Galactose (Biological Importance)

• Brain Sugar

• Milk Sugar

• Synthesized from glucose in the mammary glands (in lactating moms) for use in lactose

• Present in the chemical markers that distinguish various types of blood:

  • A

  • B

  • AB

  • O

• Each blood type is based on 3-4 monosaccharides

Blood Type O

universal donor

Blood Type AB

universal recipient

Fructose (Biological Importance)

• Levulose

• Fruit Sugar

• used as dietary sugar

• used in HFCS (High Fructose Corn Sugar)

Ribose (Biological Importance)

• Pentose Sugars

• Component of RNA and ATP and DNA molecules

Reactions of Monosaccharides

1. Oxidation

   • C1 - weak oxidizing agent

   • -ose to -onic

     • Aldonic acid (mild oxidation)

     • Uronic acid (oxidation of primary alcohol)

     • Aldaric acid (strong oxidation)

   • C1 + C6 - strong oxidizing agents

   • -ose to -aric

     • Aldaric acid

   • C6 - enzymes

   • -ose to uronic

     • Alduronic acid

2. Reduction

   • -itol

   • Produces sugar alcohols

     • Sorbitol

3. Glycoside Formation

   • Hemiacetal to acetal

   • Replacement of the hemiacetal carbon -OH group with an -OR group

     • Glucoside

     • Galactoside

4. Phosphate Ester Formation

   • Attach phosphate to C6

5. Amino Sugar Formation

   • Hydroxyl group is replaced with an amino group

   • Amino sugar is produced

   • C2

Oxidation (Weak Oxidizing Agents)

• C1 - weak oxidizing agent

• -ose to -onic

  • Aldonic acid (mild oxidation)

  • Uronic acid (oxidation of primary alcohol)

  • Aldaric acid (strong oxidation)

Oxidation (Strong Oxidizing Agents)

• C1 + C6 - strong oxidizing agents

• -ose to -aric

  • Aldaric acid

Oxidation (Enzymes)

• C6 - enzymes

• -ose to uronic

  • Alduronic acid

Reduction

• -itol

• Produces sugar alcohols

  • Sorbitol

Glycosidic Formation

• Hemiacetal to acetal

• Replacement of the hemiacetal carbon -OH group with an -OR group

  • Glucoside

  • Galactoside

Phosphate Ester Formation

• Attach phosphate to C6

Amino Sugar Formation

• Hydroxyl group is replaced with an amino group

• Amino sugar is produced

• C2

• Usually found in polysaccharides

Biochemically Important Disaccharides

1. Maltose

2. Cellobiose

3. Lactose

   1. Lactulose

4. Sucrose

Maltose

• Glucose + Glucose

• Malt Sugar

• Beer Sugar

• Alpha 1-4 glycosidic linkage

Cellobiose

• Formed from intermediate hydrolysis of cellulose

• Beta 1-4 glycosidic linkage

• Hydrolyzed by cellobiase (enzyme)

Lactose

• Glucose + Galactose

• Milk Sugar

• Beta 1-4 linkage

• Alpha form is sweeter and more soluble in water than Beta form

Lactulose

• Neither digested nor absorbed in the intestine

• Laxative

• Used to relieve/alleviate

Sucrose

• Glucose + Fructose

• Table Sugar

• sugar cane contains up to 20% by mass

• sugar beets contain up to 17% by mass

• Alpha, Beta 1-2 glycosidic linkage

Classification of Polysaccharide

• Function

  • Storage Polysaccharide

  • Structural Polysaccharide

• Product of Hydrolysis

  • Homopolysaccharide

  • Heteropolysaccharide

  • Mucopolysaccharide

Storage Polysaccharide

• Storage form

• Used as an energy source in cell

  • Starch

  • Glycogen

Starch

• Plant energy storage

• AKA glucan, amylum, gluose

• Amylose + Amylopectin

Glycogen

• Animal energy storage

• Animal starch

• Stored in liver and muscles

Glycogenesis

• Synthesis of glycogen

• Excess Blood Glucose

Glycogenolysis

• Decomposition of glycogen

• Low Blood Glucose

Structural Polysaccharide

• Serves as structural element in plant cell walls and animal exoskeletons

  • Cellulose

  • Chitin

Cellulose

• Plant cell wall component

• Most abundant naturally occurring polysaccharide

• “Woody” portions of plants

• Beta 1-4 glycosidic linkages

• 5000 glucose units

• Humans lack this

Chitin

• Gives rigidity

• Also occurs in the cell walls of fungi

• Exoskeleton of arthropods

• Polymer of N-acetylglucosamine

• Beta 1-4 glycosidic linkages

Homopolysaccharide

• On hydrolysis yield only a single type of monosaccharide

• Glucans are polymers of glucose

• Fructosans are polymers of fructose

  • Starch

  • Glycogen

  • Chitin

  • Cellulose

Heteropolysaccharide

• On hydrolysis yield a mixture of few monosaccharides

  • Mucopolysaccharides

Mucopolysaccharides

• AKA glycosaminoglycans (GAG)

• Made up of repeating units of sugar derivatives

  • Amino sugars

  • Uronic acids

    • Hyaluronic Acid

    • Chondroitin Sulfate

    • Dermatan Sulfate

• Makes them acidic polysaccharide (negative)

• Forms mucoproteins or mucoids or proteoglycans

Hyaluronic Acid

• Found in connective tissue and joint lubrication

• Indicated for knee pain

• Can induce swelling

Chondroitin

Found in cartilage

Dermatan

Found in skin and blood vessels

Keratan

Small amounts of mannose, fructose, sialic acid.

Blood Sugar Regulation

• Insulin lowers blood glucose levels

• Glucagon raises blood glucose levels

Lactose Intolerance

Deficiency in lactase enzyme

Diabetes Mellitus

Impaired glucose metabolism

Dietary Fiber

Provides the digestive tract with “bulk” that helps move food through the intestinal tract and facilitates the excretion of solid waste.